McLean Protective Cooling A guide on selecting cooling systems

McLean Protective Cooling A guide on selecting cooling systems
McLean Cooling Technology: How To Select A Protective Cooling Solution
Protective Cooling Solution Overview
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McLean Cooling Technology
How To Select A Protective Cooling Solution
Protective Cooling Solution Overview
Why Cool Electronics in the First Place?
Keeping your electronics cool is essential to extending their life and keeping your business running.
Heat Ruins Electronics
How to Select
The life expectancy of electronics is cut in half every 10 C / 18 F they
operate above room temperature. Operating electronics above
certain temperatures can void manufacturers’ warranties, making
proper cooling essential. Cooling vital electronics increases service
life and reduces capital expenses over the long-term.
Sources of Heat
Damaging heat can come from a variety of sources. Inside the
cabinet, heat can come from:
• AC power supplies
• Controllers, drives and servos
• Transformers and rectifiers
• Processors and server racks
• Radio equipment
• And other electronic components
Electronics Life Expectancy with
Every 10° C Rise over Room Temperature
Heat also comes from sources outside the enclosure such as:
• Solar heat gain
• Welding processes
• Paint oven
• Blast furnace
• Foundry equipment
Trend Toward More Damaging Heat
Moore’s Law
Named after the founder of Intel
For the foreseeable future, the trend is toward increasing levels of
heat in electronics, not less, because the market’s thirst for more
information processing capacity and speed continues to grow. This
trend is known as “Moore’s Law.”
More powerful data-processing electronics generate extra heat with
virtually every new system that is designed. There is no guarantee
that an application which did not require much, if any, cooling in the
past will not need cooling in the future. The new system likely has
more functionality and will probably require some form of cooling as
a result.
What Are the Consequences of Damaging Heat?
Heat build-up can adversely affect industrial controls and sensitive
electronic systems as follows:
• De-rated drive performance
• I/C-based devices experience intermittent fluctuations
• MTBF decreases exponentially
• Catastrophic failure
The costs when a factory line or electronic system fails can include:
• Productivity losses
• Component replacement costs
• Late shipments
• Customer dissatisfaction
• Lost revenue
• Cell phone tower outage
• Breach in homeland security
Direct costs to a business can be as much as $50,000 per hour of
system downtime.
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McLean Cooling Technology: How To Select A Protective Cooling Solution
Options for Cooling Electronics
Options for Cooling Electronics
Conductive Cooling
This is a passive way to cool electronics. It simply allows the heat to
radiate through the cabinet walls.
Conductive cooling works well with electronics systems that
have small heat loads (<50 W) and cool air around the enclosure
(<78 F/25 C).
How to Select
If heat is an issue, one option within this type of cooling is to increase
cabinet size to create more surface area to speed the transfer of
heat. However, growing cabinet size is often not a practical solution
because of space limitations and the greater heat loads associated
with today’s high-power electronics.
Fresh Air Cooling
This is an active way to manage heat in electronics applications. This
type of cooling ventilates fresh air through the cabinet, exhausting
heat away from the hot components.
Fresh air cooling may be used when the electronics system is
deployed in a relatively clean and cool environment such as an office
building, data networking center or light-duty factory. Options for
cooling electronic enclosures with fresh air include filter fans, fan
trays, motorized impellers and packaged blowers.
Fresh air cooling is known as an “open-loop system” because no
significant seal is maintained to protect electronic components from
harmful elements such as dirt, water, metal filings and corrosive
fumes.
Protective Cooling
This is another active way to cool electrical components. This type of
thermal management maintains the seal of the enclosure—using an
air conditioner or heat exchanger as examples—to remove heat from
inside the electronics cabinet.
Protective cooling is known as a “closed-loop system” because the
seal of the electrical cabinet is maintained, allowing no elements
which can damage the electronics inside the enclosure.
Protective cooling is generally required when the electronics
application:
(1) operates in high temperatures, typically over 95 F/35 C,
(2) is deployed in a harsh environment such as an outdoor telecom
base station, wastewater treatment plant, metal working operation,
oil rig platform, paper mill, foundry and/or
(3) generates a high heat load from its own components, usually
more than 500 W.
Options for protective cooling include air conditioners, air-to-air heat
exchangers, air-to-water heat exchangers, thermo-electric coolers
and vortex coolers.
Air Conditioner
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McLean Cooling Technology: How To Select A Protective Cooling Solution
Levels of Electronic Enclosure Protection
Levels of Electronic Enclosure Protection
Protection Levels
NEMA, UL and CSA Ratings Enclosure Type Descriptions for Non-Hazardous Locations
How to Select
Indoor
Type
Type 1
NEMA
Enclosures are intended for indoor
use primarily to provide a degree of
protection against contact with the
enclosed equipment or locations where
unusual service conditions do not exist.
Enclosures are intended for indoor use
primarily to provide a degree of
protection against dust, falling dirt and
dripping noncorrosive liquids.
UL
Indoor use primarily to
provide protection against
contact with the enclosed
equipment and against
a limited amount of falling dirt.
Indoor use to provide a degree
of protection against dust, dirt, fiber
flyings, dripping water and external
condensation of noncorrosive liquids.
Indoor
Type 12
Indoor
Type 12K
Enclosures with knockouts are intended
for indoor use primarily to provide a
degree of protection against dust, falling
dirt and dripping noncorrosive liquids.
Indoor use to provide a degree of
protection against dust, dirt, fiber
flyings, dripping water and external
condensation of noncorrosive liquids.
Indoor
Type 13
Enclosures are intended for indoor
use primarily to provide a degree of
protection against dust, spraying of water,
oil and noncorrosive coolant.
Outdoor
Type 3
Outdoor
Type 3R
Outdoor
Type 3RX
Outdoor
Type 4
Outdoor
Type 4X
Outdoor
Type 6
Enclosures are intended for outdoor
use primarily to provide a degree of
protection against windblown dust, rain
and sleet; undamaged by the
formation of ice on the enclosure.
Enclosures are intended for outdoor
use primarily to provide a degree of
protection against falling rain and sleet;
undamaged by the formation
of ice on the enclosure.
Enclosures are intended for outdoor
use primarily to provide a degree of
protection against corrosion, falling
rain and sleet; undamaged by the
formation of ice on the enclosure.
Enclosures are intended for indoor or
outdoor use primarily to provide a
degree of protection against windblown
dust and rain, splashing water and hose
directed water; undamaged by the
formation of ice on the enclosure.
Enclosures are intended for indoor
or outdoor use primarily to provide a
degree of protection against corrosion,
windblown dust and rain, splashing water
and hose-directed water; undamaged by
the formation of ice on the enclosure.
Enclosures are intended for use indoors or
outdoors where occasional submersion is
encountered; limited depth; undamaged
by the formation of ice on the enclosure.
Indoor use to provide a degree
of protection against lint, dust
seepage, external condensation
and spraying of water, oil and
noncorrosive liquids.
Outdoor use to provide a
degree of protection against
windblown dust and windblown
rain; undamaged by the
formation of ice on the enclosure.
Outdoor use to provide a
degree of protection against
falling rain; undamaged by the
formation of ice on the enclosure.
Not specifically defined.
Either indoor or outdoor use to
provide a degree of protection
against falling rain, splashing
water and hose-directed water;
undamaged by the formation
of ice on the enclosure.
Either indoor or outdoor use to
provide a degree of protection
against falling rain, splashing
water and hose-directed water;
undamaged by the formation of ice
on the enclosure; resists corrosion.
Indoor or outdoor use to provide a
degree of protection against entry of
water during temporary submersion
at a limited depth; undamaged
by the external formation
of ice on the enclosure.
CSA
General purpose enclosure.
Protects against accidental
contact with live parts.
Indoor use; provides a degree of
protection against circulating dust,
lint, fibers and flyings; dripping and
light splashing of non-corrosive
liquids; not provided with knockouts.
Indoor use; provides a degree of
protection against circulating
dust, lint, fibers and flyings; dripping
and light splashing of noncorrosive
liquids; not provided with knockouts.
Indoor use; provides a degree of
protection against circulating dust,
lint, fibers and flyings; seepage and
spraying of non-corrosive liquids,
including oils and coolants.
Indoor or outdoor use; provides a
degree of protection against
rain, snow and windblown dust;
undamaged by the external
formation of ice on the enclosure.
Indoor or outdoor use; provides
a degree of protection against
rain and snow; undamaged by the
external formation of ice
on the enclosure.
Not specifically defined.
Indoor or outdoor use; provides a
degree of protection against
rain, snow, windblown dust,
splashing and hose-directed
water; undamaged by the external
formation of ice on the enclosure.
Indoor or outdoor use; provides a
degree of protection against rain,
snow, windblown dust, splashing and
hose-directed water; undamaged by
the external formation of ice on the
enclosure; resists corrosion.
Indoor or outdoor use; provides a
degree of protection against the
entry of water during temporary
submersion at a limited depth.
Undamaged by the external
formation of ice on the
enclosure; resists corrosion.
• This material is reproduced with permission from NEMA. The preceding descriptions, however, are not intended to be complete
representations of National Electrical Manufacturers Association standards for enclosures nor those of the Electrical and Electronic
Manufacturers Association of Canada.
• This material is reproduced with permission from Underwriters Laboratories Inc. Enclosures for Electrical Equipment, UL 50, 50E and
Industrial Control Panels, UL 508A.
• This material is reproduced with permission from the Canadian Standards Association.
• Underwriters Laboratories Inc. (UL) shall not be responsible for the use of or reliance upon a UL Standard by anyone. UL shall not incur
any obligation or liability for damages, including consequential damages, arising out of or in connection with the use, interpretation of,
or reliance upon a UL Standard.
• Some enclosures may have multiple ratings. For instance: 4, 12—Outdoor use; able to be used indoors with modifications; 4X, 3RX—
Outdoor use; able to be used indoors with modifications; 4, 9—Can be used in both hazardous and non-hazardous locations
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McLean Cooling Technology: How To Select A Protective Cooling Solution
Levels of Electronic Enclosure Protection
IP Rating Descriptions Example Rating
If 1st IP number is...
2
(protection against solid objects)
and the 2nd IP number is...
3
(protection against liquids)
Then the IP rating is
IP23
An enclosure with this designation provides protection against touch with a
finger, penetration of solid objects greater than 12 mm and spraying water.
How to Select
First Numeral (Solid Objects and Dust)
IP
0
1
2
3
4
5
6
Protection of Persons
No Protection
Protected against contact with large areas of the body (back of hand)
Protected against contact with fingers
Protected against tools and wires over 2.5 mm in diameter
Protected against tools and wires over 1 mm in diameter
Protected against tools and wires over 1 mm in diameter
Protected against tools and wires over 1 mm in diameter
Protection of Equipment
No Protection
Protected against objects over 50 mm in diameter
Protected against solid objects over 12 mm in diameter
Protected against solid objects over 2.5 mm in diameter
Protected against solid objects over 1 mm in diameter
Protected against dust (limited ingress, no harmful deposit)
Totally protected against dust
Second Numeral (Liquid)
IP
0
1
2
3
4
5
6
7
8
Protection of Equipment
No Protection
Protected against vertically falling drops of water, e.g. condensation
Protected against direct sprays of water up to 15 degrees from vertical
Protected against sprays up to 60 degrees from vertical
Protected against water sprayed from all directions (limited ingress permitted)
Protected against low-pressure jets of water from all directions (limited ingress permitted)
Protected against strong jets of water
Protected against the effects of immersion between 15 cm and 1 m
Protected against long periods of immersion under pressure
SCCR Requirements per UL (Condensed version)
Article 409 of the 2008 National Electric Code (NFPA 70) requires industrial control panels to be marked with a short circuit current rating. As
specified in the National Electric Code, UL508A-2001 Supplement SB, the Standard of Safety for Industrial Control Equipment, provides an
accepted method for determining the short-circuit current rating of the control panel.
The SCCR rating for our air conditioners and heat exchangers has a default value of 5 kA.
You may use a 5 or 10 kVA isolation transformer between the customer’s panel and our air conditioner and not have an effect on the
customer’s 65 kA rating.
You may use a fuse or circuit breaker with a 5 kA short circuit rating on the line side of the ACU and its branch circuit protective device and
not have an effect on the customer’s 65 kA rating.
The current limiting fuse or circuit breaker used on the line side of the branch circuit protection for the ACU must have a SCCR => that of the
panel rating. Additionally for a current limiting fuse the customer would need to verify using table SB4.2 of UL 508A, that the let through
current (Ip * 10^3) of the fuse is <= 5KA. If a circuit breaker is used as feeder protection, it must be marked Current Limiting type from the
manufacturer, and the panel builder would need to verify based on the manufacturers published curves that it will let through <= 5kA.
Examples of these curves are included in UL 508A supplement SB.
You can run separate circuits for the panel and the air conditioner as long as each is labeled with their individual SCCR ratings.
(5 kA and 65 kA)
If the customer does not implement one of the options above, then the resulting SCCR rating would be the 5 kA rating of the ACU, if that is
the lowest rated component in the panel.
Testing represents another option; however, if the customer does not implement these options, then the resulting short circuit rating of the
panel is based on the lowest short circuit current rating of all power circuit components installed in the panel.
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McLean Cooling Technology: How To Select A Protective Cooling Solution
Selecting the Right Cooling Solution
Selecting the Right Cooling Solution
Cooling Solution
Since heat dissipation is often not a solution, we will limit our choices to protective vs. fresh air cooling.
Use the environmental and electronic system criteria in the table below to determine whether protective or fresh air cooling is most
appropriate for your application.
How to Select
Protective vs. Fresh Air Cooling
Specifying protective cooling that keeps your electronics components sealed from the outside environment versus using fresh
air cooling to remove damaging heat depends on the following profile of your system application (check one side or the
other for each of the six choices):
FRESH
Clean Air / Some Dust /
Dripping Water
Moderate to Low
(typically under 95 F / 35 C)
Somewhat to Well-Above
Ambient Temperature
PROTECTIVE
SYSTEM OPERATING
ENVIRONMENT
TEMPERATURE OUTSIDE
OF THE ENCLOSURE
Hot
(typically over 95 F / 35 C)
TEMPERATURE RATING
OF THE ELECTRONICS
Below to Somewhat Above
Ambient Temperature
HUMIDITY OUTSIDE
OF THE ENCLOSURE
Moderate to Low
Wide
Moderate to Low
(typically under 3000 Watts)
Dirty / Wet / Metal Filings /
Outdoors / Corrosive Fumes
High Relative Humidity
TEMPERATURE RANGE
FOR THE ELECTRONICS
Narrow / Precise
SYSTEM POWER DRAW /
HEAT LOAD
Moderate to High
(typically over 3000 Watts)
If most of your assessments fell on the fresh air side, then a filter fan, fan tray, motorized impeller or blower is probably the
correct cooling solution for your application. However, if most of your assessments were on the protective side, then an
air conditioner or heat exchanger found in the McLean Protective Cooling Catalog is likely the right cooling solution for your
electronics system.
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McLean Cooling Technology: How To Select A Protective Cooling Solution
Selecting the Right Cooling Solution
Cooling Solution Choices
Assuming that protective cooling is needed for the application, there
are two basic choices—air conditioners or heat exchangers.
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How to Select
An air conditioner should be specified when:
• The temperature inside the enclosure must be maintained at or
below the ambient temperature
• Humidity must be removed
• A moderate to high heat load is being produced by the electronic
system
A heat exchanger can be used to transfer heat from inside the
enclosure to the outside atmosphere when:
• The electronic components can operate at a temperature above
the ambient air temperature
• Humidity is not a factor
• A low to moderate heat load is being produced by the electronic
system
McLean Cooling Technology: How To Select A Protective Cooling Solution
How to Select the Right Cooling Capacity Air Conditioner
How to Select the Right Cooling Capacity Air Conditioner
Air Conditioner Cooling Capacity Overview
The cooling capacity of an air conditioner needs to match or exceed the amount of total heat load generated by the electronic system.
Total heat load comes from two sources:
(a) the electronic components themselves which is called “internal heat load” and
(b) the ambient heat outside the enclosure which is known as the “heat transfer load.”
How to Select
Most engineers and cooling suppliers determine internal heat load. However, the impact from the heat transfer load is easily overlooked.
Heat transfer load can significantly add to the total heat load of the system, especially if the outside air temperature is high and/or the
enclosure is located in the sun.
Thus, the total heat load to be removed from the electrical enclosure by the air conditioner is the sum of the internal heat load and the
heat transfer load.
TOTAL HEAT LOAD = INTERNAL HEAT LOAD + HEAT TRANSFER LOAD
Part A: How to Determine Internal Heat Load
Method 3. Incoming – Outgoing Power
There are several methods to determine internal heat load,
depending on data availability.
A third approach is to estimate the power going into the enclosure
and the power coming out of it. The difference becomes the
estimated amount of internal heat load. The amps and volts of each
electrical line going in are multiplied to determine Watts, then they’re
added together. The same is done for the electrical line(s) coming out
of the application. The outgoing Watts are then subtracted from the
incoming Watts.
Method 1. Heat Load Data from Each Electronics Component
Manufacturer
INTERNAL HEAT LOAD =
INCOMING POWER (W) – OUTGOING POWER (W)
One way to estimate internal load is to gather heat load data from
the manufacturers of the electronics components inside the cabinet.
They may know the amount of heat their equipment is generating.
If more than one control or other electronics components are inside
the enclosure, it will be necessary to add together all the estimates of
heat load to determine total internal heat load.
Example—
The internal heat load comes from the amount of waste heat
generated inside the enclosure by the electronic components and is
expressed in Watts (W).
Method 2. Component Power – Component Efficiency
A second method is to establish the Watts of power used by each
electronic component. Derive Watts of power by multiplying the amp
draw of each device by its voltage. Then subtract the efficiency of
each component from its estimated power use. Add up the outcomes
to get the total internal heat load.
An enclosure has three input lines of 230 VAC at 11, 6 and 4 A. It has
one output control line of 115 VAC at 9 A.
Incoming Power = (230 x 11) + (230 x 6) + (230 x 4) = 4830 W
Outgoing Power = 115 x 9 = 1035 W
Total Heat Load = 4830 – 1035 = 3795 W
Method 4. Automated Equipment Horsepower
This fourth method applies only to industrial automation equipment
that operates with horsepower (hp) such as variable frequency drives
(VFDs). 1 hp = 745.6 W. Thus, the internal heat load from a 3-hp VFD is
2237 W, less its efficiency which is typically 93% - 95%.
INTERNAL HEAT LOAD =
COMPONENT POWER (W) - COMPONENT EFFICIENCY
(for each electrical device)
Example—
Example—
VFD Watts = 5 hp x 745.6 x 3 = 11184
Adjusted Watts = 11184 x (1 - .95) = 559
Total Heat Load = 559 x 1.25 = 699 W
An electronic system uses two components that draw 115 VAC at 15
A. Each has a rated efficiency of 90%. Put another way, 10% of each
device is inefficient. Unused power becomes generated heat. Thus
the estimated internal heat load is:
A cabinet has three 5-hp VFDs with 95% efficiency.
1.25 is an assumed “safety” margin for other minor heat-producing
components.
Device Power = 115 x 15 = 1725 W
Total Power = 2 x 1725 = 3450
Less Efficiency = 3450 x (1 - .90)
Total Heat Load = 345 W
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McLean Cooling Technology: How To Select A Protective Cooling Solution
How to Select the Right Cooling Capacity Air Conditioner
Part B: How to Determine Heat Transfer Load Overview
Heat transfer load is the ambient heat outside the enclosure conducting itself through the cabinet walls toward the electronics (heat energy
travels from the hottest to coldest location).
When an air conditioner cools the enclosure temperature lower than the ambient air outside, additional heat load is drawn into the cabinet
which the air conditioner needs to remove. The higher the ambient temperature and/or the presence of solar heat gain (the “greenhouse
effect”) on the enclosure, the more cooling capacity is required.
How to Select
Determining heat transfer load requires that you know the total surface area of the cabinet, less any non-conductive surface area such
as the enclosure side mounted to a wall. It also requires that you determine ΔT, which is the difference between maximum ambient
temperature and the maximum temperature rating of the electronics components.
There are two methods for determining heat transfer load—the simple chart method and the equation method.
Simple Chart Method
This method is reasonably accurate for most indoor industrial
systems where there is no unusual air movement and insulation is
not typically used inside the enclosure. The process also provides
a ballpark result for outside plant and telecommunications
applications, taking into account solar heat gain. However, it does
not incorporate the impact of wind or cabinet insulation. If either is
present, then the equation method is more precise.
Step A. Determine ΔT in °F or °C.
Step B. Find the heat transfer per ft. 2 or m2 on the chart below, using
ΔT and the proper cabinet material curve.
Step C. Multiply the heat transfer per ft. 2 or m2 by the total surface
area of the enclosure that will conduct heat. (Remember to exclude
surfaces such as a side mounted to a wall.)
Example —
A painted steel cabinet has 80 ft.2 of surface area and will be located
in a maximum ambient temperature of 95 F. The rated temperature
of the electronics is 75 F.
ΔT = 95 - 75 = 20 F
Heat Transfer = 4 W/ft.2 (from chart)
Total Heat Transfer Load = 80 x 4 = 320 W
The estimate for heat transfer load ends here, unless the electronic
system will be deployed outdoors. Then solar heat gain needs to be
added to the total heat transfer load calculated above. Solar heat
gain is determined much the same way as heat transfer per ft. 2 or m2,
using a similar chart.
SURFACE AREA (ft. 2) = [2AB (in.) + 2BC (in.) + 2AC (in.)] ÷ 144
SURFACE AREA (m2) =
[2AB (mm) + 2BC (mm) + 2AC (mm)] ÷ 1000000
Total Heat Transfer Load =
Heat Transfer per ft.2 or m2 x Cabinet Surface Area
Example — The painted cabinet above is in ANSI 61 gray.
Thus, 7 W/ft.2 need to be added to the heat transfer load which is
560 W (7 x 80 ft.2). Total Heat Transfer Load consequently becomes
720 W.
The result does not include insulation which can significantly reduce heat
transfer load.
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McLean Cooling Technology: How To Select A Protective Cooling Solution
How to Select the Right Cooling Capacity Air Conditioner
Equation Method
How to Select
Heat transfer load may also be determined by equation. This method
should be used when at least one of the following criteria are found
in the electronic system:
• Moderate to high airflow within the cabinet
• Outdoor applications that involve breezes or gusty winds
• Insulation used within the cabinet to offset the impact of solar
heat gain
The governing equations for heat transfer load are:
English System (°F, inches and feet):
q = (To - Ti) ÷ [(1/ho) + (1/hi) + R]
hi = Convective heat transfer coefficient inside the cabinet
Still air: h = 1.6
Moderate air movement: h = 2.0
Blower (approx. 8 ft./sec.): h = 3.0
R = Value of insulation lining the interior of the enclosure walls
No insulation: R = 0.0
1/2 in. or 12 mm: R = 2.0
1 in. or 25 mm: R = 4.0
1-1/2 in. or 38 mm: R = 6.0
2 in. or 51 mm: R = 8.0
q = (125 - 75) ÷ [(1/6) + (1/2) + 4]
q = (50) ÷ (.16 + .5 + 4)
q = 50 ÷ 4.66
q = 10.7 BTU/hr./ft.2
Metric System (°C, millimeters and meters):
(q = (To - Ti) ÷ [(1/ho) + (1/hi) + R] x 5.67
Definition of Variables—
q = Heat transfer load per unit of surface area
To = Maximum ambient temperature outside the enclosure
Ti = Maximum rated temperature of the electronics components
ho = Convective heat transfer coefficient outside the cabinet
Still air: h = 1.6
Relatively calm day: h = 2.5
Windy day (approx. 15 mph): h = 6.0
Total Heat Transfer Load
10.7 x 72 = 770 BTU/hr. or 770 ÷ 3.413 = 226 W
Since the cabinet is outdoors, and assuming it is painted ANSI 61 gray
and located in the sun, extra solar load needs to be added to the
outcome above which is 504 Watts (7 W per ft. 2 x 72 ft.2).
Total Heat Transfer Load with Extra from Solar Heat Gain
226 + 504 = 730 W
How to Determine Total Heat Load
Total heat load to be removed from the electrical enclosure by the air conditioner is the sum of internal heat load plus heat transfer load.
TOTAL HEAT LOAD (C) = INTERNAL HEAT LOAD (A) + HEAT TRANSFER LOAD (B)
Thus, one adds together the result from Part A to the outcome from Part B.
Example—
The internal heat load from one of the examples above was 3795 Watts. The heat transfer load from the other example above was 730 W.
Therefore, total heat load is 3795 + 730 = 4525 W.
To convert Watts into BTU/hr. to determine air conditioner capacity in the English system, multiply by 3.413.
4525 W is then 15444 BTU/hr.
Power input, protection level and dimensions of the air conditioner also need to fit system requirements.
Caution! Do not simply match the nominal cooling capacity of the air conditioner model with the total heat load result above. Be sure to
know the maximum ambient temperature outside the enclosure as well as the rated temperature of the electronic components. Apply these
temperatures to the performance curves provided by the cooling manufacturer to select an appropriately sized air conditioner. Failure to do
so may under-size your air conditioner as much as 20% - 25%, thereby under-cooling the electronics and making the application vulnerable
to potential over-heating issues.
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McLean Cooling Technology: How To Select A Protective Cooling Solution
How to Select the Right Cooling Capacity Heat Exchanger
How to Select the Right Cooling Capacity Heat Exchanger
Heat Exchanger Cooling Capacity Overview
Cooling with an air-to-air heat exchanger assumes the electronic components in your system are able to operate above the ambient
temperature outside the enclosure. If this is not the case, then an air conditioner must be used.
Selecting a heat exchanger is similar to specifying an air conditioner in that the cooling capacity of the unit must remove the internal heat
load from the electrical enclosure.
Because the cooling capacity of heat exchangers is expressed in terms of Watts/°F or Watts/°C, an extra step is necessary to convert net heat
load into a result used to select the appropriate heat exchanger. Divide the net heat load by the ΔT which is the difference between the
maximum ambient temperature outside the enclosure and the maximum temperature rating of the electronic components.
HEAT EXCHANGER CAPACITY (C) = [INTERNAL HEAT LOAD (A) – HEAT TRANSFER (B)] / ΔT
How to Determine Internal Heat Load
Internal heat load stems from the amount of waste heat generated inside the enclosure by the electronic components and is expressed in
Watts.
To determine internal heat load, follow one of the four options outlined in the air conditioner “How to Determine Internal Heat Load” section
on page 12.
How to Determine Heat Transfer
In air-to-air heat exchangers, heat transfer is actually cabinet heat loss because the heat inside the enclosure is conducting itself through the
cabinet walls toward the cooler temperature outside the enclosure. That is why heat transfer is subtracted from internal heat load to arrive at
total net heat load.
To determine heat transfer you need to know the total surface area of the cabinet, less any non-conductive surface area such as the
enclosure side mounted to a wall. You must also determine ΔT which is the difference between maximum ambient temperature and the
maximum temperature rating of the electronic components.
There are two methods to determine heat transfer—the simple chart method and the equation method. The simple chart method may be
used for nearly all indoor heat exchanger applications. The equation method needs to be applied when air movement outside or inside the
electrical enclosure is high, or for outdoor applications.
Here are the steps for the simple chart method:
Step A. Determine ΔT in °F or °C.
Step B. Find the heat transfer per ft. 2 or m2 from the Heat Transfer graph on page 13, using ΔT and the proper cabinet material curve.
Step C. Multiply the heat transfer per ft. 2 or m2 by the total surface area of the enclosure that will conduct heat. (Remember to exclude
surfaces such as a side mounted to a wall.)
SURFACE AREA (ft. 2) = [2AB (in.) + 2BC (in.) + 2AC (in.)] ÷ 144
SURFACE AREA (m2) = [2AB (mm) + 2BC (mm) + 2AC (mm)] ÷ 1,000,000
Heat Transfer (Cabinet Heat Loss) = Heat Transfer per ft. 2 or m2 x Enclosure Surface Area
The estimate for heat transfer ends here, unless the electronic system will be deployed outdoors, or airflow inside or outside the enclosure is
high. Then the equation method needs to be used to determine heat transfer (cabinet heat loss).
For the equation method, follow the steps on page 13 in the air conditioner selection section. The result will be a negative number; the
negative sign should be ignored when deducting heat transfer from internal heat load.
Caution! If the result of the equation method is a positive number, then this means that you want the electronics temperature inside the
cabinet to be lower than the temperature outside the enclosure. In this case, an air conditioner should be specified for the electronics system.
Subject to change without notice
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15
How to Select
However, since the conductive cooling nature of the cabinet itself removes some of the heat from the system, heat transfer should be
subtracted from internal heat load (versus added in the case of air conditioners).
McLean Cooling Technology: How To Select A Protective Cooling Solution
How to Select the Right Cooling Capacity Heat Exchanger
How to Select
How to Determine Heat Exchanger Capacity
—Outdoor Example—
Air-to-air heat exchanger capacities are not provided in terms of
Watts or BTUs/hr. of cooling like air conditioners. Instead, they are
expressed in terms of Watts/°F or Watts/°C. Thus, the final step in
determining heat exchanger capacity is to divide the total net heat
load by ΔT. Then select the heat exchanger with the same or higher
Watts/°F or Watts/°C as the outcome of this process.
A telecom system draws a total of 5,000 W; its efficiency is 85%. It
is protected in a steel cabinet that is 72 ft.2 (6.69 m2) and painted
with RAL 7035 light-gray paint. The enclosure walls are lined inside
with 1 in. (25 mm) of insulation. The application will be deployed
in a maximum ambient outdoor temperature of 104 F (40 C) with
occasional winds reaching 15+ mph. The rated temperature of
the electronics is 114 F (46 C). Air circulation inside the cabinet is
moderate.
—Indoor Industrial Example—
An electronic system uses two components that draw 230 VAC at 7.5
A. Each has a rated efficiency of 90%. They are protected in a painted
steel cabinet that is 60 in. (1524 mm) tall, 36 in. (914 mm) wide and 18
in. (457 mm) deep. The system will be located in a maximum ambient
temperature of 80 F (27 C). The rated temperature of the electronics
is 95 F (35 C).
HEAT EXCHANGER CAPACITY (C) =
[INTERNAL HEAT LOAD (A) – HEAT TRANSFER (B)] ÷ DELTA ΔT
HEAT EXCHANGER CAPACITY (C) =
[INTERNAL HEAT LOAD (A) – HEAT TRANSFER (B)] ÷ ΔT
Total System Power = 5000 W
Less Efficiency = 5000 x (1 - .85)
Internal Heat Load = 750 W
Internal heat load (A) may be determined using the “Component
Power – Component Efficiency” method on page 12, given the
available information. In this example, the estimated heat load is:
Device Power = 230 x 7.5 = 1725 W
Total Power = 2 x 1725 = 3450
Less Efficiency = 3450 x (1 - .90)
Internal Heat Load = 345 W
Internal heat load (A) is determined using the “Component Power
– Component Efficiency” method on page 12. In this example, the
estimated heat load is as follows:
Heat transfer (B) is derived using the equation method, since this
is an outdoor application. For brevity, we will assume the English
system (°F, inches and feet).
q = (To - Ti) ÷ [(1/ho) + (1/hi) + R]
“q” is heat transfer per surface area. For an explanation of the other
variables, see “Equation Method” on page 14.
Heat transfer (B) is derived using the simple chart method, since this
is an indoor industrial application. Both cabinet surface area and ΔT
q = (104 - 114) ÷ [(1/6) + (1/2) + 4]
are needed to determine heat transfer. Cabinet surface area is 54 ft. 2
or 5.02 m2 (from surface area formula on page 13). ΔT is 15 F (8 C)—the q = -2.14 W/ft.2
difference between ambient temperature and the rated temperature
of the electronics.
Total Heat Transfer = 2.14 x 72 ft.2 = 154 W
(negative sign is ignored)
Heat Transfer (Cabinet Heat Loss) =
Heat Transfer per ft.2 or m2 x Enclosure Surface Area
ΔT is 10 F — the difference between ambient temperature and the
Using the painted steel curve on the Heat Transfer chart on page 13,
heat transfer per ft.2 or m2 is 3 W/ft.2 or 32.5 W/m2.
Heat Transfer = 3 W/ft.2 x 54 ft.2 = 162 W
rated temperature of the electronics.
HEAT EXCHANGER CAPACITY (C) =
[750 W (A) – 154 W (B)] ÷ 10 F
Now that we know internal heat load, heat transfer and ΔT, we can
determine heat exchanger capacity as follows:
HEAT EXCHANGER CAPACITY (C) = 60 W/°F
HEAT EXCHANGER CAPACITY (C) =
[345 WATTS (A) – 162 WATTS (B)] ÷ 15 F (or 8 C)
As in the indoor industrial example, the above result is minimum
heat exchanger capacity. If no heat exchanger model is similar to the
result, choose the next largest size to ensure adequate electronics
cooling.
HEAT EXCHANGER CAPACITY (C) = 12 W/°F or 22 W/°C
The result is minimum heat exchanger capacity. If no heat exchanger
model is similar to the result, choose the next largest size to ensure
adequate electronics cooling.
Power input, protection level and dimensions of the heat exchanger
also need to fit the system.
Power input, protection level and dimensions of the heat exchanger
also need to fit the system.
16
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McLean Cooling Technology: How To Select A Protective Cooling Solution
How to Select the Right Cooling Capacity Heat Exchanger
Notes
How to Select
Subject to change without notice
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