By: Wayne Perry
A well-maintained, double-acting, water-cooled piston compressor is a marvel of efficiency, but these days it’s rare to find these machines in operation. We’ll take a brief look at how to measure and compare their performance to rotary screw compressors.
We recently met with a prospect at an 80-year-old company with 70-year-old compressors. All are water-cooled, two-stage, double-acting reciprocating compressors from 1949 and 1952. Two are 75 hp and one is 125 hp. Because of the scarcity and cost of parts and expertise to maintain these machines, the owner is considering replacing them. At the same time, the owner is confident that no modern compressor will beat the efficiency of these venerable machines, and he is hesitant to give them up. Operating in southern California where power is very expensive, the company is very tuned in to energy efficiency and wants to leverage local utility incentives to reduce the cost of new equipment.
Looking at the electrical efficiency of just the compressor, the specific performance of the water-cooled double-acting, two-stage is very strong compared to newer screw compressors. On paper their specific performance reaches down to the 15-16 kW/100 cfm range. By comparison, the most energy-efficient screw compressors are in the 16-17 kW/100 cfm range.
There are not too many of these machines out there running anymore, so this is a rare dilemma. But since very few of us in the industry today know much about these machines, it is an interesting case and a learning opportunity.
Qualifying for utility incentives typically requires some comparison metrics. In this case, you need to measure actual energy consumption. Having not seen one of these machines in operation, our field rep contacted our System Design & Engineering team to discuss how to measure performance on this type of machine. With fixed speed rotary screw compressors with known cfm flows, it is usually sufficient to measure power in kW (not Amps) and collect data on when the unit is loaded, unloaded, and off.
Unlike rotors in most screw compressors, piston rings and valve seals wear. As they wear, the volume they produce decreases. Besides the rings and valves, intercooling is critical to good performance. Failing to keep the intercoolers clean and fed with cool water will reduce the efficiency and longevity of the compressors. So for large reciprocating units like these, we record actual kW (again, not Amps) and for the flow it is better to install a flow meter after the dryer and as far downstream as possible to reduce the effect of pulsations. The motor signal (on/off) should also be logged, as well as a synchronized pressure input. With the actual flow and kW in hand, the specific performance (kW/100cfm) of these legacy machines can be compared to the CAGI Data Sheets for screw compressors that could replace these units.
(On a side note, if you want to assess the performance of a small shop piston compressor, a simple pump-up test is a convenient way to check the current output of the compressor.)
Knowing the specific performance of the compressor isn’t the whole picture, however. These are water-cooled machines, so a true energy cost comparison must include the energy used in the cooling water process. Adding in the power costs for cooling circuits tends to narrow or eliminate the lead over screw compressors. This is also true when comparing performance of water-cooled to air-cooled screw compressors. Water cooling helps gain some efficiency in the compressor but presents some additional costs for electricity, water, water treatment, and cooling system maintenance.
The market moved away from double-acting reciprocating compressors to rotary screw compressors not because of increased efficiency, but because of lower overall operating costs. These were great machines, but the efficiency of many of today’s large screw compressors has greatly improved during their 50 or so years on the market. When you add the power costs related to cooling water to the power of the compressor itself the efficiency difference narrows. Then add in the higher maintenance costs for the compressor and the maintenance of a cooling water system and the overall life cycle cost comparison will usually shift in favor of a properly sized and controlled system with screw compressors.