When designing cooling systems, simplicity is key.



With energy costs going up, process system and building owners are taking a greater interest in reducing operating costs. It may seem that the only way to do this is to start over with a ground-up redesign, but how often does that happen? Fortunately, it is possible to cut operating costs without such a drastic approach, and this method is applicable both in retrofit and new system design situations. Even better, it can be done with only a minor impact to the initial cost.

Suppose a cooling tower is used to supplement a refrigeration-based system during the peak season. This also means that the main system is shut off in the shoulder season (the time between the peak- and low-cooling-demand seasons of a process), and only the cooling towers are used. Using only the cooling tower can save a lot of energy because there are only pump operating costs, compared to the pump plus compressor costs with the main system. The cooling towers permit a lower operating cost for the same cooling. Figure 1 shows where the cooling comes from over the course of the cooling season, using a hypothetical city.

Figure 1. A cooling tower is used to supplement a refrigeration-based system during the peak season. Also, the cooling tower provides cooling during the shoulder season to reduce energy demands.

The key to this approach is to employ an efficient heat exchanger. Typically, a shell-and-tube heat exchanger has been used because it is proven and reliable. With this design, the typical approach temperature used is 5°F (2.8°C). While that is not bad, the technology of heat exchangers has advanced, providing better options.

Modern gasketed plate heat exchangers can handle a 2°F (1.1°C) approach temperature, and advanced models can achieve a 1°F (0.6°C) approach temperature. This means that for the same cooling water temperature coming in from the cooling tower, cooler temperatures can be delivered to the internal loop - without paying more.

How does this help? Going to a more efficient heat exchanger extends the cooling tower’s capability to supply more cooling, reducing the operation time of the refrigerant-based system. As shown in figure 2, the blue section showing operating period for the refrigerant-based system is smaller. The “shoulders” become bigger, handling more load with the system with lower operating costs and reducing operation of the refrigeration system.

Figure 2. Adding a heat exchanger with a close approach temperature extends the cooling tower’s ability to supply more cooling, further reducing the operation time of the refrigerant-based system.

How much money can be saved? If the cooling requirements and the capabilities of the cooling tower are known, it is a simple matter to find out from the heat exchanger manufacturer what the heat exchanger can deliver, as well as the pressure drop and flow rate. Use that to “fill in” the orange space in figure 2, then compare any additional pump operating costs (if there are any) with the energy saved by not running the refrigerant-based system for that time.

This method is simple, straightforward, and unlike a lot of energy-savings claims, verifiable in advance. An added benefit is that it can be applied to a retrofit or a system design without a ground-up change.

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