Cabinet Cooling Thermal Calculations

How to Calculate the Required Thermal Resistance for a Cabinet or Enclosure

Heat exchanger manufacturers usually present thermal performance data as a function of heat load and incoming air and water flow rates. This works well for applications where the heat exchanger is used to cool water with air, as you can simply plug in your heat load, air temperature, and liquid temperature to determine if it offers sufficient thermal performance.

Mass Flow Rate Calculation

Cabinet cooling applications use the heat exchanger in the opposite configuration - cold water flows in the liquid circuit and warm air from the cabinet is cooled as it passes across heat exchanger fins. In cabinet cooling applications, you usually need to know the temperature of air as it enters the cabinet, and the maximum temperature that air in the cabinet will reach. Neither of these can be read directly from heat exchanger performance curves.

The usual way to calculate the temperature change of air is to use the mass flow rate calculation.

This can be time-consuming and prone to error.

To avoid these calculations, Aavid developed charts to quickly estimate temperature rise in common heat transfer media at various heat loads. Graphs are available for air, water, oil, and 30/70 ethylene glycol-water (EGW). To calculate temperature change, simply select the appropriate graph, look up your flow rate and heat load, and read off the temperature change. In our technical library under thermal reference you can view a pdf or our temerpature change graphs.

When used in conjunction with product performance curves, these offer a quick and simple way of calculating the temperature of cold air entering the cabinet, and maximum air temperature in the cabinet.

Heat Exchanger 6310 Graph

Example Cabinet Cooling Calculation

You are evaluating a 6310 heat exchanger with an Ostro fan for cooling an electronics cabinet. The water entering the heat exchanger is at 20°C and a flow rate of 1 gpm. The heat load, Q, is 2400W.

What is the temperature of cooled air entering the cabinet (i.e. the temperature of air leaving the heat exchanger) and what is maximum temperature in the cabinet (i.e. the temperature of warm air entering the heat exchanger)?

First check the performance curve of 6310 in the catalog. You will see that with a 1gpm water flow and the Ostro fan which supplies approximately 250 cfm, its performance is 80W/°C.

Since we know that Q is 2400W and Q/ITD is 80°C/W, we can calculate Initial Temperature Difference (ITD).

ITD = 2400W ÷ 80°C/W = 30°C

We also know that incoming water temperature is 20°C. We can therefore calculate incoming air temperature:

The incoming air temperature = 20°C + 30°C = 50°C.

To determine outgoing temperature of the air, we use the 'Air Flow' chart using parameters 250 CFM and 2400 W.

We find the change in temperature is approximately 17°C. The outgoing air temperature is 50°C - 17°C = 33°C.

We know that this heat exchanger with the Ostro fan will cool air to 33°C, and the hottest temperature air in the cabinet will reach is 50°C.

To determine the outgoing temperature of water we use the 'Water Flow' chart.

Air Flow Chart
Water Flow Chart

At 1 gpm and 2400 W, this shows that the change in temperature is approximately 9°C. Therefore, the outgoing water temperature is 20°C + 9°C = 29°C.

Graphs for air, water, oil, and EGW are available in downloadable PDF format. These are helpful for sizing heat exchangers and cold plates and are also useful in a variety of other temperature change calculations.

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