Stick to the Design and Keep Coils Clean for Maximum Efficiency
A case in point may be seen in a problem with a supermarket distribution system in Ohio that I was asked to check several years ago.
The produce room was designed to operate at 35°F, yet the eight evaporator units operating on a typical ammonia liquid recirculation fed system operating at 20°F suction could not maintain the desired room temperature.
The first thing I checked was the cleanliness and condition of the finned coils that were spaced 6 or 8 fins per inch. Using a flashlight, I could see they were so dirty and clogged with dirt and cardboard fibers, due to lack of proper cleaning and maintenance, that the flashlight beam could barely be seen through them.
A special crew was then dispatched to thoroughly clean each unit. The cleaning process consisted of pumping each unit out and enclosing it with Visqueen from the top of the unit to the floor, to protect all nearby boxed products from the over-spray and use of the power sprayer. Then each unit was power-sprayed with just enough pressure to clean the dirt off the entire depth of the coil without bending the fins, using mild soap without chemicals that would attack the galvanized surfaces, a good supply of warm water, carefully and thoroughly cleaning the coil of each unit and flushing its drain line. The coil was dried by running “fans only” for several hours.
But even after all this was done and the units started up, desired temperature could still not be attained and was about 5 to 8 degrees too high. So next I checked the inlet air and outlet air temperatures of two or three units with a two-point electronic thermometer. The temperature reduction was not even close to what it should have been; i.e. for a 35°F room and 23°F coil temperature (allowing 3°F loss due to pressure loss from coil to compressor suction), one would expect a 27°F to 29°F discharge air temperature. Temperature readings of the discharge air were only about 40% of what they should have been.
Upon further discussion with the operator we found that he had throttled all the hand expansion valves of all the units to only one-half turn open, due to an unnecessary and unjustified fear of over-feeding the units with liquid and perhaps causing high liquid levels and possible compressor slugging. I explained how the units are circuited and designed to operate with a surplus amount of liquid ammonia (4:1 overfeed rate) that assures all inside surfaces of all the tubes are thoroughly wetted with boiling liquid ammonia to produce a maximum inside-the-tube film coefficient that results in the maximum heat transfer.
Then, using the two-point electronic thermometer, we adjusted all the manual hand expansion valves to the number of “turns open” that produced the most cooling, recorded those settings with instructions to keep them set “as is.” The result was that the operating pump’s amperage increased because we were now circulating close to the design mass flow rate for a 4:1 overfeed amount of liquid, the accumulator’s “normal level” remained the same, and “miraculously” the room temperature fell 6°F to 8°F.
More recently, a flip side of that situation was brought to my attention by some contractors who knew of jobs where well-meaning but uninformed operators and production managers are operating liquid recirculation systems with both normal and stand-by pumps running together and then fully opening the hand expansion valves at all system evaporators.
The thinking there, that “more is better,” doesn’t agree with the heat transfer reduction in the evaporators that occurs due to the liquid ammonia “brining” effect.
Note that the maximum cooling occurs at a certain hand expansion valve setting determined by a trained technician, taking inlet and outlet temperature measurements.
If the expansion valve is closed or “pinched off” from the optimum setting, the coil receives less cold liquid ammonia than it is designed for and this results in less heat transfer, because some inside tube surfaces are not fully “wetted” with cold boiling refrigerant, but rather with cold ammonia vapor. The resulting “inside the tube” heat transfer film coefficient at those areas is much lower. If the hand expansion valve is opened several turns more than the optimum or, worse yet, fully opened, the pump pressure drop is reduced and the flow increases. This produces a “brining” flow much like chilled water flowing through an air conditioning coil, where, due to the excess liquid, refrigerant boiling does not occur. Here again, the “inside the tube” heat transfer film coefficient is less and the capacity is reduced.
The important message here is twofold:
Note that recirculating systems built by experienced, qualified contractors and manufacturers have two pumps, both sized only for the flow required to feed all the connected evaporators for the selected over-feed rate, usually 4:1 or 3:1. One is meant to operate continuously and the other is for stand-by duty, for example, if the motor of the operating pump fails.
Also, the liquid piping is sized for the design flow only, not for twice that amount.
Once the optimum hand expansion-valve setting is established and properly recorded by an experienced mechanic or service technician, leave it alone and do not run the stand-by pump. All it does is waste energy and reduce the desired results.
