Maintenance Tip: Monitoring Heat Exchanger Temperature
Gardner Denver Nash uses two types of heat exchangers - Shell and Tube, and Plate and Frame. As the names imply, the Shell and Tube heat exchanger transfers heat through tubes, while the Plate and Frame transfers the heat via plates. These plates are similar to a car radiator.
If a customer contacts me about performance issues with their equipment, and I know that their equipment has a heat exchanger, I first reference the "Temperature Check Sheet.”
Let's start here - what is a heat exchanger? Seems like a good place to start, right? Well, a heat exchanger is a device in which two fluids flow along opposite sides of a solid boundary wall, which keeps the fluids separate while at the same time letting heat to pass from the hot fluid to the cold. A heat exchanger is a vital component to maintaining the temperature, and thus the performance, of your equipment.
There are three basic “approaches” for our customers - 2.5°F, 5°F and 10°F. What this means is that if everything is working correctly, the "C" reading of the water temperature going to the pump has a 2.5°F, 5°F or 10°F difference from the cooling water supply coming in at "A."
A single-stage pump will have a 5°F or 10°F “approach” heat exchanger, while a two-stage vacuum pump will have a 2.5°F or 5°F “approach.” Whichever it is, each “approach” ensures that the pump is able to run as cool as possible. This is especially important in hot weather. Compressors generally have 10°F “approach” heat exchangers, because the water temperature isn't affected as much as it is with vacuum pressure. High water temperature will affect the vapor pressure and prevent good vacuum performance.
When testing, record all of the temperatures on the check sheet. Always record the date. As we all know, temperatures changes with the seasons - after all, you don't want to compare January and August readings without being aware of the time frame. Almost all heat exchangers run hotter in the summer because the cooling water supply is warmer. There are exceptions, such as when a customer uses chilled water.
The “check sheet” is based on the following temperature points:
Shell and Tube heat exchanger example:
You may be thinking “this sounds great in theory, but what does it look like in practice?” Here is an example of a good reading on a 2.5°F “approach” for a Shell and Tube heat exchanger:
Some key differences between a 2.5°F and a 10°F “approach” is the heat exchanger size and cooling water flow, especially on the Shell and Tube exchangers. Note that on a 10°F “approach” exchanger, the “B” reading is 10°F higher and thus gives us the higher “approach” at “C”. Therefore, “C” would be 10°F higher than “A” in this example.
If you have full water flow from “A” to “B”, it will be taking the heat out with the cooling water and not have much difference. If there is a low cooling water flow, “B” will have a higher temperature - the lower the water flow, the higher the temperature. This will raise the reading at “C”, the seal water temperature.
If there isn't any difference between “A”, “B” and “C”, but “D” is hot, the problem is most likely a lack of heat transfer due to calcium or scale buildup on the tubes. A .010 -.015 buildup of calcium is a good insulator for heat transfer, as are rust and scale. Because the tubes are easier to clean, dirty water should be going through the tubes, while the cleanest water should be going through the shell.
If you have a high temperature change from “C” to “D”, the strainer or orifice may be plugging up. Be suspicious of this whenever you see a temperature difference between “C” and “D” that is 20°F or more.
Temperature "E" should be recorded in case the suction temperature is hot enough to affect pump performance or the other readings.
These are the “rules of thumb”, but if you want to have 100% peace of mind, it's always a smart idea to check with your supplier. Gardner Denver Nash maintains a full-time aftermarket staff of NASH Certified® Service technicians who are able to troubleshoot any question or concern you have. Our team is available around the clock and across the world, so to find out more, please contact us at gdnash.com.
To download your own Water Temperature Check Sheet and check your equipment's temperature yourself, click here.