Four-Way Reversing Valves And Efficiency
The forum, “Are Internal Leakage and Heat Transfer Required in a Standard for Method of Testing for Capacity of Four-Way Reversing Valves,” asked these questions:
It was pointed out that in the action of a four-way valve, the slide moves away from the ports in either the low- or high-pressure stream. “They interfere with each other,” an engineer said, resulting in a loss of both heat and mass transfer.
“You lose mass flow and reduce suction pressure drop.” This, in turn, affects the efficiency and capacity of the compressor.
The only measures in the proposed standard right now are flow rate and suction pressure drop, another engineer said. The committee should add heat leakage, heat transfer, and mass transfer. However, this could hold up the publication of the standard.
A retired engineer commented that the committee should be discussing a means to measure the penalty of using a four-way valve.
Pressure loss in the suction flow path and four-way valve is not the greatest loss to system efficiency, an equipment designer commented. Leakage and heat transfer could be much greater losses and penalties. “We need to address three major losses.”
In addition, “Not all four-way valves slide back and forth,” he said in further defense of measuring penalties. “Who knows what some bright young engineer might design?”
One manufacturer of reversing valves stated that as it is, the standard could be misleading to end users. “Not all valves are the same. Suction drop and flow capacity don’t tell the whole story.”
Another valve manufacturer commented, “Most valves have very low leakage when they’re new.” And even as they start to wear, current valves have pressure-loaded seals so that when pressure goes up, leakage decreases.
Then there are differences in valve materials. Plastic-to-metal seals have different sealing characteristics than metal-to-metal seals. Large valves, at least, have been studied in this respect; in a 10-ton system, plastic-to-metal seal valves had less effect on capacity than metal-to-metal seals, commented the manufacturer.
Transfer LossesOne engineer asked what specific heat transfer areas are associated with four-way reversing valves. The answer is mass flow loss that results in heat loss.
It was reported at the meeting by an engineer that an agency is testing 3-ton heat pumps under ARI frosting test conditions. Thermocouples put around the reversing valve have picked up 4 degrees to 5 degrees F superheat just due to the reversing valve, he said. “I measured this effect and it’s real.”
“You can correlate the suction line temperature rise to leakage.”
So, what is the system penalty? “How much bigger compressor do I have to buy? The standard just can’t focus on cooling-mode operation,” an engineer stated.
“If you choose based solely on pressure drop, you will choose a larger valve, which in turn will leak more.”
Publication date: 03/24/2003