Harris Environmental - Design Guide Page #2

Temperature Uniformity

Temperature Uniformity In an environmental room, the air temperature must be very uniform at the working surfaces and uniform throughout the whole room. Room manufacturers, however, make a distinction between the terms uniformity and gradient.

Uniformity refers to the deviation from a specified temperature maintained across the room at working height. Gradient usually refers to the maximum difference in temperature from any point in the room to any other.

When uniformity is important, it must be clearly specified as the set point temperature and the allowable variation. For example, 21ºC ±0.5% means the temperature at one end of the working surface can be 20.5ºC, but no other point at the working height can exceed 21.5ºC, even when all the specified temperature and moisture loads are active in the space at the same time.

In rooms used for storage, temperature uniformity is generally not important, and since storage shelves impede airflow, uniformity is difficult to achieve if any heat loads are active. Be aware that specifying uniformity in a cold storage room is likely to increase its cost by 30 to 40%.

With environmental rooms, there are two typical uniformity specifications:

• • ±1.0ºC
• • ±0.5ºC

In theory, it should be easier and less costly to maintain ±1.0ºC uniformity than ±0.5ºC, but, in fact, the controls and equipment are about the same for these conditions. Moving from ±1.0ºC to 0.5ºC will increase the room cost by about 5 percent - not a large cost impact when tighter uniformity is essential to the process.

When specifying uniformity, most designers require the room manufacturer to submit documentation describing how the specification will be achieved. This should include the planned uniformity, proposed airflow patterns to achieve it, anticipated heat loads and cooling system capacity.