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# Selecting a Heat Exchanger

#### heat exchangers | thermal management

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

## Select the Cooling Liquid

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 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 incoming air from the temperature of 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.

Step 5: Select the Appropriate Heat Exchanger Model

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 56 W/°C at 7.5 LPM (2 gpm) (using a standard fan) would be acceptable. As shown in the following graph, Heat Exchanger 6210 meets the required performance by reaching 56 W/°C at the intersection of the 60 Hz Marin fan line.

Step 6: Determine the Liquid Pressure Drop

From the data given, we know our pump needs to supply water at 2 gpm (7.5 LPM). Using the liquid side pressure drop chart for the 6210 curve, the point where a vertical line at the 2 gpm (7.5 LPM) point on the x-axis intersects with the 6210 curve reveals that the liquid pressure drop through 6210 is 8 psi (0.55 bars). The pump selected must overcome this pressure drop to ensure a 2 gpm (7.5 LPM) 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 6210 is 0.24 inches of water (55 pascals).

## 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.

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.

Step 4: Select the Appropriate Heat Exchanger Model

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.

Heat Exchanger Section to view and compare our options and their performance capacities.

## Fan Considerations when Adding a Heat Exchanger

### Integrating a Heat Exchanger into your System

When designing a liquid cooling loop, there are several considerations relating to mating the fan and heat exchanger and installing the assembly into your system. This application note examines how these considerations, namely the use of a plenum, flow direction, and volumetric and mass flow rate, affect fan selection and integration.

#### Plenum

The plenum distances the fan from the heat exchanger fins to ensure that the air is distributed across the entire face of the heat exchanger.

If the fan is placed too close to the heat exchanger, it reduces the effective size of the heat exchanger to approximately that of the fan (Figure 1). Since the air is now passing through a smaller area, the result is a higher air-side pressure drop and a reduced air flow. The combination of the smaller effective heat exchanger area and reduced air flow results in less heat transfer.

When placed the correct distance from the heat exchanger (see Figure 2), the fan moves the air across the entire fin area of the heat exchanger. Since the air flow is spread out over a larger area it results in a lower pressure drop, therefore greater air flow and better performance.

To obtain maximum performance from your heat exchanger, it is also important that the junctions between the fan, plenum and heat exchanger are airtight to avoid air leakage and ensure that all the air flows through the heat exchanger.

Most of Boyd’s standard heat exchangers feature an integral fan plate and plenum at the optimum distance for good air flow. This improves performance when integrating the heat exchanger into your system.

### Fan Placement

Several conditions, including performance, fan life, and noise, impact the fan placement.

#### Performance

Provided that there are no external restrictions on air flow, a fan moves the same amount of air across a given resistance, regardless if it is pushing or pulling. This means that if you are simply attaching a fan to a heat exchanger in an open space, there is little performance difference whether you push or pull the air across the heat exchanger. If the fan is pushing the air across the heat exchanger, there may be a slight temperature rise in the air entering the heat exchanger and therefore decrease in performance due to heat generated by the fan. In most cases this is marginal.

However, where the air path is constrained like in a cabinet cooling application, one direction may be less restrictive than the other, resulting in a performance difference. Such situations need to be evaluated on a case-by-case basis.

#### Fan Life

Like all electrical devices, the motor of the fan will last longer when exposed to cooler temperatures. There can be as much as a 55% reduction in life when fans are operated in 60°C air as opposed to 20°C. If you are cooling the liquid, it is best to push the cool air across the heat exchanger so that the cooler air passes over the motor of the fan. Conversely, if you are cooling the air, fan life and performance will be improved if the fan draws the air across the heat exchanger.

#### Noise

Orienting the fan on the side of the heat exchanger furthest from the operator, exhausting the air away from the operator, provides the quietest operation. Other factors that can affect the noise level of the fan include overall airflow, blade size and design, and the speed at which the fan operates. Larger, slower moving fans are quieter than small, high-speed fans.

#### Volumetric Flow and Mass Flow

Cooling capacity depends on the mass flow rate. A fan provides a constant volume flow, not a constant mass flow. Mass flow and volume flow are related by the density of the air. Denser air affords a higher mass flow rate and therefore offers improved heat exchanger performance.

The density of air is determined by pressure and temperature. At a temperature of 59°F and a pressure of 14.7 psia, the density of air is 0.076 lb/ft3. Increasing the temperature or decreasing the pressure results in a lower density. When operating at elevated temperatures and altitudes, more volumetric flow is required to compensate for this lower density.

For example, our 6210 heat exchanger equipped with a Marin Fan has an air flow rate of 225 cfm. At 59°F and a pressure of 14.7 psa, this is equivalent to a mass flow rate of 17.1 lb/min. However, at an altitude of 20,000 ft, the mass flow rate is less than half of this value. Figure 3 shows how this mass flow rate varies with altitude and temperature.

Figure 3: Volumetric flow rate vs mass flow rate of our 6210 heat exchanger with a Marin Fan at various temperatures and altitudes.

## Conclusions

Generally, when installing a heat exchanger and fan into your system, you should:

• Use a plenum to give good air distribution and therefore optimum performance
• Consider the system configuration, noise requirements, and fan life to decide whether to push or pull the air through the heat exchanger.
• If you are operating at elevated temperatures or altitudes, take the air density into consideration to ensure that the selected fan is adequate.

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