Lytron - Total Thermal Solutions
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Tools & Technical Reference

 
 

Selecting a Heat Exchanger

1. Cooling Liquid

In order to select the correct Lytron heat exchanger or oil cooler, you must first determine the required thermal performance for your application. Use the example shown below:

Step 1: Application Data

Liquid Type: Water
Required Heat Load (Q): 3,300 W (11,263 BTU/Hr)
Temp. of Incoming Liquid (Tliquid in): 80°C (176°F)
Temp. of Incoming Air (Tair in): 21°C (70°F)
Rate of Liquid Flow: 2 gpm (7.6 lpm)

Step 2: Select the heat exchanger product series

Choose an aluminum, copper or stainless steel heat exchanger based on the fluid compatibility. Aluminum tubing is usually used with light oils, or ethylene glycol and water solutions. Copper is normally used with water. Stainless steel is used with deionized water or corrosive fluids.

Step 3: Calculate the initial temperature difference

Subtract the temperature of the incoming air from the temperature of the incoming liquid as it enters the heat exchanger.

ITD = Tliquid in - Tair in
= 80°C - 21°C = 59°C or (176°F - 70°F = 106°F)

Step 4: Calculate the required performance capability (Q/ITD)

Divide the required heat load (Q) by the ITD found above in step 3.

Heat Exchanger Equation 1  

Heat Exchanger Air Flow GraphStep 5: Select the appropriate heat exchanger model

Refer to the thermal performance graphs for the heat exchangers selected (See performance graphs for copper heat exchangers - 6000 series, copper heat exchangers - OEM Coils, stainless steel heat exchangers - Aspen Series, stainless steel heat exchangers - 4000 Series and oil coolers). Any heat exchanger that exceeds 56 W/°C at 2 gpm (using a standard fan) would be acceptable. As shown in the following graph, Lytron’s 6210 exceeds the required performance.

Step 6: Determine the liquid pressure drop

From the data given, we know our pump needs to supply water at 2 gpm. Using the liquid side pressure drop chart for the 6210 curve, the point where a vertical line at the 2 gpm point on the x-axis intersects with the 6210 curve reveals that the liquid pressure drop through the 6210 is 8 psi (0.55 bars). The pump selected must overcome this pressure drop to ensure a 2 gpm flow.

Step 7: Determine the air pressure drop

The vertical line on the thermal performance chart indicates the air flow rate (190 CFM for the Marin fan) as provided by our standard fans at 60 Hz. The intersection point of this air flow rate and the 6210 graph on the air side pressure drop reveals that the air side pressure drop through the 6210 is 0.24 inches of water (55 pascals).

 Heat Exchanger Liquid Side Pressure Drop Graph

 Heat Exchanger Air Side Pressure Drop Graph

2. Cooling Air

In cabinet cooling applications, the air is hotter than the 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 the temperature rise using the heat load and the temperature of the cool air entering the cabinet.

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)
Max.temp of air in cabinet (Tair in): 55°C (131°F) — This is the temperature of the 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 the incoming liquid from the temperature of the 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.

 Heat Exchanger Equation 2

 Heat exchanger air flow graph

Step 4: Select the appropriate heat exchanger model

Refer to the thermal performance graphs for the heat exchangers selected (See performance graphs for copper heat exchangers - 6000 series, copper heat exchangers - OEM Coils, stainless steel heat exchangers - Aspen Series, stainless steel heat exchangers - 4000 Series and oil coolers). Any heat exchanger that exceeds 68.6 W/°C at 2 gpm (using a standard fan) would be acceptable. Using water as the coolant, a copper heat exchanger is recommended. As shown in the following graph, Lytron’s 6310 exceeds the required performance, offering a Q/ITD of approx. 96 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 the 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

 Temperature Change Graph for Air

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 the water we use the ‘Water Flow’ chart to find that the change in temperature is approximately 5°C. Therefore the outgoing water temperature is 20°C + 5°C = 25°C.

 Water flow graph

Alternative sizing equation

The general heat transfer equation can be used to calculate the heat load and the fluid temperature change given the fluid flow rate and specific heat.

 Heat Exchanger Equation

dot_over_m can be calculated for water and air using the following equations:

 Equation

The temperature change graphs 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.

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