Chiller Loop Volumes
Small Chiller Loop Volumes
The question is asked, “Why are there so many problems on small (10 – 100) ton chiller installations that do not seem to arise on larger installations?”
Many problems tend to become less of an issue as a system gets larger, loop volume and air management as two examples. The simple fact is, small chiller installations require more attention to proper design than the large installation. Certainly, more sins seem to be committed in small chiller design & installation than in any other cooling application
The total volume of water in the chilled water loop can be a critical factor. This has nothing to do with water flow; we are talking now strictly about the total mass of water in the loop. Positive displacement (reciprocating, scroll and screw) compressor chillers need a certain amount of water in the chilled water loop to avoid short cycling & temperature swing issues. If a small chiller is directly coupled to an air handling unit, there is often insufficient loop volume to provide smooth operation and good compressor life. A storage tank or oversized piping are typical solutions when a loop has inadequate volume. No chiller should ever be installed with less than 3 gallons of loop volume per ton of chiller capacity, and a better design figure for small chillers (especially single compressor units) is 6 gallons per ton. In industrial applications where water temperatures need to be tightly controlled, or for 100% outside air cooling applications, 10 gallons per ton is a good design figure.
Some manufacturers of small chillers also offer pump and tank packages that are integrally installed in the chiller. Be sure that you know whether this is an open or closed tank! There are very few HVAC applications where an open tank is desirable. Imagine an open or non-sealed tank with a chiller located on grade and an air handling unit on the roof; you can’t solve that flood with check valves in the piping (although I’ve seen people try)! The tank should be installed in the loop in the chilled water supply line leaving from the chiller to the building. The tank should also be baffled to ensure there is no stratification and that the fluid entering tank is adequately mixed with fluid in the tank.
Air Management & Water Fill Considerations
Small systems need an expansion tank. An automatic air vent should be installed at all piping high points, and at least one manual valve is needed, preferably at the highest point. A pot feeder is necessary for closed loop chemical treatment. A proper water make-up system with PRV (and backflow preventer, if not already present) is required; a bypass (“fill”) line with manual valve should bypass the PRV; and there should be an adequately sized drain valve (or valves) located at the low point(s) of the system. Without all of this, the system can not be properly commissioned or maintained correctly.
The installation needs to be chemically cleaned, then drained & flushed, then refilled. The air needs to be bled and the pot feeder used to install both the initial chemical cleaner charge and then the final anti-corrosion chemicals.