Modeling Chilled Beam Systems in HAP

Modeling Chilled Beam Systems in HAP
HAP e-Help
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Modeling Chilled Beam Systems in HAP
This HAP e-Help provides a high-level overview of chilled beams, the different types available, how they work and how to model
them in HAP. Please consult specific manufacturers’ application and product literature for application and design information.
Carrier offers the 36CB series chilled beam system. Application information is available for downloading at
www.commercial.carrier.com or by contacting your local Carrier sales representative.
Europe has successfully applied Chilled Beam systems for a decade even though it is relatively new technology in North
America. The inherent design and operational advantages influenced the popularity of chilled beam systems in North America
We discuss these advantages later in this e-Help.
There are two types of Chilled beam systems: active and passive.
Resulting supply
air to room @
64-66 °F
Primary air from
DOAS unit enters
beam @ 44-55 °F
Room air is
induced over
beam coil
Water coil inside
beam cools/heats
space loads
Figure 1 – Active Chilled Beam
Active chilled beams (ACB) use pre-cooled (and dehumidified) primary air using chilled water in a quantity necessary to meet
the room latent load and ensure good air quality for the occupied area. The cooled and dehumidified primary air absorbs the
space latent load; resulting in the chilled beam coil operating without condensation. The chilled beam then cools or heats the
induced air to meet the room sensible load and respond to the room thermostat requirements.
Primary air discharged
through slots, induces
room air
Figure 2 – Active Chilled Beam Cut Away
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Modeling Chilled Beam Systems in HAP
Active chilled beams operate using the induction process. During induction, the primary air discharges under pressure through
nozzles located within the device as illustrated in Figure 2. This high velocity incoming primary air creates a negative pressure in
the inlet portion of the beam thereby inducing room air through the beam coil where it mixes with the cold primary air. This
mixed air is then discharged through the outlet slot of the beam into the room, resulting in a total airflow quantity 3 to 4 times
greater than the primary airflow. We refer to this ratio of total air to primary air as the induction ratio.
Passive chilled beams Passive chilled beams (PCB)
work using natural convection. Air cooled by the coil
inside the beam becomes denser than the surrounding
room air and therefore flows downward into the room.
The difference in density combined with the height of the
beam induces room air down through the beam coil.
Thus passive beams mainly provide a downward airflow
in the room, as shown in Figure 3. This downward flow
induces air from the room upward to the ceiling level and
then through the beam coil. Unlike an active chilled
beam, the passive chilled beam delivers treated primary
ventilation air directly to the space and not through the
chilled beam. Nevertheless, like the active chilled beam,
this ventilation air must be sufficiently dehumidified to
meet the entire latent room load.
Figure 3 – Passive Chilled Beam Airflow Pattern
To avoid drafts in spaces with low ceilings do not locate
passive beams above workstations with sedentary occupants. Both remixing and displacement terminal devices provide good
comfort in the room in combination with passive beams.
What are the advantages of chilled beam systems over conventional designs? Chilled beam systems are suitable for use
in high sensible cooling load applications or where individual temperature control is required. Compared with a system where
the cooling duty is supplied entirely by air (all-air systems), a chilled beam system reduces the fan power requirements and
space needed for air-handling plant equipment and ducting. Chilled beam systems have the following advantages:
•
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•
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High sensible load capacity (offices, schools, computer rooms)
Decouples ventilation load from room sensible + latent loads resulting in better temperature control and fan energy savings
Typical supply air temperature is 64-66 °F exiting the chilled beam maximizing occupant comfort. Conventional systems
deliver cold air at 55 °F with the potential of creating drafts if poor mixing occurs
Constant volume ventilation air eliminates potential air dumping as compared to varying airflows in VAV system.
Reduced fan power requirements (100-250 CFM/ton)
Increased space ventilation effectiveness (1.0) due to the good mixing (high induction ratio) of room air and supply air
Reduced plenum space required (units are ~12” tall), good for retrofit applications
Easily integrates with T-bar (false) ceilings
Some units incorporate fluorescent lighting fixtures and fire sprinkler heads
Some units offer directional air flow pattern control, optimizing comfort and preventing drafts
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Modeling Chilled Beam Systems in HAP
What are the typical design conditions used for the chilled water and primary air for a chilled beam system and how
are they controlled?
Assuming applications with normal ceiling heights (9-10 ft), apply chilled beams in applications with cooling loads of 25-30
Btu/ft² of floor area. This capacity limitation is primarily due to the maximum possible air velocity in the space. Chilled beams use
warmer chilled water from the secondary side of the cooling plant (or from mixing of primary and secondary water) at an inlet
temperature from 57-61 °F to prevent condensation from occurring on the beam coil. This equates to a primary supply air
temperature of approximately 65 °F. Install special humidity (condensation) sensors on the chilled beam coil that close the water
control valve if the RH gets to 90% on the incoming chilled water pipe. Alternatively, use an atmospheric RH sensor in
combination with an air temperature sensor to reset the supply water temperature upward closer to 60 °F during conditions
when condensation might occur.
That is why it is important for the DOAS system to adequately dehumidify the primary ventilation air such that it can absorb all of
the latent loads in the zone. The DOAS leaving air temperature (LAT) should be in the 44-55 °F dry bulb range with
approximately 44-45 °F dewpoint (DP) to ensure there is no condensation on the beam coil. The lower end of the ranges should
be used when the zone latent loads are higher, such as conference rooms, school classrooms, etc. while the higher end of the
range may be used in applications with low zone latent loads.
How do you model a chilled beam system in HAP?
An accurate equivalent model for chilled beam systems can be created in HAP using the 4-pipe fan coil system type, as
described in the following example. Even though HAP offers a "CAV - 4-Pipe Induction" system, the 4-pipe fan coil system
offers some additional features that make it better as an equivalent model for chilled beams.
For our modeling example, assume the chilled beam system is a 4-pipe cooling & heating system with cooling and heating
provided in the beam. The system serves four separate office zones. A dedicated outdoor air system (DOAS) preconditions
outdoor ventilation air and supplies the treated air to the chilled beam units. The DOAS will also use an air-to-air heat recovery
device to increase system energy efficiency.
In the HAP air system properties form, select Terminal Units as the Equipment Type and 4-pipe Fan Coil as the System
Type. Then select the Common Ventilation System radio button as indicated in Figure 4. "Common Ventilation System" is
HAP's term for the DOAS unit.
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Modeling Chilled Beam Systems in HAP
Figure 4 – HAP System General Tab Input Screen
Next, under the Vent System Components tab select the items as shown in Figure 5.
Figure 5 – HAP Vent System Cooling Coil Input Screen
As mentioned previously, chilled beam units operate as sensible-only devices (no latent) therefore the DOAS must supply air to
the chilled beams at conditions capable of absorbing all of the zone latent load thereby preventing condensation on the beam
coil. This means the DOAS cooling coil LAT must be selected at a dry bulb temperature and dewpoint less than zone-neutral
conditions. Let us assume a room design setpoint of 75 °F/50% RH (~ 55 °F DP) and use an assumed cooling coil LAT of 55 °F.
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Modeling Chilled Beam Systems in HAP
For the Heating Coil setpoint from the common ventilation unit, assume a LAT slightly less (2-3 °F) than the cooling coil,
otherwise the cooling and heating coils will fight each other and operate simultaneously anytime the DOAS is operating. See
Figure 6.
Figure 6 – HAP Vent System Heating Coil Input Screen
Since the cooling coil LAT approaches saturation, (90-100% RH) additional dehumidification of the ventilation air is often
required. This is basically a reheat coil as modeled in HAP. On the Vent System Components tab, check the box labeled
Dehumidification, and specify 80% supply air RH, as indicated in Figure 7.
DOAS supply air
RH, not zone
humidity setpoint
Figure 7 – HAP Vent System Dehumidification Input Screen
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Modeling Chilled Beam Systems in HAP
Keep in mind that for a Common Ventilation System the dehumidification maximum RH% input is for the supply airstream, not
the relative humidity setpoint for the zone. The 80% RH setpoint means anytime the supply air RH% goes above 80% the
cooling coil will be activated to further dehumidify the air while the heating coil in the reheat position will be activated to maintain
no greater than 80% RH in the supply air.
You might have to use a trial-and-error approach and
change the cooling LAT and dehumidification RH%
values until you arrive at an optimum and reasonable
result. As a general rule, use the highest LAT and
RH% that maintains the zone at 40-50% RH, which
ensures no condensation and minimizes energy
usage.
Hourly Zone
Loads Report
The Hourly Zone Loads Report shown in Figure 8
displays the resulting room relative humidity and
zone dry bulb temperatures for each hour in each
zone. This report should be used to check these
values for each zone.
For our example 4-zone building, we generated the
Hourly Zone report for the peak design month. Also
consider checking the zone RH% during off-design
times like September or October. This report can be
generated for any off-design month. If the Hourly
Zone reports indicate a maintained zone 40-50% RH,
then the common vent system LAT and RH are
acceptable.
Figure 8 – Ask For HAP System Design Hourly Zone Loads Report
Hourly Zone Load
Report Displays
Resulting Zone RH
Figure 9 – HAP System Design Hourly Zone Loads Report
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Modeling Chilled Beam Systems in HAP
The last three settings for the Vent System should be entered appropriately for your conditions. The vent fan and exhaust fans
are the fans inside the DOAS, not the zone terminals (which have no fans in our case). The fan power settings must include all
static losses including the recovery unit, the supply/return duct losses and the static losses through the beam units.
Next we move to the Zone Components tab, as shown
in Figure 10. Here you assign the spaces to the zones.
In our case we have four zones, each with one space.
Also set the thermostat settings and schedule.
Under Common Data, we will enter a cooling supply air
temperature of 65 °F, which is typical for a chilled beam
system. The heating coil setting will be 90 °F, which will
minimize stratification during heating and maximize the
ventilation efficiency.
Finally, for the Terminal Units settings you can leave
the Terminal Units minimum zone airflow at zero. The
primary airflow will be set properly based on space
ventilation requirements. Because we are modeling the
chilled beam as a fan coil system, the fan static must
be entered as zero. This eliminates the effect of the
fan from the system. While the program will calculate
supply airflow thinking the system is a fan coil, the
sensible BTU/h provided will be the same no matter
what this supply airflow is and there is no impact on fan
heat or energy use because there is no fan.
Figure 10 – HAP Zone Components Common Data Input
Screen
Figure 11 – HAP Zone Components Terminal Units Input Screen
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Modeling Chilled Beam Systems in HAP
What about the Chilled water Plant Model?
The chilled water plant for chilled beam systems
needs to operate at two supply water
temperatures. The DOAS needs to receive cold
water at a supply temperature like 44 °F in order
to condense sufficient moisture out of the
outdoor air stream to handle the space latent
load. The chilled beam units need to receive
warmer water, typically in the 57 to 61 °F range.
This is achieved by producing cold chilled water
at 44 °F LCHWT and supplying it first to the
DOAS coil. Outlet water from the DOAS unit is
then typically blended with water from the return
side of the plant to reach the desired 57 to 61 °F
temperature for supply to the chilled beam units.
To develop an equivalent plant model for this
configuration in HAP, first define the chillers
according to actual system specifications. That
is, supplying 44 °F LCHWT and with flow rates
appropriate for the actual delta-T. This delta-T
is often in the range 16 to 20 °F. Connect the
Figure 12 – Chilled Water Plant Modeling For Chilled Beam
chillers to a chilled water plant. Specify the
plant design LCHWT as 44 °F.
On the
Distribution Tab (Figure 12) the best results are
obtained by specifying "Primary Only/Constant Speed" or "Primary Only/Variable Speed" as the system type. Specify the Coil
Delta-T at design as your overall system design delta-T - such as the 16 to 20 °F value.
In conclusion, chilled beams are becoming more popular. They provide many operational and design advantages, as discussed.
By decoupling the ventilation loads from the zone sensible and latent loads, the terminals are designed to handle lower total
airflow quantities, thereby using smaller equipment while resulting in uniform air distribution, high air-change rates and uniform
temperatures in the zone with fewer drafts. It is important to account for the zone latent loads in your design considerations and
to ensure proper sizing of the DOAS including the ventilation air loads plus the zone latent loads, while the chilled beams are
sized to handle the zone sensible loads only.
As mentioned previously, you should consult manufacturer’s specific design application literature when attempting to design a
chilled beam system.
References:
1) Design Considerations For Active Chilled Beams; Alexander, Darren, P.E. & O’Rourke, Mike; ASHRAE Journal, Sept. 2008;
pp. 50-58
2) DOAS & Humidity Control; Larranaga, Michael D., Beruvides, Mario G., Ph.D., P.E., Holder, H.W., Karunasena, Enusha,
Ph.D. & Straus, David C, Ph.D.; ASHRAE Journal, May 2008; pp. 34-39.
3) 36CB Active & Passive Chilled Beams Product Literature; Form 36CB-1PD; August 2008; Carrier Corporation, Syracuse,
NY.
4) 36CBA, CBP Active & Passive Chilled Beams Application Data; Form 36CB-1XA, August 2008; Carrier Corporation,
Syracuse, NY.
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