06-26-2011, 03:33 PM
Mastercool question: I have been using Mastercool for about 25 years and have two 6500cfm units on my roof. I have a problem with one unit this summer that is a mystery to me. I have new pads (40x28x12). They are completely soaked with water (ie: the pump is working fine). I have a new belt on the system so that the two speeds work fine. The pads are tight at the front so that air can't get around them. The unit is closed up properly. The duct work is intact and not broken in anyway. The barometic damper is out of the system so that there are no restrictions to the airflow. Everything seems perfect... except it's not cooling very well. The other unit (same water supply, same exposure etc.) is cooling great, but this unit is about 10-12 degrees warmer. Any ideas what to look for? I can't figure it out! :(
Runs with bison
07-01-2011, 01:38 AM
I've not worked on these type of coolers, so bear with me... I have used demister pads and water sprays to increase the capacity of large chemical plant multistage centrifugal air compressors in East Texas so I think I understand the principle even if the equipment differs greatly. Granted, Phoenix is much drier than East Texas.
Sounds like you've already checked the things that make the most sense. So let's assume that the pad/duct/water supply systems are behaving the same. What about the air flow? Can you stick an anemometer in the duct work at the same point on both systems to measure feet/sec or some such? (I've done this with my home HVAC before using a cheap hand held anemometer to see what the blower was really doing.) Does the one with the lower temp (higher delta T) show a substantially lower flow? If so then the problem might be one of air flow rather than total duty. Perhaps the one with the low delta T is actually pulling through substantially more air? Q (duty) might be similar but the overall flow is much different and therefore so is temp. This seems unlikely, but without seeing the blowers and ductwork of the two units I can't rule it out. I've seen subtle differences in gear in "identical" plants result in notably different operation that perplexed sharper engineers before me on the same unit (one of them became CEO of our $5 billion dollar/yr operation), before I figured out what differed that had been missed before.
Have you considered swapping the pad sets between the two units to see if relative performance changes? (I suggest this because I suspect it is easy to do and will give good info about where to look.) This seems like a longshot, but perhaps the configuration or orientation of the pads differs in some subtle way. If it doesn't make any difference in performance after the swap then you can rule out the pads/configuration.
Measuring water supply temp and water flow is straightforward. Have you done this? Yes it is probably a waste of time, but bucket flow measurements can turn up all sorts of surprises (been there, done that...sometimes a flowmeter orifice or other device is FUBAR or doesn't match its calibration.)
Something differs a lot if the temp delta you reported is representative. The question is "what differs?"