Ian Wilson of Strathbrook Engineering provides a quick insight into the different types of compressors and their place within the cooling spectrum.
There are many different types of compressors in the refrigeration industry, each with its strengths and weaknesses. Some are best suited to small systems, while others are only ever seen in very large industrial systems. Some work well with all refrigerant types and others have major restrictions placed on them.
In this article I will attempt to describe which technologies work best in particular applications and which one you should stay clear of.
Hermetical variations
The most common compressor type is the hermetically sealed piston compressor.
This is a small piston compressor sealed inside a light gauge pressed metal pressure vessel. Typically seen in domestic fridges and similar small cooling applications,
they are the cheapest compressor to manufacture and have very few moving parts. They are suitable for most refrigerants with the only restrictions being high pressure refrigerants such as carbon dioxide because the pressure is too high for the pressure vessel to operate safely; and corrosive gases such as ammonia which eats yellow metals such as copper and bronze, leading to the internally housed copper motor windings failing very quickly.
This type of compressor has reasonably high energy efficiency across most operating ranges and most synthetic gases. It can suffer, however, when used at very low temperatures with air cooled condensers as the high compression ratio makes the compressor inefficient and the thin suction vapour may not be able to provide the compressor cooling that is required to keep the compressor running.
They can be single piston or multiple pistons and typically operate very reliably with cheap low tolerance parts.
As the compressors tend to be mounted on springs inside the pressure vessel, never
sit the compressor on its side, if it comes off its spring mounts it will fail due to internal pipe fracture.
These compressors generally do not have capacity control as an option and have a splash feed oil system.
If a compressor fails for any reason they are generally not worth repairing as the replacement part is less expensive than the cost of labour to repair it.
A sealed pressure vessel requires very specialised skills if it is to be resealed after inspection.
Semi hermetic piston compressors are similar in operating principle to their smaller hermetic brothers. Being housed in a cast iron block which is sealed up with cast iron bolt-on covers, however, makes the compressor repairable.
Generally the layout of the semi hermetic compressor is a horizontal crankshaft with a direct driven motor. The crankshafts can be similar to an internal combustion engine with cast crank throws carrying bolt-together piston rod assemblies.
A less expensive option is a straight crankshaft with a series of eccentric rings mounted on the crank to provide the up reciprocating motor of the one-piece piston rods which are slipped onto the crankshaft during assembly. This reduces the cost of the connecting rods and the amount of time it takes to assemble.
The valve plates bolt to the main casting, which allows for more accurate assembly of the moving parts and finer tolerances between the piston and the valve plate during operation. These smaller clearances increase efficiency, improve energy consumption and reduce compressor heat.
This style of compressor can range from the very small to extremely large and can operate with all gases (except ammonia) across a vast speed range. Bitzer currently offers a range of compressors with bolt-on suction gas cooled variable speed drives that can operate between 30 and 87Hz.
The internal electric motor is normally a four-pole three-phase unit, which can be wound as a single motor or split across a pair of winding packs which are started separately (1/2 of a second or faster apart) to reduce motor starting current. These motors are cooled by the refrigerant vapour and can have greatly reduced physical size as a result.
The oil systems can be pump driven from the main crankshaft via an oil pick-up tube and a pressurised oil gallery network; or splash fed via a rotating disc that picks up oil and delivers it to an oil reservoir, which then feeds the moving parts.
The big mover in terms of compressor sales over the last 10 years is the hermetically sealed scroll compressor. The scroll refers to a pair of matched circular scrolls that have a rib cast into a flat disc that gets smaller and smaller as the rib moves around the center point of the male and female halves. One of the scrolls is fixed to the compressor casting and the second rotates around a small eccentric shaft.
As the moving rotor turns it traps gas in the space between the ribs of the two rotors, and the gas trapped in the pocket formed between the ribs is forced into a smaller and smaller space until it is discharge out of the centre.
This type of compressor doesn’t need valves so it is not affected by changing compression ratios in the way a piston compressor is, but it does have losses that increase as the compression ratio rises and the gas leaks back between the two ribs. Scroll compressors have the same limitations as piston compressors when gas selection is considered, mostly related to the materials used, but scroll compressor losses are higher at low temperatures and overheating can be a problem.
On the other hand, they perform very well with the low compression ratios that high suction pressures require and have low vibration levels at high speed. This makes them popular in the air conditioning package market. An added bonus for packaged AC units is that the scroll compressor is tall and narrow, which helps when designers are trying to cram large capacities into small cabinets.
Capacity control generally works by driving the two rotors apart, which allows some of the gas to remain in the rotors while they rotate. Speed control is also popular with many inverter driven units available.
One major limitation on scroll compressor design is that the mass of the rotating scroll will make the compressor vibrate once the size of the compressor exceeds a given limit. This is why you don’t see 50 horse power scroll compressors competing with other compressor designs.
The oil systems tend to use a splash design feed from oil in the sump of the compressor.
Open drive
Open drive piston compressors are similar to the semi hermetic models but with the internal motor replaced with a shaft seal and open drive air cooled motor with a flexible coupling transferring power from motor to crankshaft.
This arrangement is generally used with larger compressors and is a must for ammonia.
The open drive motor allows the compressor to be started much more often.
Oil systems are normally pump-driven and the oil system generally includes an oil cooler and often water-cooled head and sump covers as well.
Compressors can range from twin cylinder up to 16 cylinders, with the larger machines having elaborate hydraulic capacity regulation systems that use small oil-driven pistons to close suction valve assemblies on individual heads.
The open drive compressor was the father of the all-refrigeration compressors with the first open drive compressors dating back to the mid 1800s, working with a horizontal configuration and double-acting heads which compressed gas in both directions to improve efficiency.
Early Australian manufactured compressors such as the Lightfoot were fitted with reversing heads which allowed an experienced operator to reverse the direction of flow through the compressor so the condenser could be pumped into the evaporator for service. These slow speed monsters had two 12 inch diameter pistons operating at up to 300rpm full speed and driven by flat leather belts from a steam engine via a six foot diameter compressor pulley.
Mobile
Transport refrigeration has required some unique compressor designs such as the swash plate compressor.
This design employs multiple piston/rod assemblies fixed to a rotating plate which is mounted so that the pistons move back and forward as the plate rotates. This packs a large displacement into a small area, important when mounting a compressor inside an engine bay.
It is important to remember that a vehicle air conditioning system is only about 20 per cent as efficient as a stationary system, so a large capacity is necessary to deliver cooling in the difficult environment of a mobile space.
These compressors generally have a magnetic clutch driven directly from the car or truck’s engine, so the compressor speed must be able to operate at whatever the engine revs.
Screw
The screw compressor is always big, and sometimes extremely big.
Normally used in large industrial systems, the screw compressor packs the largest possible displacement into the smallest possible casting.
They are generally direct driven with an open drive configuration and range in size from 15kW to well over 2000kW. Screw compressors compress gas by direct volume reduction and generally operate at 3000rpm or higher.
Gas entering the compressor casting is drawn into the gaps between the lobs on the two rotors. As the two rotors mesh together, the space that the gas is trapped in is sealed
off at the suction end and the volume is then progressively reduced until the gas is forced out the discharge port.
These compressors have very few moving parts and only move in one direction, so they have a continual compression process. They are particularly well suited to very large loads with multiple compressors.
The oil systems are either pump driven (oil pumped from the oil separator directly into the compressor’s bearings and rotors) or pressure driven, where the discharge pressure forces the oil to flow into the compressor through oil galleries that are vented to suction pressure.
The oil both lubricates and cools the compressor and oil pressure is also used to operate the capacity control valve which moves along the length of the rotors, moving the point at which the suction vapour enters the rotor and therefore the volume that is compressed.
Typically, capacity control is between 10 and 100 per cent but for the best efficiency the fully loaded position is used. As a rule the more the compressor is unloaded the less efficient it is.
Smaller commercial screw compressors use fixed unloaded stages that rely on pistons venting partly compressed gas to suction. While this arrangement is cheap to produce, the fixed capacity stages are far less flexible than the slide valve arrangement, which has infinite capacity steps.
Screw compressors can have economised operation which means the compressor will have two suction ports, each at a different suction pressure. The main suction is at the lowest pressure and will take vapour from the main suction line. The economiser suction port takes up to 10 per cent of the main suction’s volume but at a higher suction pressure. This higher suction pressure of the economiser port allows the system to draw gas off the top of a refrigerant receiver, which produces a cooling effect in the refrigerant and reduces the amount of flash gas that is experienced at the point of expansion. This economiser can increase the compressor’s performance by 20 per cent, while only increasing the motor load by 10 per cent, resulting in a 10 per cent net gain in efficiency.
Even with the economiser port in operation, the performance of a screw compressor at full load is equal to a piston compressor, so an unloaded screw will be less efficient than a piston compressor.
The advantage of a screw compressor is the very small size of the compressor compared to an equal capacity piston compressor. This is most important in a large plant where capacities can exceed 10mW.
Centrifugal
The centrifugal compressor has been very popular over many years and is particular well suited to large loads at low compression ratios such as large building AC water chiller systems. They are very efficient when used in their best application range and are made by many different manufacturers.
They have recently had something of a revival in smaller capacity chillers and a number of manufacturers are now producing compressors with magnetic bearings which allow the compressor to operate at very high revolutions per minute.
Centrifugal compressors can have multiple stages which extends the application range, but tends to reduce the overall system efficiency once the compression ratio is stretched.
The operating principal is basically that of a high static pressure fan and they compress by throwing the gas down the discharge line using speed rather than volume reduction. But with the correct refrigerant they can operate very reliably for many years, due to the low stress and very few moving parts.
Historical
The rotary vane compressor has nearly passed into history, but they are still in operation in small numbers in older industrial plants.
They are traditionally used as the low pressure compressor in a multiple stage ammonia system where very large displacements are required to compress very thin gas, often in a vacuum to a slight positive pressure. This design used long, flat, fibre paddles that rotate in a central shaft. The shaft is mounted off-centre so that it spins inside a cylindrical space. The flat paddles or ‘vanes’ are fixed in slots so that a set of springs force the vanes against the compressor casting. As the compressor shaft turns, the vanes move in and out, trapping the gas in the space between where it is compressed until it is forced out the discharge port. These compressors are fragile at best and are susceptible to vane failure in the event of excessive compression ratios.
This fragile nature has seen the rotary vane compressor fade into the past, to be replaced by high speed screw compressors.
There are other compressor designs available but they tend to be very specialised and very few of us in the trade will get the opportunity to work on them.
All the designs I have covered are made by multiple manufacturers in many shapes and sizes. In the end all a compressor has to do is suck gas in one end and blow it out the other, but the details behind the design and the gas that is being circulated make the difference between how long they will last and how much power they will use to get the job done.