Chilled Water Systems Rev2
HVAC Clinic
Chilled Water
Systems
Table Of Contents
Introduction ............................................................................................................................................ 3
Chilled Water System Design................................................................................................................ 3
Application Considerations................................................................................................................. 19
System Variations ................................................................................................................................ 25
HVAC Clinics
Draft - Not For Distribution
Introduction
The purpose of this clinic is to discuss modern chilled water system design as it pertains to modern HVAC systems.
Chilled water systems comprise the majority of larger built-up applied commercial cooling systems. A properly
designed chilled water system can offer unparalleled comfort at very low energy consumption. In addition, the
maintenance associated with most chilled water systems can be reduced compared to most other comparable
systems (heat pump systems, variable refrigerant flow, packaged rooftop systems, etc.). Finally, a properly designed
chilled water system can last in excess of thirty years before major equipment requires replacement. This compares
favorably to an expected 15-20 year lifetime with most rooftop, water source heat pump and variable refrigerant
volume systems.
Chilled Water System Design
A chilled water system comprises of (figure 1):





Load terminals
Chiller
Chilled water and condenser water pumps
Cooling tower
Distribution piping & accessories
Figure 1. Chilled Water System
The load terminals typically consist of chilled water coils for most commercial applied systems. However, other load
absorbing devices, such as chilled beams, may be utilized. The chiller maintains the chilled water loop at a
temperature that allows the load terminals to properly absorb the HVAC heat load. The chiller may be water cooled or
air cooled. If the system is air cooled, a cooling tower and condenser pumps will not be required. The chilled water
and condenser water pumps create the pressure differential required to maintain the required flow rates. The cooling
tower rejects the heat produced by the chiller. The distribution piping connects the pumps, tower, chiller and load
terminals. Finally, the accessories may include expansion tanks, air separators, heat exchangers, controls valves and
any devices in order to maintain proper operation of the chilled water and condenser water loop.
This section is dedicated to the discussion of the chilled water side of the system. The chilled water system
comprises of the chiller, pump, load terminals and distribution piping (figure 1). This section applies equally to air and
water cooled chillers. The function of the chilled water system is to transport the cold water produced by the chiller to
the load terminals and back.
The load terminal absorbs the heat from the airside system and transfers the heat to the chilled water system. In
order to absorb heat, the chilled water supplied to the terminal must be less than the air temperature being
conditioned. The controls measure the temperature of the supply air or space air and then modulate the capacity of
the coil to match the load.
WN Mechanical Systems
Chilled Water Systems
Page 3 of 31
HVAC Clinics
Draft - Not For Distribution
Three types of load modulation devices are typically used. Those are two way modulating valves, three way
modulating valves and face and bypass dampers (figure 2).
Figure 2. Load Terminals
Two way valves modulate the flow through the coil by increasing the pressure drop at the outlet of the coil. The water
flow through the coil modulates in near proportion to the load. Assuming the chilled water temperature rise across the
coil at part load remains constant; the chilled water flow would be proportional to load. However, the heat transfer
characteristics of a coil often dictate that the chilled water temperature difference of the coil at part load is actually
greater than the chilled water temperature difference at full load. This can unique characteristic of two way
temperature control valves lends itself well to waterside energy recovery. This subject will be discussed at greater
length later in this clinic.
Figure 3. Two Way Valve Control
Three way control valves modulate the flow through the valve, similar to that of two way control valves. However, the
quantity of water that is reduced through the coil at part load is re-mixed downstream of the load device through the
bypass pipe (figure 4). Three way control valves ensure constant water flow through the distribution system serving
the devices. In contrast, hydronic arrangements that serve systems with two way valves must be variable flow
systems. Hydronically, the two systems function exactly the same at the coil. The water flow is reduced, in near
proportion to the load, as the load decreases. The primary difference is that at part load, the mixing of the bypass
water and coil water will ensure that the water temperature difference at part load is always less than the water
temperature difference at full load.
Figure 4. Three Way Valve Control
WN Mechanical Systems
Chilled Water Systems
Page 4 of 31
HVAC Clinics
Draft - Not For Distribution
The hydronic system serving load terminals with three way valves will be constant flow systems. Constant flow
systems require precise balancing at each load terminal in order to maintain the required flow the all the terminals at
all loads. Two way systems, in contrast, do not require precise balancing due to the nature of the system. Coils with
two way valves require that the system will be variable flow.
Face and bypass dampers are another method of load control that is generally utilized for systems that require
humidity control. Face and bypass dampers maintain the load by modulating the airflow across the coil (figure 5). As
the coil damper modulates, the air is diverted through a bypass damper. The airflow across the coil modulates in
direct proportion to the load. The water flow to the coil remains constant at all load conditions. No coil water flow
modulation is required. This is referred to as a wild coil. At part load, the water temperature modulates in an identical
fashion to a three way valve. As the load decreases, the water temperature difference across the coil decreases.
Figure 5. Face & Bypass Dampers
Face and bypass dampers are better able to maintain humidity control at part loads compared to two or three way
valves. At part load, as the airflow decreases across the coil, the temperature of the air leaving the coil will decrease.
This assumes that the water flow rate at the coil remains constant. This lower temperature air is able remove
moisture from the unconditioned air. In contrast, when utilizing two or three way valves, the temperature of the air
leaving the coil will always increases at part load.
Summarizing the three methods of load control:



Two way modulating Valve
o Variable water flow
o Constant or increasing return water temperature at part load
Three way modulating valve
o Constant water flow
o Variable system return water temperature always less design return temperature
Face and bypass damper
o Constant water flow (wild coil)
o Variable system return water temperature always less design return temperature
o Improved dehumidification
Before choosing any of these three methods of load control, the effect on the system must be evaluated.
Several methods exist for connecting chillers to the load devices. The systems that will be discussed in this clinic:



Direct Coupled
o Single pump, single chiller
o Single pump, multiple chillers
o Multiple pumps, multiple chillers
o Single pump, chillers in series
o Multiple pumps, chillers in series
Primary-Secondary (Decoupled)
o Single pump, single chiller
o Multiple pumps, multiple chillers
o Single pump, chillers in series
o Multiple pumps, multiple chillers in series
o Multiple pumps, preferentially loaded chillers
o Multiple pumps, sidecar chiller
Variable Primary
WN Mechanical Systems
Chilled Water Systems
Page 5 of 31
HVAC Clinics
o
o
Draft - Not For Distribution
Multiple pumps, multiple chillers
Single chiller, single pump
Each of these methods will be discussed in detail. Additional variations exist beyond those shown in figure above.
However, a discussion of those systems is beyond the scope of this clinic.
Direct Coupled Arrangements
Constant volume, direct coupled single chiller systems are generally utilized for smaller commercial applied HVAC
systems. Often, single chillers systems, due to the inherent smaller size, utilize air cooled chillers. A single chiller
system uses a single pump to maintain flow to the coils. A sufficient number of coils will be equipped with three ways
coils in order to maintain relatively constant flow back to the chiller.
Figure 6. Direct Coupled System
Directly coupled, single chillers systems have the advantage of being cost effective when compared to other chilled
water system designs. Due to the simple operation of the system, the controls are vastly simplified. In fact, many
single chiller, directly coupled chiller plants utilize stand-alone control systems. Communicating controls are not
required.
Multiple chiller plants present two primary advantages when compared to single chiller designs. Those advantages
are:


Redundancy
Efficiency
Multiple chillers are utilized in larger HVAC systems for the same reason a commercial aircraft utilizes several
engines; redundancy. Put simply, redundancy provides reliability. Should a chiller, pump or tower fail, the building
has one or multiple backup chiller systems. For many building owners today, redundancy is a necessity.
In addition, multiple chiller plants provide an increased opportunity to realize plant energy savings. Large systems
operate largely at part load. At part load, chillers and their associated pumps and tower fans can be turned off.
Energy savings can be realized with larger chiller plants by first turning off accessories (pumps, fans, etc.) before
unloading the chiller itself.
For example, let’s compare a system with two 250 ton chillers operating with their own dedicated pumps and tower
fans to a system with a single 500 ton chiller of equal efficiency with its own pumps and tower. While the part load
efficiency of the 500 ton chiller may be as consume as little as 60% of its full load efficiency at 50% load (250 tons),
utilizing a single chiller at 100% load (250 tons) and turning off the other chiller, pumps and towers will generally result
in greater energy savings. This may seem counterintuitive given the improved part load efficiency of a chiller at 50%
load. However, a chiller has a very high COP. COP is a unit less measurement. It is the ratio of work or useful
WN Mechanical Systems
Chilled Water Systems
Page 6 of 31
HVAC Clinics
Draft - Not For Distribution
output to the amount of work or energy input. Table 1 below summarizes the COP of the various components utilized
in chilled water systems.
Table 1. Component COP's
Component
COP
Water Cooled Chiller
6-7
Evaporator and Condenser Pumps
.7
Tower Fans
.4
Upon inspection of table 1, it become apparent that chillers are very efficient energy consuming devices compared to
pumps and tower fans. As such, it behooves the Engineer to design a system in which these accessories can be
readily turned off. Unless the part load efficiency of the chiller is phenomenal compared to the full load efficiency (as
see with some magnetic levitation chillers), a plant will consume less energy if power consuming accessories such as
pumps and fans are turned off in lieu of unloading larger chillers.
The most basic type of directly coupled multiple chiller plant is the single pump (constant speed), multiple chiller
system (figure 7). While this system is simple to implement and control, it has a major drawback. As the load
decreases and a chiller is de-energized, the flow through both chillers must remain. If an isolation valve is used to
isolate the disabled chiller, the activated chillers flow rate will be nearly doubled. Even if the chiller can operate under
those conditions, the leaving chilled water temperature will increase.
Figure 7. Direct Coupled Single Pump
Conversely, if the flow remains through both chillers, the activated chiller will make its setpoint temperature while the
deactivated chiller will simply supply the system return water temperature. These two temperature streams will mix at
the return header.
WN Mechanical Systems
Chilled Water Systems
Page 7 of 31
HVAC Clinics
Draft - Not For Distribution
In either scenario, we find that we have a variable temperature being supplied to the load terminals. This temperature
will always be greater than the design temperature. While the building is at part load, some load terminal may not be
able to satisfy their individual loads at this higher temperature. Internal zones are examples of zones which may have
high cooling loads, even when the building is at a reduced load. It is very likely that this system would not be able to
maintain the loads in these types of zones. For this reason, directly coupled single pump, multiple chiller systems are
very rarely implemented.
Another variation of directly coupled, multiple chiller systems is to dedicate a single pump to each chiller (figure 8). In
this scenario, the temperature mixing issue experienced with the single pump, multiple chillers is resolved. However,
at part load, this system presents a new dilemma. At part load, as a chiller is de-activated, its associated pump is
turned off. This prevents mixing of dis-similar temperature streams at the return header. However, the flow to the
system is reduced.
Figure 8. Direct Coupled Dedicated Pumps per Chiller
In order to understand this dynamic, we must understand the interaction of multiple parallel pumps with the system
curve (figure 9). When two pumps are operating, we get 100% of the design flow. However, when a pump is turned
off, the flow is reduced by 35-45%. In the example below (figure 9), two Taco KV4013 pumps are selected. With one
pump operating, the reduction in flow is 41%.
Figure 9. Pump Example
WN Mechanical Systems
Chilled Water Systems
Page 8 of 31
HVAC Clinics
Draft - Not For Distribution
This loss of flow can have a tremendous negative impact on the operation of the system. Assuming the load devices
are designed with three way control valves, the system is now operating with 35% less flow. While we are at part
load, some of the load devices may be starved of flow. Similar to the previous example of the single pump, multiple
chiller scenario, some interior zones which remain highly loaded may be starved of flow.
In addition, as chillers are enabled and disabled, they will see an instantaneous change in flow. For example, when a
chiller is de-activated and its associated pump is turned off, the chiller see an increase in flow of 30%. Most chillers
will not be able to run with an instantaneous 30% change of flow. The chiller will turn off based on internal pressure
safeties.
One very effective method of combating the issues presented thus far associated with directly coupled systems is to
utilize chillers in series (figure 10). Piping chillers in series allows the designer to experience the advantages
associated with redundancy while solving the temperature mixing and flow issues discussed in the previous
examples.
Figure 10. Chillers in Series
However, chillers in series do have their disadvantages. If the system is designed for a typical 10 oF temperature
delta, each chiller will share equally in the delta. That is, each chiller will operate at a 5 oF temperature delta at full
load. This means, each chiller will see double the flow in order to maintain design capacity (load ≈ flow x ∆T).
However, being that the pressure drop of most chillers is optimized at a 10oF ∆T, we will see a quadrupling of
pressure drop across the chiller barrel (recall ∆P ≈ flow2). Between the two chillers, we would see 4x the pressure
drop experienced with a typical two chiller parallel plant.
There is a solution. What would happen if the system was designed for a 20 oF ∆T? At a 20oF ∆T, each chiller would
pick up a 10oF ∆T and the system would experience the same pressure drop as a typical two chiller parallel plant.
Note however, that at part load with one chiller de-activated, the pump still must overcome the pressure drop
experienced by both chillers. In addition, in order to maintain capacity at the load terminals, the mean temperature
must be the same as a 10oF ∆T system. For example, if a 10oF ∆T system was designed at 55oF-45oF, a 20oF ∆T
system would have to be designed at 60 oF-40oF in order to maintain the same effective load making capability at the
coils. In this scenario, the chillers may take an efficiency penalty. The upstream chiller would be sized at 60 oF-50oF,
increasing its relative efficiency compared to either chiller in a two chiller parallel configuration plant sized at 55oF45oF. However, the downstream chiller would be sized at 50oF-40oF, decreasing its relative efficiency compared to
either chiller in a two chiller parallel configuration plant sized at 55oF-45oF. While it is possible the efficiencies of the
two chillers will balance, it is more likely there will be a small decrease in overall chiller efficiency. This can easily be
overcome by selecting a more efficient chiller downstream. In addition, the warmer temperatures experienced by the
upstream chiller make it idea for utilizing alternate chiller types. For example, an absorber, which runs with much
higher COP’s at higher supply water temperatures, would be idea as an upstream chiller.
Increasing the design temperature difference in the system has an added advantage. Pump energy is:
WN Mechanical Systems
Chilled Water Systems
Page 9 of 31
HVAC Clinics
Draft - Not For Distribution
Thus, by utilizing a 20oF ∆T system that cuts our flow rate in half, we will experience an 87.5% decrease in pump
energy. Considering that pumps have a very low COP, the overall system energy savings utilizing chillers in series
can be tremendous.
Series chillers can be controlled using one of two methods. The first method involves creating equal leaving chilled
water setpoints for each chiller (figure 11). In this scenario, the upstream chiller will be preferentially loaded. For
example, assume two equally sized chillers and a 60 oF-40oF temperature split. Both chillers are set to maintain a
40oF supply temperature. At loads less than 50%, the upstream chiller will be able to maintain the 40oF supply
temperature and the downstream chiller will remain off. As the load increases above 50%, the upstream chiller can
no longer maintain the 40oF setpoint. At that point, its supply temperature will rise above 40oF and the downstream
chiller will be activated.
Figure 11. Equal Set Points
Series chiller arrangements with equal setpoints are ideal for heat recovery chillers. Because the upstream chiller is
always at full load when the building load is above 50%, the upstream chiller can always produce hot water. In
addition, heat recovery chillers operate more efficiently and can produce hotter water at higher supply water
temperatures.
Alternately, the chillers can be set with staggered chilled water setpoints (figure 12). This would preferentially load the
downstream chiller. For example, assume two equally sized chillers and a 60 oF-40oF temperature split. The
upstream chiller is set to maintain 50oF. The downstream chiller would be set at 40oF. At part load (<50%), the
upstream chiller would remain off so long as the return water temperature is less than 50 oF. The downstream chiller
would operate. As the load increases beyond 50% and the return water temperature rises above 50 oF, the upstream
chiller is activated.
Figure 12. Staggered Set Points
Series systems with staggered setpoints are also idea for chiller of differing efficiencies. A less efficient upstream
chiller will benefit from the increased leaving chilled water temperature associated with staggered setpoints. This in
WN Mechanical Systems
Chilled Water Systems
Page 10 of 31
HVAC Clinics
Draft - Not For Distribution
effect increases the efficiency of the upstream chiller. The more efficient downstream chiller is ideally suited to make
colder water.
Similarly, absorption chillers will benefit in the upstream position in a series system with staggered setpoints. An
absorption chillers COP increases dramatically as the leaving chilled water temperature is increased. An electric
chiller is then assigned as the downstream chiller. This type of plant can not only benefit from tremendous efficiency
advantages, but it also functions as a dual fuel type central plant.
Finally, a heat pump chiller is ideal suited in a series arrangement as the upstream chiller. Recall that a heat pump
chiller controls to the condenser water setpoint. The chilled water temperature is wild and is thus not controlled. In a
series arrangement with a heat pump chiller upstream, the heat pump maintains the heating water setpoint and
benefits from seeing the increased return water temperature. This vastly improves the heat pump chiller efficiency.
The downstream chiller maintains the system chilled water setpoint.
Due to the pressure drop associated with piping chillers in series, it is generally impractical to run more than two
chillers in a series configuration. However, a designer could run multiple series chillers in a parallel direct coupled
arrangement (figure 13). These could be run with a single pump arrangement, similar to that of figure 13. However,
the same challenges exist with regards to maintaining the supply water temperature, but to a lesser degree. As a
single chiller is turned off on the first branch, the other chiller on the same branch will maintain the design supply
water temperature. Similarly, as a second chiller is turned off on the second branch, the second chiller on that branch
will maintain the supply water temperature. The problem occurs as the third chiller is turned de-energized. As that
chiller is turned off, the system can no longer maintain the supply water temperature. It is at this point, the system
may become unstable.
Figure 13. One Pump with Multiple Chillers in Series
With regards to direct coupled plants, running single pump with multiple series chillers in parallel operates much better
than a single pump with single chillers in parallel. However, it is still far from ideal. In the four chiller example, the
system may become unstable at loads less than 25%.
Much like running chillers in parallel, each with their own dedicated pumps; running the chillers in a series
arrangement does improve the operation of the system (figure 14). The first two chillers on each branch can be deenergized without any adverse effects to the system. It isn’t until the third chiller is turned off that the problems begin.
As the third chiller is de-energized, a pump is turned off. As soon as the pump is turned off, the flow will decrease by
35%. It is at this point, the system become unstable.
WN Mechanical Systems
Chilled Water Systems
Page 11 of 31
HVAC Clinics
Draft - Not For Distribution
Figure 14. Two Pumps with Multiple Chillers in Series
Chillers in series are an excellent design option for a two chiller, direct coupled plants. They give the designer
redundancy, simplicity of controls and excellent efficiency. However, with regards to directly coupled plants with more
than two chillers, series configurations improve upon the challenges presented with single chillers piped in parallel,
but they are not without their limitations. Thus, when designing a direct coupled plant, either a single chiller or two
chillers in series should be employed.
Primary-Secondary Configurations
Due to the inherent limitations and system instability associated with direct coupled plants utilizing more than a single,
non-series chiller arrangement, an alternate piping arrangement was needed. In 1954, Bell & Gossett’s Gil Carlson
introduced what has become known as the primary-secondary pumping scheme to address this problem. A primarysecondary pumping scheme, also known as a decoupled chiller plant, divides the chilled water system into two distinct
loops that are hydraulically separated by a neutral bridge. These loops are known as the production or primary loop
and the distribution or secondary loop. The inherent separation of the primary and secondary loops allows variable
flow through the distribution system to match the cooling load, while maintaining constant flow through the chillers
(figure 15).
Figure 15. Primary Secondary Configuration
WN Mechanical Systems
Chilled Water Systems
Page 12 of 31
HVAC Clinics
Draft - Not For Distribution
The coils in a primary-secondary configuration are controlled with two way valves. As the load at the coils decreases,
the system pressure increases. The control system responds to the increase in system pressure and in turn
modulates the distribution pump.
The bypass pipe is common to both the production and distribution loops. The bypass pipe hydraulically decouples
the production and distribution pumps. There should not be any control valves, check valves or any other accessories
in the bypass pipe. Because water can flow freely between the supply and return pipes for both loops, a change in
flow in one loop does not affect the flow in the other loop.
The actual extent of the hydraulic decoupling depends on the amount of pressure drop in the bypass pipe. Perfect
decoupling is accomplished only if the pressure drop in the bypass pipe is zero. Because there will be some pressure
drop, some degree of pump coupling will occur. Bypass pipes are typically sized for a velocity of between 10 ft/s and
15 ft/s. The bypass pipe should be sized for a flow rate equal to that of the largest chiller in the system. While the
bypass pipe should length should be minimized so as to prevent coupling, a minimum distance of 5-10 pipe diameters
is required in order to prevent random mixing of the supply and return water streams.
Primary-Secondary Rules:
 Sized for minimum pressure drop, generally 10-15 ft/s
 Allow 5-10 pipe diameters to avoid random mixing of supply and return water streams
 No check valve or other obstructions
The production pumps circulate the chilled water through the chiller, supply tee, bypass pipe and return tee (figure
16). This pressure drop is relatively small compared to the pressure drop in the distribution loop. In addition, each
pump only operates when its respective chiller is operating. Not only is the production loop decoupled from the
distribution loop, but each chiller is decoupled from one another. This allows a tremendous amount of flexibility with
regards to chiller selection. The chillers can be of any type, size or age.
Figure 16. Primary Loop
A headered pump arrangement can be used for the production pumps (figure 17). However, a headered arrangement
hydraulically couples the chillers on the production loop. Headered pumps allow redundancy. In addition, headered
pumps can be used headered with chillers of varying capacities so long as some means of pump flow control is
utilized, such as variable frequency drives.
However, manifolding production pumps does have its drawbacks. Manifolded pumps hydraulically couple the chillers
on the production loop. If a chiller isolation valve is opened while one pump is running, the flow rate will drop in the
operating chiller. This instantaneous drop in flow could cause the chiller to fault and turn off. Conversely, if a pump is
energized before opening the second chillers isolation valve, the operating chiller will see a sudden increase in flow.
This can cause water hammer and chiller operation issues. Variable frequency drives or pumps with ECM motors can
be used to help alleviate the flow variations caused when enabling and disabling chillers. For example, before a
second chiller is enabled, the disabled pump can be slowly ramped up to 50% flow while the enabled pump is slowly
modulated down to 50% flow. Then, as the second chiller’s isolation valve is slowly opened, the variable frequency
drives are allowed simultaneously ramp up to 100% of the design flow for the two chillers. While similar sequences
WN Mechanical Systems
Chilled Water Systems
Page 13 of 31
HVAC Clinics
Draft - Not For Distribution
can be effective at softening the flow surges associated with starting and stopping chillers, it complicates the controls
and increases the cost associated with the system.
Figure 17. Manifolded Pumps
The distribution loop circulates water from the supply tee, through the building load and back to the return tee (figure
18). All of the building load terminal should be equipped with two way valves. The distribution loop is designed to be
a variable flow loop. The distribution pumps should utilize variable capacity pumps. As the two way valves modulate,
the distribution pumps modulate to match the required flow at the load terminals.
While a primary second system requires additional pumps, slightly more complex controls and a bypass pipe, often
the overall cost associated with the system will actually decrease compared to a traditional direct coupled system.
First, each pump is sized for the decreased head associated with the production and distribution loop. More
importantly, the distribution pump is sized for the block flow rate. In a traditional direct coupled system, the pumps
must be sized for the flow of the sum of the peaks flow rate. Depending on the building diversity, this can decrease
the flow rate for the production loop by as much as 30%. This decrease in head, flow rate and the associated
increase in efficiency of the pump selections often will offset the additional cost associated with the bypass pipe and
controls.
Figure 18. Secondary Loop
Rather than utilizing a single distribution pipe and variable frequency drive, multiple distribution pumps may be used.
A primary secondary arrangement allows the selection of multiple pumps on the distribution loop without affecting the
pumps on the production loop. This allows for selection of multiple pumps which may operate more efficiently at part
load and increased redundancy.
WN Mechanical Systems
Chilled Water Systems
Page 14 of 31
HVAC Clinics
Draft - Not For Distribution
Primary secondary pumping arrangements lend themselves well to campus type environments. A single or multiple
pumps could be assigned to each building. The returns from each building would be headered before entering the
bypass pipe (figure 19). Using this configuration, each building would hydraulically operate independently of the other
buildings. In addition, the use of a primary secondary configuration allows the each building pump(s) to operate
without affecting the production loop pumps.
Figure 19. Campus Environment
An important characteristic to remember when designing primary second systems is that the return water temperature
upstream of the bypass will always be equal or greater than the design ∆T. As discussed previously, load terminals
with two way valves will generally see higher return water temperatures at part load. Being that a primary secondary
system should be designed with two way valves on all load terminals, the return water temperature should always
increase at part load. This becomes very advantageous when waterside free cooling is required. More on this topic
will be discussed later in this clinic
The temperature in the return tee is a great indicator of excess or deficit flow in the production loop. For example,
consider the situation described in figure 20. Assume the system has a design supply temperature of 45oF with a
55oF return and two 1000 gpm chillers. If deficit flow is present in the system, the temperature on both sides of the
return tee will be equal to the system return temperature (in this case 60oF). Because the distribution loop requires
1,500 GPM of flow, water will flow through the bypass into the supply tee. The warm return water will mix with the
cold production water and dilute the supply water delivered to the distribution loop.
Figure 20. Deficit Flow
When deficit flow is present, the system is short of capacity and another chiller must be enabled. In the example
above, because the return water temperature is equal on both sides of the return tee, we know we need to turn on a
WN Mechanical Systems
Chilled Water Systems
Page 15 of 31
HVAC Clinics
Draft - Not For Distribution
second 1000 GPM chiller. When the second 1000 ton chiller and pump is enabled, we create a situation in which we
have excess flow in the bypass line (figure 21). Under these circumstances, water flow will flow in the direction of the
return tee. This excess water flow will dilute the return water temperature on the production side of the return tee.
This will cause water temperature on the production side of the return tee to be less than the water temperature on
the distribution side of the tee.
Figure 21. Excess Flow
Measuring the water temperature in the bypass and the water temperatures on both sides of the return tee is an
excellent way to ascertain when to start and stop chillers. Knowing the flow through the plant, the flow through the
bypass can be calculated as:
Where:
GPMb = flow rate through bypass (gpm)
GPMp = flow rate through plant (gpm)
Trs = Return Temperature in secondary loop (oF)
Trp = Return Temperature in primary loop (oF)
Tb = Temperature in bypass (oF)
Given the previous equation, the flow rate can be determined at any load. This allows the control system to easily
determine the flow rate at any load, without the use of expensive flow measuring devices. Based on the flow in the
bypass pipe, the control system can then determine when to add and subtract chillers and pumps.
When the control system adds or subtracts pump-chiller combinations based on the flow in the bypass pipe should be
carefully considered. For example, if the a chiller is added too quickly and the load causing the excess flow in the
bypass was due to a transient effect, a chiller will need to be dropped quickly in order to prevent over production of
flow in the bypass. In addition, enabling chillers due to transient loads can waste valuable energy. Conversely,
dropping chillers too quickly can result in a loss of temperature control at the load.
WN Mechanical Systems
Chilled Water Systems
Page 16 of 31
HVAC Clinics
Draft - Not For Distribution
Table 2 below summarizes when to add or subtract chillers based on the flow in the bypass
Table 2. Primary Secondary Sequencing
Flow in bypass
Condition
Action
Deficit
Deficit for 15 – 30 minutes
Start another pump-chiller
Excess
Excess flow for 110%-115% of flow produced by next
pump to be turned off
Stop
Excess
Excess flow < next pump/chiller to be turned off
Do Nothing
Based on the flow in the bypass, chillers should not be subtracted unless there has been deficit flow in the system for
between 15 and 30 minutes. This ensures that the load increase was not simply a transient effect, preventing overcycling. The length of the timer is a function of the size and mass of the load. Larger building should utilize longer
timers. Very little loss of temperature control at the load should occur due to the use of the timer. Should there be
excess flow in the bypass that exceeds the next pump-chiller combination to be turned off, that pump-chiller
combination should not be de-activated until the flow in the bypass exceeds its design flow rate by 110% to 115%.
This prevents short cycling due to a slight increase in load and potential increased demand charges by the utility.
Variable Primary Configurations
Throughout the 21st century, chillers were considered constant flow evaporator devices. However, the flow would
always change slightly due to systems effects. These effects include the interaction of the pump with the system
curve and variations of flow as chillers are enabled and disabled along with their associated pumps. Modern chilled
water systems commonly employ variable flow through the evaporator. This further enables designers to achieve
valuable pump energy savings.
Figure 22. Evaporator Flow
In a variable primary system (figure 23), the flow of water through the entire system is allowed to modulate. The plant
is directly coupled to the load. The distribution pumps are eliminated. Two way valves at the load terminals, similar to
that of a primary secondary system, are required. Three way valves should not be used. The production pumps are
selected to run as variable flow rate pumps. As the two way valves at the load terminals modulate, the variable
capacity pumps modulate to match the required flow at the coils. The production pumps are controlled to maintain a
minimum system pressure in order to maintain flow at remote load terminals.
WN Mechanical Systems
Chilled Water Systems
Page 17 of 31
HVAC Clinics
Draft - Not For Distribution
Figure 23. Variable Primary
A bypass pipe, similar to that of a primary-secondary system, is still required. However, the bypass pipe is installed
with a two way modulating control valve. All chillers have minimum and maximum evaporator flow rates, as well as
defined rates of change of flow per minute that can be tolerated without adversely affecting the operation of the chiller.
The function of the bypass pipe and valve is to maintain the minimum flow rate required by the chiller. As the
minimum flow rate of the chiller is approached (typically around 30-40% of design flow), the bypass valve is controlled
to maintain the chiller minimum flow.
The bypass pipe should be designed such that:







Size for minimum flow through one chiller
Bypass valve shall be the normally open type
Valve head ratings must be higher than pump dead head
Linear characteristic - valve position % equals flow %
Fast actuator
Valve control range 100:1
Flow meter controls bypass to maintain chiller minimum flow
Variable primary systems simplify the design of primary-secondary systems by eliminating the need for the distribution
pumps along with their associated electrical and piping connections. In addition, there is a reduction in operating
costs by eliminating any constant speed pumps and by utilizing warmer return water temperatures at the plant (when
the bypass control valve is closed). However, the controls associated with variable primary systems are slightly more
complex. With primary-secondary systems, the flow in the bypass pipe can be used to determine when to add and
subtract chillers. However, being that the flow in the bypass of a variable primary system is often zero; some other
means of determining flow must be incorporated. A flow device (ultrasonic, magnetic, vortex, turbine or pressure
based) located on the distribution side of the return tee is used to control the two way bypass control valve.
Additionally, the flow device can be used to determine the total system load. Knowing the flow, the system load can
be determined using the following equation:
WN Mechanical Systems
Chilled Water Systems
Page 18 of 31
HVAC Clinics
Draft - Not For Distribution
Based on the system load, the chillers can be appropriately sequenced. Chiller sequencing of variable primary
systems will be discussed later in this clinic.
Figure 23 shows show a multiple chiller variable primary arrangement. However, single chiller variable primary
systems are not only becoming relatively prevalent with modern HVAC systems, but offer significant energy savings
compared to traditional direct coupled single chiller plants (figure 24).
Figure 24. Variable Primary with Three Way Valve(s)
Multiple chiller variable primary system can be somewhat complex to control due to the added complexity involved
with determining when to add and subtract chillers and maintaining a fixed rate of change of flow. This involves
installing one or several flow measuring stations and added programming complexity. However, single chiller variable
primary systems are much more straightforward to control. The logic required to add and subtract chillers is not
required. Some means of flow measurement is required in order to operate the bypass control valve. Otherwise, the
control sequence is nearly identical to that of a primary-secondary system. Yet, the added energy savings make
single chiller variable primary systems a very practical energy efficient solution for many small to medium sized chilled
water plants.
Application Considerations
Waterside Energy Recovery
Several methods exist for waterside energy recovery of chilled water systems. Waterside energy recovery is
especially useful in climates with relatively low ambient wet bulb temperatures or applications in which airside energy
recovery is not practical (i.e. hotels with fan coils). Applied correctly and in the right ambient conditions, systems
utilizing waterside energy recovery can realize a very short return on investment.
Traditionally, waterside economizers utilizing plate and frame heat exchangers were placed in a location that allowed
them to operate parallel to the chillers in the plant (figure 25). The plate and frame heat exchanger would operate
whenever the ambient wet bulb enabled the plate and frame heat exchanger to produce the desired supply water
temperature to the load devices. For example, assume a system with a 10oF degree tower approach and a 3oF heat
exchanger approach. Assuming the load devices require 45oF chilled water, the plate and frame heat exchanger will
operate whenever the ambient wet bulb is less than 32oF (45oF - 10oF tower approach - 3oF h/x approach). If chilled
water reset is utilized, which was common with older direct coupled chilled water plants, waterside energy recovery
could be exploited for a slightly larger percentage of operating hours.
WN Mechanical Systems
Chilled Water Systems
Page 19 of 31
HVAC Clinics
Draft - Not For Distribution
Figure 25. H/X in Parallel
However, the advent and proliferation of primary-secondary and variable primary pumping systems changed the
dynamics of how waterside energy recovery is implemented. First, temperature reset should not be applied to
systems with variable capacity production pumps. As discussed earlier, the COP of chillers is very high when
compared to pumps. Chilled water reset attempts to save chiller energy in lieu of pumping energy. The chiller is reset
to a higher temperature while the pumps operate at a relatively high flow rate. However, this is counterproductive
considering the COP disparity between chillers and pumps. If anything, chillers should generally be run at higher
loads or lower temperatures while trying to offset pumping energy.
In addition, primary-secondary and variable primary system with load terminal units utilizing two way control valves
create higher return water temperatures at part load. If we were to re-locate the position of the heat exchanger to
return leg of the distribution loop, we would not only be able to take advantage of that warmer return water
temperature at part loads, but we would be able to utilize free cooling whenever the heat exchanger is able to reduce
the temperature of the return water. We would no longer be constrained to using the heat exchanger only during
those select hours when the ambient wet bulb is low enough to produce the distribution supply water temperature.
This type of system variation is called a sidecar plate and frame piping arrangement (figure 26). In this arrangement,
the plate and frame can run concurrently with the system’s chillers. Whenever the ambient wet bulb is sufficient to
reduce the return water temperature of the system, a three way valve is opened and the heat exchanger pump is
activated, enabling free cooling.
WN Mechanical Systems
Chilled Water Systems
Page 20 of 31
HVAC Clinics
Draft - Not For Distribution
Figure 26. Heat Exchanger in Secondary
For example, assume a plant is sized to supply 45oF chilled water with a 55oF design return water temperature. At
50% load, the return water temperature is 58oF (recall that return water temperature will rise at part load upstream of
the bypass). Assume a 10oF degree tower approach and a 3oF heat exchanger approach. If the outdoor ambient wet
bulb is 40oF, the heat exchanger can reduce the return water to 53oF (40oF wet bulb + 10oF tower approach + 3oF h/x
approach). In this scenario, we have just reduced the load to chillers by 38% ([58 oF - 55oF]/[ 58oF - 45oF]). If the heat
exchanger was piped in parallel, the ambient wet bulb would not have been sufficient to allow any amount of free
cooling. The chillers would have to condition the entire load from the building without the benefit of free cooling.
Controls
When controlling chilled water plants, a number of variables should be considered:




When to turn a chiller on
When to turn a chiller off
How to optimize efficiency
How to optimize reliability
This discussion will focus on the control of multiple chiller systems only. Single chiller plants utilize many of the same
concepts as multiple chiller plants, but generally require simplified control sequences. In addition, being that multiple
direct coupled chiller systems are rarely used in modern HVAC system, discussion of controlling those systems will be
excluded as well.
Chiller sequencing involves when to turn on an additional chiller, when to turn off a chiller and which chiller to turn on
or off (figure 27).
WN Mechanical Systems
Chilled Water Systems
Page 21 of 31
HVAC Clinics
Draft - Not For Distribution
Figure 27. Chiller Sequencing
Table 2 below summarizes the most common methods of sequencing multiple chiller plants.
Table 3. Methods of Sequencing
System Type
Primary-Secondary
W/ Constant Speed
Chillers
Primary-Secondary
w/ Variable Speed
Chillers
Variable Primary
Systems
Turning On Chiller
Deficit Flow in
bypass for 15 – 30
minutes
Chiller to be turned
On/Off
Chiller to be
turned On/Off
Asymmetrical
Chillers
Equal Size
Chiller
Pump-chiller
combination results
in highest plant
loading
Run hours on
stop, starts on
run
Turning Off Chiller
Excess flow 110%-115%
of flow produced by next
pump to be turned off
Based on change of optimum chiller combination
to maximize plant efficiency (see figure 28 for
example) for 15 – 30 minutes
Pump-chiller
combination results
in highest part load
efficiency
Run hours on
stop, starts on
run
Primary-secondary systems employing constant speed chiller technologies are generally controlled by measuring the
flow in the bypass. As discussed earlier in this clinic, the most common method of determining flow for primarysecondary system is measuring the temperature at both sides of the return tee and at the temperature in the bypass.
Assuming constant speed compressors are utilized, chillers are added when deficit flow exits in the bypass for 15- 30
minutes. Constant speed chillers are disabled when there is excess flow in the bypass equal to 110% to 115% of the
flow produced by the next pump chiller pair to be disabled. This method ensures that the constant speed chillers are
loaded to their maximum potential.
Determining which pump-chiller pair to start or stop is a little more complex. If the plant utilizes asymmetrical constant
speed chillers in a primary-secondary arrangement, it is generally best practice to turn off the pump-chiller
combination that will result in the fewest number and smallest pump-chiller combinations being fully loaded. Recall
that while chillers are generally more efficient at part load, the accessories connected to them operate at much lower
COP’s (pumps, towers, etc).
WN Mechanical Systems
Chilled Water Systems
Page 22 of 31
HVAC Clinics
Draft - Not For Distribution
If the plant utilizes constant speed chillers of equal size in a primary-secondary arrangement, it is generally best
practice to turn off the chiller with the highest number of run hours. Conversely, if a chiller needs to be started, it is
best to prevent cycling and start the chiller with the lowest number of starts. Chillers are machines which prefer to
run. Starting and stopping chillers creates much more wear than running chillers. Thus, when starting chillers, always
try to optimize based on the number of times each chiller has started.
Systems utilizing primary-secondary systems with variable speed compressors often generally operate more
efficiently with multiple chillers at part load compared to fewer chillers at full load. The energy usage associated with
the pumps will not offset the improved part load efficiency of the chillers. For example, a system utilizing two 500 ton
machines and a 500 ton load will likely operate more efficiently with two 500 machines at 50% load compared to a
single 500 ton machine operating at 100% load. Generally speaking, this would not be true with constant speed
chillers. While this is somewhat of a generalization, the improved part load efficiencies of variable speed machines,
especially those utilizing magnetic levitation technology, will more than offset the energy associated with any constant
speed pump when operating at part load.
This operating characteristic remains true when running constant speed chillers in a variable primary arrangement.
Recall that as a pump unloads to 50% flow, it consumes 12.5% of its full load energy. In addition, a constant speed
chillers part load efficiency continues to improve, compared to its full load efficiency, until about 40% capacity. Thus,
like primary-secondary systems with variable speed chillers, which chiller to turn on an off should be a based on the
optimum chiller combination which results in the maximum plant energy efficiency. The same method of control
remains true with variable primary chillers utilizing variable speed compressors.
Which chiller to turn on and off with variable primary systems depends on the type and efficiency of the chillers
selected. If the plant utilizes an asymmetrical design, it is generally best to turn off the chiller that will result in the
highest part load efficiency. Modern control systems will determine the instantaneous plant COP for each chiller
combination. For example, assume a 2300 ton plant that consists of a 500 ton, 800 ton and 1000 ton chiller. The
control system should determine the plant efficiency for each chiller combination (figure 28) and then run the chillers
which minimize plant energy consumption.
Figure 28. Load versus Efficiency
It should be noted that the plant efficiency curves and chiller load making capability will change based on entering
condenser water temperature. As the condenser water temperature drops as a function of ambient wet bulb
temperature, the chillers individual load making capability will increase and the plant efficiency curves will increase.
The control system should be able to calculate these changes and optimize the ideal chiller combination curves as a
function of ambient wet bulb.
The rate of change of flow through chillers in variable primary systems is of critical importance to control accurately.
Chillers have defined flow rate changes that they can withstand without adversely affecting the operation of the chiller.
Controlling the rate of change of flow rate is especially important when turning on additional chillers.
Finally, three general system timers are commonly employed in the control of chilled water plants. Those timers are:



Load Confirmation Timer
Staging Interval Timer
Minimum Cycle Timer
WN Mechanical Systems
Chilled Water Systems
Page 23 of 31
HVAC Clinics
Draft - Not For Distribution
The load confirmation timer provides a delay before turning on an additional chiller. This timer prevents prematurely
starting a chiller during load transients. The staging interval timer provides the system time to respond after a chiller
has been turned on before activating another chiller. This timer prevents additional chillers from being activated
during periods of rapid load variation or pull down. Finally, the minimum cycle timers prevents over cycling of chillers.
Rapid cycling of chillers can prematurely wear critical components, leading to equipment failure.
Based on the hydraulic configuration of some plants, the control system may programmed to unload chillers before
starting or stopping machines. Recall that if chillers are served by headered pumps, the flow rate through one chiller
can change dramatically when the isolation valve to another parallel chiller is opened. For example, when a chiller is
enabled and its associated isolation valve is opened, there will be an overall drop in flow in the system before the
second chillers pump is enabled (figure 29). If the first chiller is unloaded before the second chillers isolation valve is
opened, it will be better able to withstand the sudden drop in flow that will accompany the opening of the valve.
Figure 29. Unload Before Start
Finally, the control system should optimize the energy consumption of the chiller operating in conjunction with the
cooling tower. As condenser water temperature increases, chiller efficiency decreases. Conversely, as condenser
water temperature increases, tower efficiency increases. For a given cooling tower, condenser water temperature is a
function of ambient wet bulb temperature. For a given chiller, efficiency at part load is a function of load, leaving
evaporator water temperature and entering condenser water temperature. Thus, for every ambient temperature and
load condition, there is an optimum condenser water temperature at which the energy consumption of the plant is
optimized (figure 30).
120
100
POWER
80
Chiller Energy Consumptioin
60
Tower Energy Consumption
40
Chiller + Tower Energy
Consumption
20
0
50
60
70
80
TOWER LEAVING WET BULB (F)
Figure 30. Performance @ Constant Wet Bulb Temperature & Load
WN Mechanical Systems
Chilled Water Systems
Page 24 of 31
HVAC Clinics
Draft - Not For Distribution
Plants that operate in drier climates and which utilize variable frequency drives tend to operate and higher efficiencies
at lower condenser water temperatures. Plants that operating in wetter climates and that utilize constant speed
compressors tend to operate at higher efficiencies at somewhat higher condenser water temperatures.
Putting it all together, the chiller plant controls should optimize the total energy consumption of the plant while
ensuring stable and consistent operation of the chillers, pumps and cooling tower. The meter is not just connected to
the chiller. Proper and efficient operation of the chilled water plant require a thorough understanding of the system,
the full and part load efficiencies of all of the components, the operation of those components and the needs of the
building owner.
Condenser/Cooling Tower Piping
Cooling towers require a relatively constant flow of water in order to maintain fill saturation. In addition, in colder
climates, ice formation in towers is inversely related to flow rate. Thus, it is recommended that a constant speed
tower recirculation loop be installed in order to maintain the towers minimum flow rate and prevent ice formation
during winter operation (figure 31).
Downstream of the tower recirculation pump, a two position three way valve should be installed. During startup in
cold ambient conditions, the water in the basing may be very near freezing. Under these conditions, the three way
valve allows water to be recirculated back into the basin, allowing the water temperature to increase before it is
circulated through the tower. As the water temperature increases to an acceptable level, the three position valve
opens, allowing the water to flow through the cooling tower.
Figure 31. Cooling Tower Piping
The chiller condenser water pumps should be selected with variable speed drives. The condener water pumps
modulate to maintain differential pressure during startup and operation of the chiller. Finally, if free cooling is
required, the heat exchanger is installed upstream of the chillers. In this position, the heat exchanger pre-heats the
condenser water before entering the chiller. This helps maitain minimum condenser water temperatures in the event
chiller operation is required to supplment free cooling operation and maintain the load. In the event the heat exhanger
meets the load (no chiller operation required), a two way chiller bypass valve diverts the water around the chiller to the
inlet of the tower recirculation pump.
System Variations
Heat Recovery Chillers
Systems with simultaneous heating and cooling demands or that require domestic hot water can benefit from utilzing
chillers with heat recovery. Heat recovery chillers utilze the heat energy produced by the condenser to produce
warm/hot water. Two types of heat recovery chillers exist, those with


Auxilary heat recovery condensers
Heat pump chillers
Auxilary heat recovery chillers utilize a second heat heat exchanger bundle (figure 32) in order create hot water. The
auxiliary heat exchanger is preferentially loaded, receiving the hot gas directly discharged from the compressor,
maximizing the hot water generation capability. After the hot, high pressure gas discharging the compressor
WN Mechanical Systems
Chilled Water Systems
Page 25 of 31
HVAC Clinics
Draft - Not For Distribution
generates hot water, the slightly cooler condenser refrigerant vapor is routed to the primary condenser. There is is
condensed to a high pressure liquid and the refigeration cycle completes.
Figure 32. Heat Recovery Chiller
The chiller is controlled based on leaving chillled water temperature. The heat capacity generated from the auxiliary
heat exchanger is not controlled. The heat generated to the auxiliary condenser is directly related to the chilled water
capacity generated by the chiller. Auxilary heat recovery chillers are able to produce hot water in the rage of 95oF to
115oF.
A heat pump chiller, like an auxilary heat recovery chiller, is capable of generating hot water. Unlike an auxilary heat
recovery chiller, a heat pump chiller does not utilize a separate secondary heat exchagner in order to generate heat.
A heat pump chiller is optimized to generate high levels of compressor lift, utilzing its condenser bundle to produce
heat. Heat pump chillers utilize 100% of the heat energy that would otherwise be rejected at the cooling tower,
utilzing that heat to meet the building heating demand (figure 33). Heat pumnp chillers are able to produce hot water
in the rage of 120oF to 170oF. However, the total lift generate by the compressor is generally limited to 65 oF to 100oF.
Thus, at higher hot water temperatures, the chilled water temperature produced by the chiller is limited.
Figure 33. Heat Pump Chiller
Heat pump chillers are controlled to maintain the hot water load. The chilled water capacity is not controlled. Being
that the cooling load is the dependent variable, the chilled water capacity is directly related to the heating capacity
required. Thus, a heat pump chiller is commonly one of multiple chillers in the plant. The heat pump chiller is base
loaded. Any additional chilled water demand would be satisified by one or multiple dedicated cooling only chilers. If
the heat pump chiller cannot maintain the heating load, a supplemental boiller may be installed to meet any additional
heating demand.
In order to determine the overall efficiency benefit of a heat pump chiller compared to a conventional chiller plus boiler
plant, figure 34 depicts the efficiencies that could realistically be expected operating a heat pump chiller.
WN Mechanical Systems
Chilled Water Systems
Page 26 of 31
HVAC Clinics
Draft - Not For Distribution
Figure 34. Heat Pump System Example
The conversion of COP to chiller kW/Ton is:
kW/Ton = 3.516/COP
If a chiller has an efficiency of 1.0 kW/Ton, the COP would be 3.5. Thus, assuming chiller with an efficiency in heat
pump mode of 1.2 kW/Ton, the cooling COP is 2.9. That equates to .35 units of input work to 1 unit of output cooling.
A typical R-134 heat pump chiller can produce 150oF to 180oF hot water. In this example, assume 150oF hot water
and 44oF chilled water with a cooling COP of 2.9 (1.2 kW/Ton) and a heating COP of 3.8. For the purposes this
example, this is a very conservative figure. Most modern R-134a heat pump chillers can operate at an efficiency
greater than 1.2 kW/Ton.
Recall that COP is a unitless value relating output capacity to input work. The heat pump chiller will generate about
25-45% more to account for the heat of compression. In this example, we will assume the heat of compression is
accounts for 35% of the heat generated by the chiller. That is to say for every 1.35 units of heating energy produced,
the chiller will produce 1 unit of cooling energy. This give us a heating COP of 3.8.
This results in a combined COP of the plant of 6.7 (2.9 + 3.8).
A similar chiller plus boiler system may incorporate a chiller with a COP of 6.1 (0.57 kW/Ton where k/W/Ton =
3.516/COP) and a boiler operating at similar conditions at a .95 COP (95% efficient condensing boiler). The overall
COP for this plant would be:
Comparing the two systems (figure 35):
WN Mechanical Systems
Chilled Water Systems
Page 27 of 31
HVAC Clinics
Draft - Not For Distribution
Figure 35. Heat Pump COP System Efficiency
the heat pump system uses 5 times less energy than the chiller plus boiler system (6.7 COP / 1.48 COP). Upon
further anlysis of the two systems, it becomes readily apparent why heat pump systems can offer designers an
opportunity to attain tremendous energy savings.
In order to realize the energy savings potential afforded by a heat pump chiller, the system must have simultaneous
heating and cooling loads. For example, larger buildings with a higher percentage of interior zones would generally
create simultaneous loads. The ASHRAE 2008 Handbook reads:
Another common example of such a system would be a variable air volume operating during the warmer spring,
summer and fall months. VAV systems are typically designed with some type of reheat at the air terminal units.
These reheat terminal require some form of heat energy, even during the warmest months of the year.
Figure 36. VAV Reheat
The ratio of heat energy to cooling energy produced by a heat pump chiller is typically very close to 1.35 to 1. The
difference in heating energy to cooling energy is a product of the heat produced by the refrigeration cycle (heat of
compression). However, rarley will the building load profile follow the ratio of heating to cooling energy produced by
the chiller. Thus, heat pump chillers are generally run in conjunction with supplemental boilers and chillers.
Heat pipe chillers should always be piped such that the heat pump chiller experiences the warmest return water
temperatures. This facilitates the chillers ability to produce hot water effectively and efficiently. Two possible
scenarios are described in the next section, “Preferentially Loading Chillers.” Both of those scenarios describe piping
the heat pump or heat recovery chiller upstream of the bypass in a primary secondary arragement. Alternately, in a
variable primary arrangement, piping the chillers in series with the heat pump upstream is also a phenomincally
efficient method of piping heat pump chillers (figure 37). The heat pump chiller experiences an elevated return water
WN Mechanical Systems
Chilled Water Systems
Page 28 of 31
HVAC Clinics
Draft - Not For Distribution
temperature at part load wehenver the bypass valve is closed. The dowstream chiller provides the leaving chilled
water plant setpoint.
Figure 37. Heat Pump Piping
Preferentally Loading Chillers
A chillers position relative to the bypass in primary secondary systems can determine how it is loaded relative to the
other chillers in the plant. For example, if a chiller is positioned upstream of the bypass (figure 38), that chiller will be
preferentally loaded.
Figure 38. Preferential Chiller Loading
WN Mechanical Systems
Chilled Water Systems
Page 29 of 31
HVAC Clinics
Draft - Not For Distribution
To understand this dynamic, recall that the return water temperture upstream of the byass will, at part load, always be
warmer than the design return water temperature. The temperature downstream of the bypass, at part load, is always
less than the return water temperature. If a chiller is placed upstream of the bypass, that chiller will experience
warmer return water temperatures than the chillers downstream of the bypass. Thus, this chiller will be preferentailly
loaded compared to the other chillers in the plant. Chillers in the upsream position will remain decoupled from the
production plant.
Preferential chiller loading particually useful when applied to chiller that benefit from higher return water temperatures.
Examples of chillers which benefit from warmer return water temperatures include heat recovery chillers, heat pumps
chillers and abosrption refrigeration chillers.
Sidestream Chillers
The drawback to the upstream configuration shown if figure 39 is that the chiller must be able to produce the design
supply water temperature. However, it is often beneficial to take advantage of the warmer temperatures afforded by
placing the chiller upstream of the bypass without having to supply the design supply water temperature. A system
employing a heat pump chiller is a prime example of just such an application. Heat pump chillers are able to produce
elevated hot water temperatures at higher chilled water supply and return temperatures.
Figure 39. Sidestream Chiller
A sidestream configuration places the chiller upstream of the bypass with the chilled water supply tied back into the
production loop return. In this position, the chiller does need to produce the design supply water temperature.
A heat pump chiller is ideally suited for a sidestream piping arrangement. In a VAV system, the heat pump chiller is
sized to meet the peak reheat load and potentially the domestic hot water load. The chillers downstream of the
bypass are sized to meet the remaining cooling load. As the heat pump chiller modulates to maintain the heating load
associated with any reheat load or domestic hot water load, the return water temperature is cooled before entering the
chillers downstream of the bypass. This pre-cooling effect reduces the load on the downstream chillers and
maximizes the overall COP of the plant.
In cooler climates where chiller operation is not required during winter operation, or in climates where the cooling load
can be maintained utilizing waterside or airside energy recovery, a boiler is sized for the total heating load. During the
summer and shoulder months, the boiler is disabled and the heat pump chiller maintains the heating load. If designed
properly, the overall efficiency benefit of heat pump chillers employing similar hydronic arrangements will afford
building owners a very quick payback on investment.
WN Mechanical Systems
Chilled Water Systems
Page 30 of 31
HVAC Clinics
Draft - Not For Distribution
Production Pump Control Based On Valve Position
Traditionally, the production pumps in primary-secondary and variable primary pumping systems have been controlled
based on system pressure. The pressure is measured at one or several critical locations in an attempt to maintain the
required flow to remote load terminals. However, many newer control valves are available with position feedback
(figure 40). These types of valves offer a hidden opportunity to further improve pump energy savings.
Figure 40. Pressure Reset
Rather than controlling the variable capacity production pumps based on an arbitrary value such as pressure, the
pumps are controlled based on valve position. All of the systems control valves are monitored for valve position. The
valve with the highest feedback value becomes the indirect control setpoint. If the valve position is less than a set
value (typically 95%), the pressure in the system is increased via a reset schedule until the valve position achieves
the predetermined maximum setpoint. If the valve modulates above the maximum value, the system pressure is
increased by increasing the capacity of the production pumps.
This method of pump control is far less arbitrary than direct pressure measurement. Not only does this method of
control ensure that all of the load terminals are receiving the required water flow, but the system often presents
significant pump energy savings compared to system pressure control.
WN Mechanical Systems
Chilled Water Systems
Page 31 of 31
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement