There are certain minimum specifications that everyone must answer before the selection of a T.E. device can begin. Specifically there are three parameters that are required. Two of these parameters are the temperatures that define the gradient across the T.E. device. The third parameter is the total amount of heat that must be pumped by the device.

The gradient across the T.E. device (Actual DT) is not the same as the apparent DT (System DT). The difference between these two DT's is often ignored, which results in an under designed system. The magnitude of the difference in DT's, is largely dependent on the type of heat exchangers that are utilized on either the hot or cold sides of the system.

Unfortunately, there are no "Hard Rules", that will accurately define these differences. Typical allowances for the hot side of a system are:

1. finned forced air: 10 to 15°C
2. free convection: 20 to 40°C
3. liquid exchangers: 2 to 5°C above liquid temperature

Since the heat flux densities on the cold side of the system are considerably lower than those on the hot side, an allowance of about 50% of the hot side figures (assuming similar types of heat exchangers) can be used. It is good practice, to check the outputs of the selection process to reassure that the heat sink design parameters are reasonable.

The third parameter that must be identified for the selection process, is the total heat to be pumped by the T.E. device. This is often the most difficult number to estimate! To reduce the temperature of an object, heat must be removed from it, faster than heat enters it. There are generally two broad classifications of the heat that must be removed from the device. The first is the real, sensible or "active" heat load. This is the load that is representative of what wants to be done. This load could be the I²R load of an electrical component, the load of dehumidifying air,or the load of cooling objects. The "other" kind of load is often referred to as the parasitic load. This is the load due to the fact that the object is cooler than the surrounding environment. This load can be comprised of conduction and convection of the surrounding gas, "leak" through insulation, conduction through wires, condensation of water, and in some cases formation of ice. Regardless of the source of these parasitic loads they must not be ignored.

There are other things that may be very important to a specific application. Things such as physical dimensions, input power limitations or cost. Even though these are important, they are only secondary. Melcor's approach to thermoelectric device selection / recommendation utilizes a computer aided design program AZTEC™ which selects an optimized thermoelectric design for the given operating hot side temperature, desired cold side temperature, and the total heat load to be pumped over the actual DT.

We have enclosed a "check list" to assist you in defining your application's existing conditions. If you should require any further assistance please contact one of our engineers.