Cooling Air Characteristics
In cabinet cooling applications, air is hotter than liquid. In this case, the ITD is the difference between the hot air entering the heat exchanger and the cold liquid entering the heat exchanger. You may need to calculate temperature rise using the heat load and the temperature of cool air entering the cabinet.
Step 4: Select the Appropriate Heat Exchanger Model
Example: Cabinet Cooling Application
You are cooling a cabinet containing electronic components that generate 2400 W of heat. The air in the cabinet must not exceed 55°C. What heat exchanger should be selected, and what is the temperature of the cool air entering the electronics cabinet?
Step 1: Application Data
Liquid Type: Water
Required Heat Load (Q): 2,400 W (8,189 BTU/Hr)
Temp. of Incoming Liquid (Tliquid in): 20°C (68°F)
Maximum Temperature of air in cabinet (Tair in): 55°C (131°F) — This is the temperature of hot air entering the heat exchanger
Rate of Liquid Flow: 2 gpm (7.6 LPM)
Step 2: Calculate the Initial Temperature Difference
Subtract the temperature of incoming liquid from the temperature of incoming air as it enters the heat exchanger.
ITD = Tair in - Tliquid in = 55°C – 20°C = 35°C (or 131°F – 68°F = 63°F)
Step 3: Calculate the required performance capability (Q/ITD)
Divide the required heat load (Q) by the ITD found above in step 2.
Refer to the thermal performance graphs for heat exchangers selected (See performance graphs for copper heat exchangers - 6000 series and OEM Coils, stainless steel heat exchangers - Aspen Series and 4000 Series and aluminum heat exchangers - ES Series). Any heat exchanger that exceeds 68.6 W/°C at 2 gpm (7.6 lpm) (using a standard fan) would be acceptable. Using water as the coolant, a copper heat exchanger is recommended. As shown in the following graph, 6310 exceeds the required performance, offering a Q/ITD of approx. 76 W/°C using our Ostro fan.
Liquid and air pressure drop can be determined the same way as in the previous example.
Step 5: Calculating the Temperature of the Cool Air Entering the Cabinet
Now, to calculate the temperature of cool air entering the cabinet, use the temperature change graph for air. With a heat load of 2,400 W, and a flow rate of 250 CFM (the flow rate of the standard Ostro fan recommended for use with the 6310) we can see that the temperature change is 17°C. This means that the cool air entering the cabinet will be: 55°C – 17°C = 38°C
Please Note: These graphs offer a simple graphical way of estimating fluid temperature change if you know your heat load and flow, without having to do calculations. The graphs for water, air, 50/50 ethylene glycol/water and oil allow you to calculate temperature changes for air and liquid for all types of heat exchangers.
Step 6: Calculating the Outgoing Water Temperature
To determine the outgoing temperature of water we use the ‘Water Flow’ chart to find that the change in temperature is approximately 5°C. Therefore, outgoing water temperature is 20°C + 5°C = 25°C.
Alternative Sizing Equation
The general heat transfer equation can be used to calculate the heat load and fluid temperature change given fluid flow rate and specific heat.
ṁ can be calculated for water and air using the following equations:
The temperature change graphs found in our thermal reference guide in the technical library plot the above equation for common heat transfer media (air, water, oil, and a 50% EGW mixture) providing a simple way to look up ΔT if you know your heat load and fluid flow rate.