Thermoelectric Air Conditioner
Notes
Things you need to know to start sizing an
air conditioner:
Temperatures: The ambient is the air temperature around the enclosure,
often the room temperature. The enclosure temperature is the temperature range
you wish to maintain in the enclosure. The difference between the two is the
design temperature differential (delta T). Look at these temperatures with
care. What is the real maximum ambient? What is the real maximum enclosure
temperature? How can I define these? Ask maintenance or those who work in the
area. Look at the temperature specs of the equipment inside.
You may want 22 C in your enclosure but is it really necessary? Would 35 C
be just as acceptable with occasional excursions to 40 C under worst case
conditions? Do not impose unrealistic demands.
Heat Loads Active and Ambient: We define an active load as any source of
heat inside the enclosure. Waste electric heat or exothermic reactions are
examples. This can be determined in several ways.
The first is by simply adding up the amount of heat generated by each
component. This sounds easy but the information is often not readily available
in component spec sheets and requires direct inquiries to manufacturers.
A second method is to apply a control volume approach and to measure the
total electrical input and output, subtract the two and assume the remainder
has been turned into heat.
Another method requires knowledge of the thermal characteristics of the
enclosure, how many degrees does it rise given a defined amount of internal
heat generation. This can be found by monitoring internal and external temperatures
and varying a known internal load while bringing the system to equilibrium
between each step and recording the values. Use these numbers to determine the
degrees per watt enclosure characteristic. Once this characteristic of the
enclosure is known the system can be operated to steady state, the ambient and
enclosure temperatures measured and a quick calculation made to estimate the
amount of internal heat generated. An ambient load is that amount of heat added
to the enclosure due to ambient conditions. The usual ambient load is that
caused by the temperature difference between the enclosure and the ambient. It
is a function of the enclosure thermal characteristics (size, insulation,
seals, windows etc…).
The same value determined experimentally as described above can be used to
estimate the ambient load. Outdoor applications often have an additional solar
component. Indoor applications can have something similar if for instance they
are in close proximity to a heat source such as a furnace.
Performance Curve: The total load and temperature differential (dT) can be
applied to the performance curves of the air conditioners to determine if the
capacity is sufficient.
Things you should consider when selecting
an air conditioner:
Purpose: What is the real need for cooling: maintaining electronics
temperatures, precision temperature control, maintaining sample temperatures,
cooling a process? Answering these questions will ascertain the need for an air
conditioner and help in selecting the control types and methods.
Temperature Control: Several control options are available. The most widely
used controls are the TC-6F cool only temperature control set to its 35 C
setting and the TC-3F heat/cool control. This setting provides a comfortable
temperature for the electronics, minimizes chances of condensation and an
efficient duty cycle for the air conditioner. For tighter control, air
conditioners set up for remote control via an umbilical cord are used in
conjunction with TC-3400/3500/4300/4600 or customer supplied controls. These
units typically require a 3 to 32 VDC drive signal to turn the heating or
cooling on. Buck heating control where the air conditioner is on 100% of the
time and control is provided through a separate heater is another option. P,
PI, PID and PWM control schemes have all been used with success.
There is one caution regarding input surges when trying to get tight
temperature control with AC input units. Each application should be evaluated
independently to assure safe and proper control.
Environment: IP and NEMA both define the types of environments one might
find. We’ve included those types of designations with the air conditioners to
help you select the right one for your environment. In general our standard air
conditioners can handle factory and office environments, “XE” style are
appropriate for many wet factory environments and outdoors, “X” style can
handle a bit more rugged environment, the “XM” style employ military fans
throughout and have been customer tested to survive severe shock and vibration
in all axes while “XP” units can handle harsh indoor/outdoor hazardous
locations like CI D2 explosion proof.
Power Input: Air conditioners requiring 110 VAC, 220 VAC, 110/220 VAC, 12
VDC, 24 VDC and 12/24/48 VDC are available.
Cooling Medium: Are you looking for an air (fan) cooled air conditioner
(AHP and FHP products)? Is a liquid cooled version appropriate for your application
(LHP products)? Perhaps the environment rules out both fan and liquid cooled.
Is compressed air available (AHP-CXP products)?
Enclosure size and characteristics: How big is it? The bigger it is the
more heat it transfers in and out. Is it insulated? Does it have any features
that will interfere with mounting an air conditioner? Is it appropriate for the
environment? Does it have windows and access ports which will affect the amount
of cooling required? Can you protect it from external loads by using solar
shades or shields? Will the enclosure, its contents and the air conditioner
mesh well together?
Air Conditioner Mounting: These air conditioners can be mounted in any
orientation, wall mounted is usually best. There are two types of mounting
styles available, through mount where the cold side of the air conditioner
extends into the enclosure and flush mount where there is no or minimal
protrusion into the enclosure. Regardless of which style chosen it is important
to orient the air conditioner in such a way that it compliments rather than
hinders internal air circulation. Internal fans (cold side fans) typically run
continuously to provide a constant internal air flow. External fans cycle with
demand.
Condensation Concerns: The best time to
address condensation concerns is in the selection of the air conditioner. Will
conditions be ripe for condensation to occur? Condensation may form on the fins
when their surface temperature goes below the dew point temperature. Use the
“Cold Sink” equations provided for each product with your total load value in
this equation to determine the delta T and therefore the actual temperature of
the cold side heat exchanger. Compare this value to the expected dew point to
see if condensation might occur.
The 35 C setting of the TC-6F temperature
control goes a long way in minimizing moisture. Set it to the 25 C or
continuous on position and you may be setting yourself up for problems because
the cold side fins will get that much colder. Side, front, or back mounting is
recommended if there are condensation concerns. Many flush mount units come
with condensate control systems which soak up moisture and transport it away.
There are drip pan accessories for through mount air conditioners to collect
and remove moisture.
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