Considering refrigeration and HVAC challenges unique to tall and complex buildings.

In December and January, this column looked at system tightness. We focused on leakage of refrigerant to the outside world, but there is another form of tightness that gets much less attention although it can be equally damaging to system performance.

This is the internal leakage past control valves, solenoid valves and internal pressure relief valves, and it has been likened to grand larceny.

In simple terms, all of these potential leak points are included in the system design for a reason, and if they fail to perform then the plant will probably not be at its best, resulting in a shortfall in capacity or an excess of power input one way or another. Here are a few examples of how things can get out of hand and result in significant losses.

One of the most significant sources of internal leaks in a cold storage system is the hot gas defrost setup. This requires high pressure gas to be fed from the condensers to the evaporators so that as the gas condenses and gives up its latent heat, the frost that has built up on the evaporator fins is melted. There can be several control valves for each evaporator, some feeding the gas to the coil and others controlling large gas-powered shut off valves to close off the evaporator from the main suction line. If any of these valves leak in the 23 or so hours per day that the evaporator is not defrosting, the valves will provide a direct path for gas to bypass the condenser and evaporator and go straight to the suction line. This requires additional compressor capacity and additional power to provide enough cooling to satisfy demand. In some cases, it is estimated that as much as 20% of a plant’s capacity could be dealing with gas bypass.

Internal relief valves, sometimes known as “overflow valves,” where the valve relieves from the high side of the system to the low side, provide a parallel path. These might be compressor relief valves, which usually feed straight from discharge to suction, or they could be from pumped liquid lines or oil recovery systems, discharging to the low side receiver. A leaking compressor relief valve ought to be obvious because the suction superheat will be high, but sometimes this can be masked by increased oil cooling.

When the leak is on the slide valve control gear of a screw compressor, it can cause a machine to get stuck on part loading instead of fully loading. This makes the compressor less efficient, but might also prevent an economizer from working properly since the economizer port will be at suction pressure. If the Vi slide sticks then the compressor could lose up to 10% efficiency through over- or under-compressing.

All of these internal leaks can be difficult to spot although the warning signs will be there if you know what to look for. Just as it is possible to spot fraud in a finance office if you monitor the right signals, these refrigeration efficiency thefts can be identified and the culprits brought to justice. At the IIAR annual show last month, one valve manufacturer launched a system for monitoring hot gas defrost valves for this reason and reckons that they could save thousands of dollars per year with their device. I wish them well, and I hope they shine a spotlight on this hidden problem.

*Edward G. Robinson played ex-con Pressure Maxwell in the 1942 movie “Larceny, Inc.”