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 72 F in your enclosure but is it really necessary? Would 95 F be just as acceptable with occasional excursions to 104 F 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 Curves: 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. Complete details on this process can be found on TECA Catalog Page 12.

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. Inquiries for other inputs are welcome.

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. See page 70 of TECA Catalog, Air Conditioner Accessories for more details on drip pans.

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