There is an old axiom among ammonia refrigeration engineers: "In efficient, safe, ammonia refrigeration systems, wet return piping must have no trapped liquid."
Seems pretty obvious to me, but unfortunately, many designers do not consider the effects of trapped liquid properly in overfeed (liquid recirculation) return piping in low temperature refrigeration systems. Liquid trapped in wet return piping lowers the evaporator temperature. As a result, there is a greater loss in the system performance and efficiency.
So, how do you analyze static head penalty? I'll look at a hypothetical situation to show you.
The Facts. Ammonia refrigerant at -40oF (-40oC) is trapped at the bottom of a column with a 10' lift. Data from ASHRAE refrigerant tables indicate that ammonia at -40oF has a pressure of 10.41 psia and weighs 43.07 lb/ft3. Thus a 10' column, 1' square, weight 430.7 lb and has a static pressure of
430.7 / 144 in2/ft2 = 2.99 psi
This must be overcome for the vapor from the evaporator to reach the compressor suction. Therefore, based on the information in the refrigerant tables:
10.41 psia - 2.99 psi = 7.42 psia (-51oF)
That assumes no slip in the column of ammonia. The suction temperature to obtain -40oF at the coil outlet is -51oF (-46oC).
As an example, assuming no slip, the coil suction temperature at the coil with
-40oF at the top of the riser, the assumed compressor suction is:
10.41 psia + 2.99 = 13.4 psia = -31oF (-35oC)
Some piping experts say there is a slip factor of about 0.6 (i.e., all the bubbles reduce the effective weight of the liquid and reduce the penalty accordingly). The loss then would be +/-6oF, in lieu of +/-10oF at these temperatures. I think that is probably optimistic. Applying this data to the above example indicates that the temperature at the coil outlet would vary from -35 to -31oF (-37 to -35oC) in rapid succession.
If you check a screw compressor (booster) rating, the performance drops drastically when a compressor has to be sized for 10oF below the necessary evaporating temperature at the coil.
Here's another example. Some years ago, I had a call from a freezing plant with an interesting capacity problem. The conversation went like this:
"We have large spiral freezers, each should have a capacity of 5,000 pounds an hour, with a suction temperature of -40oF. We have ample compressors and they operate at -45oF [-42oC] or lower at times. The recirculator is on the second floor, near the low stage compressors, and the freezers are about 25 feet below the recirculator's wet return. The air temperature ranges between about -10oF and -15oF [-23 and -26oC]. What is wrong? We cannot produce an acceptable frozen product at a rate any where near 5,000 pounds per hour."
A quick calculation indicated that there was a loss of 15 to 20oF (8 to 11oC) in the trapped wet return. A job site visit and a few temperature checks indicated that the calculations were on target. The actual loss due to static head was measured at 17oF (9.4oC) max. Obviously, options such as double risers and pumped liquid return systems may have solved this problem. However, when considering some major plant additions and the existing recirculator size and piping, a third option proved to be the most efficient and cost-effective solution. The plant layout was such that the recirculator could be moved from the second floor to a shallow pit adjacent to and below the spiral freezers' evaporators. The equipment room was directly above the spiral freezers. After considerable discussion, the owner agreed to make this costly change. In any one of the cases, rearranging control valves and piping around the spiral freezer evaporators was necessary.
This rearrangement eliminated the static head penalty caused by the trapped wet return. The coils were top fed, so piping modifications were made that directed the wet return downward to the recirculator return connection. This modification eliminated the trapping and solved the temperature problem. The capacity problem in the spiral freezers also was solved. It reduced the ammonia refrigerant inventory and returned the plant to design conditions. While making the changes, the suction and wet return stop valves were converted from globe to angle to further reduce pressure drop in the system.