Thermal Design of an Air Cooler

In this article we are going to perform a thermal design of an air cooler using ASPEN EDR software.

Quick Links

• 1 – Problem Definition
• 2 – Air Cooler Thermal Design
• 3 – Modifying the Design to Achieve the Required Thermal Performance
• 4 – Conclusion

Problem Definition

Let say we have a footprint of 20mx7m available to us at a plant. We want to design an air cooler that fits into that footprint and has a capacity of 9,500 kW (minimum).

In order to achieve the required design, we first need to perform a thermal analysis using some assumptions in order to get an estimation of the required air cooler dimensions.

Air Cooler Geometry Assumptions/Data:

Airside assumptions:

Air Cooler Process Data (usually available from process datasheet):

The process fluid composition is also given in the process datasheets (or PIDs).

Air Cooler Thermal Design

First, we are going to define the general application options. As shown below, ASPEN provides various options for different types of design. The most important design selection here is the “outside tube application” (as marked below). Here we should specify the outside tube stream. It can be one of the three options:

1) Dry Air
2) Humid Air
3) Gas

Figure 1 – Outside tube application

Here, we are going to use “dry air” as the outside tube stream.

Then, we need to specify the tubeside process information. As you can see in the below screenshot, the “Heat Exchanged” is specified to be 9,500 KW as required by the design and specified in the problem definition.

Figure 2 – Heat Exchanged

Other design data, such as the inlet air temperature, the allowable pressure drop (if provided by process datasheet), and process flow composition are also entered into the corresponding sections of the software.

Looking at the tubeside stream properties calculated by ASPEN (shown below), we can see that we have two-phase fluid in the tubes.

Figure 3 – Vapour Mass Fraction

Now we are defining the unit geometry as shown in the below screenshot. We have selected the “Counter-current” flow orientation for a better efficiency.

Figure 4 – Flow Orientation

After inserting all the remaining information/assumptions regarding the tube dimensions, material, fin material, type and size, bundle information, tube layout angle and tube support, we can define the software optimization parameters.

This is the final step (and very important, as it will directly affect the final geometry and thermal performance of the air cooler). In this step, we specify the limits/constraints used by the software for optimizing the design variables and to get to the required thermal performance. These limits are: Geometry limits, Process limits, and Optimization options (as shown below).

Figure 5 – Process Limits

Figure 6 – Optimization Options

It is very important to use the engineering judgment based on the previous experiences to put in the realistic limits in this step. Using non-realistic limits will cause the divergence in the thermal calculations and makes the required trial and error steps that follows the initial design much more time consuming and error prone.

Running the study, we get the following results in API sheet format.

Figure 7 – First Run Results

As we can see in the above API sheet, the overall dimensions of the air cooler are 16.6×8.3m. So, the length is acceptable, but the width is beyond what is available to us at the plant. Also, the heat exchanged is 8,180.4 KW, which is below the required 9,500 KW. So, we need to do some updates to the design to achieve the design requirements.

Modifying the Design to Achieve the Required Thermal Performance

To increase the heat capacity of this air cooler, we can increase the tube rows as well as the tube fin frequency. We can also increase the tube length (as the current 16.6m length of the air cooler is less than the available length of 20m at the plant). We also need to decrease the bay width (our current air cooler width of 8.3m is above the 7m available width at the plant). We can reduce the tube pitch to increase the number of tubes without significantly increasing the air cooler dimensions.
Implementing the above changes, and rerunning the simulation, the new API sheet is shown below. The new heat exchanged in the air cooler is 9,528.8 KW and new air cooler dimensions are 17.7×5.55m, which are perfectly in the required design criteria.

Figure 8 – Second Run Results

ASPEN also generates the air cooler setting plan and tube layout drawings (shown below).

Figure 9 – Setting Plan Drawing

Figure 10 – Tube Layout Drawing

The tubeside temperature is shown below.

Figure 11 – Tubeside Temperature

The tube metal surface temperature is shown below.

Figure 12 – Tube Metal Surface Temperature

Conclusion

As shown in this article, using only minimal process information, we can do thermal analysis and design of an air cooler whilst taking into account the size limitations and keeping the thermal performance required to accomplish the process duties of the air cooler.

The setting plan generated by ASPEN (shown above) help us to verify the final overall dimensions of the air cooler, as well as the number of bays, bundles, and fans, and to make sure that the design is appropriate based on the footprint available to us.

The tubeside fluid temperature and metal surface temperature plots (shown above) generated by ASPEN are extremely helpful to ensure that the effective temperature difference required for the process efficiency is met and the mechanical design temperature of the tubes is above the maximum metal surface temperature given in the tube-side metal surface plot.