How To Size A Waterside Economizer Part 6: The Key To Efficiency
/By Chad Edmondson
Approach temperature and an AHRI 400 rating are probably the two most critical factors when it comes to selecting a plate and frame heat exchanger for maximum efficiency and guaranteed performance. Do yourself and your client a favor and (1) insist that the plate and frame has been tested to the 2015 standard and (2) specify a 2°F approach temperature.
Let’s take approach temperature first, since it is largely the reason for the development of AHRI 400.
In case you’ve forgotten, approach temperature (in an economizing application) is the difference between the chilled water temperature leaving the heat exchanger and the temperature of the condenser water entering the heat exchanger while operating in economizing mode.
Why specify such a small approach temperature? Besides the obvious fact that you want to transfer as much heat as possible from the chilled water to the condenser water, it’s important to understand that increasing the approach temperature by just a few tenths will have a surprisingly large impact on the size and capability of the heat exchanger. Increasing the approach from 2°F to 2.3°F for example could decrease the size of the heat exchanger by 15%. While a smaller heat exchanger may sound like a good thing in terms of cost, in the long run you’re not doing your client any favors. This resulting reduction in capacity will absolutely translate into higher operating cost (more chiller work) for the owner.
Let’s look at the numbers. The example below shows the required heat exchanger surface for a plate and frame heat exchanger with the design parameters in the upper right, including a design approach temperature of 2°F. As you can see, we’ve done the math for you to come up with a required heat exchanger surface of 2083 ft².
But look what happens if we increase the approach by just few tenths:
As you can see the required area drops significantly with just a slight increase in approach temperature. (Feel free to check our math!) And while a smaller heat exchanger may sound like a good thing, all it really means is that your chiller is going to have to work harder to compensate for the reduced capacity of the heat exchanger. A 2°F approach will keep you in the sweet spot between upfront cost and performance while also ensuring your heat exchanger can do the job.
Keeping Everyone Honest
Specifying an AHRI 400 certified heat exchanger for your waterside economizer ensures all manufacturers are designing and selecting to the same performance criteria. When tenths of a degree matter any rounding of temperature or pressure drop, or difference in tolerances, can result in a smaller heat exchanger that will not perform to real life conditions.
Realizing this is the case, AHRI set out to design a test that removed any “play” in the numbers – eliminating the chance that a heat exchanger specified for a 2°F approach temperature might actually have an approach somewhere between 2.0°F and 2.5°F or a higher than published pressure drop. That’s why it is so important that your base specification for this equipment include the requirement that the heat exchanger be certified under AHRI Standard 400. This certification is also required by ASHRAE 90.1 (Sections 6.4.1.4).
