The Compressor Guide

The compressor guide

 

There are many different types of compressors in the refrigeration industry, each with its strengths and weaknesses.  Some are best suited to small system, while others are only ever seen in very large industrial system. 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.

 

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 refrigerant with the only restrictions being high pressure refrigerants such as Carbon dioxide ( pressure too high for the pressure vessel to operate safely ) and corrosive gases such as ammonia. (ammonia eats yellow metals such as copper and bronze , which means the internally housed copper motor windings would fail very quickly)

This type of compressor has reasonably high energy efficiency across most operating ranges and most synthetic gasses, but can suffer when used at very low temperatures with air cooled condensers as the high compression ratio makes the compressor inefficient and the thin suction vapor 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, as 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 labor to repair it, and the sealed pressure vessel requires very specialized skills if it is to be resealed after inspection.

Semi hermetic piston compressor are similar in operating principle to the smaller hermetic brothers but are housed in a cast iron block which is sealed up with cast iron covers which bolt on, and are therefore removal , making 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, which 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 and 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 offer a range of compressor 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 vapor 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 pressurized oil gallery network, or splash feed 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 ten 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 half’s.  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 if forced into a smaller and smaller space until it is discharge out of the center of the scrolls.

 

This type of compressor doesn’t need valves so it is not effected by changing compression ratios in the way a piston compressor is , but is does have losses that increase as the compression ration rises. In the case of the scroll compressor, 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 work 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 dive 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 the power from the motor to the 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 as the small volume internal motor has limitations on multiple starts in quick succession.

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 sixteen cylinders , with the larger machines  having elaborate hydraulic capacity regulation system that use small oil driven pistons to close suction valve assembles on individual heads.

 

The open drive compressor was the father of the all refrigeration compressor 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 revering 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 300 rpm full speed and driven by flat leather belts from steam engine via a 6 foot diameter  compressor pulley.

 

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 design packs a large displacement into a small area which is important when mounting a compressor inside an engine bay.

It is important to remember that a car air conditioning system is only about 20% as efficient as a stationary system, so a large capacity is necessary to deliver cooling in the difficult environment of a mobile system.

These compressors generally have a magnetic clutch driven directly from the car or trucks engine, so the compressor speed must be able operate at whatever revs the engine requires.

 

The screw compressor is always large and sometimes extremely large. Normally used in large industrial system 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 20 horse power to 3000 horsepower and larger. Screw compressors compressor gas by direct volume reduction and generally operate at 3000 rpm 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 compressors 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 vapor enters the rotor and therefore the volume that is compressed.

Typically capacity control is between 10 and 100%, but for the best efficiency the fully loaded position is used. As a rule to 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 compressor can have economized 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 vapor from the main suction line. The economizer suction port takes up to 10% of the main suctions volume but at a higher suction pressure. This higher suction pressure of the economizer port allows the system to draw gas off the top of a refrigerant receiver, with produces a cooling effect in the refrigerant and reduces the amount of flash gas that is experienced at the point of expansion. This economizer can increase the compressors performance by 20% , while only increasing the motor load by 10%. ( a 10% Gain on efficiency).

 

Even with the economizer 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 a equal capacity piston compressor. This is most important in a large plant where capacities can exceed 10 megawatts.

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 compressor very thin gas, often in a vacuum to a slight positive pressure. This design used long flat fiber paddles that rotate in a central shaft. The shaft is mounded off center 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 vane move in and out trapping the gas in the space between the vanes 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.

 

 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 difference manufactures.    

They have recently had something of a revival in smaller capacity chillers, and a number of manufactures are now producing compressors with magnetic bearings which allow the compressor to operate at very high RPM .  Centrifugal compressors can have multiple stages, which extends the application range, but is 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.

There are other compressor designs available but they tend to be very specialized 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 manufactures 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.

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