United Trades Exclusive
compressors: an overview
By Dale Yelnich
In modern HVAC technology, the compressor is part of the air conditioning cooling system. It compresses the refrigerant coolant into a super heated vapor before it passes into the condenser and turns back into a liquid. This thermal heat transfer is what makes an air conditioner work the way it does, and without a compressor, the cooling process would not occur. But the basis of compression, and it’s modern use in HVAC systems did not begin with a liquid, it began by compressing air.
The first known mechanical compressor was a bellows. By expanding a flexible bladder, air would be drawn in through a nozzle. When the bladder was then compressed, the air would shoot out of the nozzle at a high rate of speed. This original air compression technology was used when forging metals. By blowing compressed air into a fire, the temperature would increase and higher melting points could be achieved, which became particularly useful when creating metal alloys for stronger weapons.
In 1758, Benjamin Franklin experimented with cooling alcohol as a means to lower the temperature of an object. The bulb on a mercury thermometer was dipped into grain alcohol, and compressed air was blown over the alcohol from a bellows. The ambient temperature was 64 degrees, but the bulb temperature quickly plummeted to below freezing, as a thin film of ice appeared on the thermometer bulb. Franklin realized the implications of what had just occurred, and stated, “”From this experiment one may see the possibility of freezing a man to death on a warm summer’s day.” The basis of mechanical cooling had just been discovered.
The steam engine was the next breakthrough in compression technology. Super heated water expands when turned to steam, and the resulting pressurized water vapor is channeled into a cylinder. When the steam enters the cylinder, it is is initially compressed by the piston, but the water vapor expands rapidly and pushes against the piston face, causing it to move backwards.
The piston is connected by gearing which turns a wheel, and as the piston moves back and forth, the wheel begins to rotate. A valve at the back of the cylinder is opened when the piston reaches its maximum reverse travel, and the steam exits the valve. The piston once again moves forward from kinetic energy, the compressed steam expands as it enters the cylinder, propelling the piston backwards, and the entire process begins again. This is the mechanical process of the first liquid compressors, only done in reverse, with the exhaust valve allowing a low pressure gaseous refrigerant to flow in. As the piston compresses the gas, it squeezes the molecules together through friction and creates heat. The compressed gas is bled off into a condenser where it releases the built up heat. This is the exact function of a reciprocating compressor.
Ammonia was the first refrigerant used, where the low pressure evaporated liquid was compressed into a gas, then fed into a condenser and allowed to turn back into a liquid. This process was used to make ice in the mid-19th century, and from there the modern air conditioning compressor was formed. After that, the basic idea of compression remained the same, with the modern refrigerant Freon being invented in 1928.
Compressor technology began to evolve, with the first modern types still being based on a piston inside of a cylinder. These reciprocating compressors, these were the hallmarks of the HVAC industry for decades, but are quickly losing favor to more efficient and reliable ways to compress refrigerants.
One of the most common types of compressors used in rooftop units, heat pumps and even small water chillers, is a scroll compressor. This design uses two interleaved spirals, or scrolls, where one remains stationary and one revolves inside of the other. This action compresses the refrigerant gas and discharges it out of the center valve where it moves to the condenser. The efficiency is comparable to a reciprocating compressor, but since a scroll compressor has about 70% fewer moving parts, it is inherently more reliable overall.
Helical-Rotary compressors are a variant of the scroll compressor. The spirals are elongated to look more like interleaved screws, and as these screws turn together, they feed the compressed gas through a valve and into a condenser. These compressors need to be manufactured with small tolerances in both the helical rotors and the rotary chamber, and as such are initially a bit more costly. But they provide up to 35% increased efficiency and they are very reliable. Rotary compressors are most commonly used in water chillers from 70 to 450 tons.
The last of the major compressor types is called a centrifugal compressor. Unlike the other three types, which use positive displacement compression to shrink the volume of the refrigerant, a centrifugal compressor uses a rapidly spinning disk to compress the refrigerant vapor.
Although very reliable, centrifugal compressors become more efficient when compressing larger amounts of refrigerant. They are generally only used in commercial or industrial applications for prefabricated water chillers up to 3000 tons, or field assembled units up to 8500 tons.
Compressors have come a long way since the first bellows was used to compress air, but even in the most modern compressors available, the basic principles of compression, from air to HVAC refrigerant, are still being employed.