Harris Environmental - Design Guide page 3

Temperature Gradient

Temperature Gradient In the context of environmental room specifications, the term gradient can have several meanings:

The change in temperature divided by the change in distance over which it is measured.
The maximum temperature differential between the floor and ceiling of the room.
The maximum temperature difference between any two points in the room.

The first definition is perhaps the most technically correct, but is seldom used. The second is often used by suppliers, but seldom desired by the end user. The third is what end users generally mean when they request a specific gradient - the temperature should not be very different anywhere in the room, even when heat-producing operations are in progress. Temperature Gradient

Assuming this is what you need, you should specify the third definition to avoid confusion.
The gradient is generally specified in degrees. For instance: 21ºC room temperature with a uniformity of ±0.5ºC and maximum gradient of 1.0ºC. The smaller the gradient, the more expensive the room will be, because more air must be circulated to evenly distribute and remove the heat.

A gradient of more than 3ºC is quite generous, and most rooms constr5ucted for cold storage can achieve this specification quite readily. Gradients of less than 3ºC alert the supplier to expect environmental room-grade requirements. The cost will increase because the mechanical system must be larger, and room architecture must be adapted to accommodate air distribution plenums or ductwork.

The graph at left shows how heat loads and allowable gradient combine to influence the size--therefore the cost-- of mechanical systems. The required airflow capacity of the system increases geometrically as the load increases and the allowable gradient decreases. Smaller gradients are more costly and so are larger heat loads.

For normal operations in life sciences environmental rooms, a gradient of 1ºC is easily maintained because process loads seldom exceed 15 watts per square foot. In rooms with high heat loads from large centrifuges, shakers or large pumps--be aware that it will cost more to maintain the same gradient.