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HVACR 317 – Refrigeration Core. System Components Compressors. Compressor Types. There are five types of compressors: Screw Centrifugal Reciprocating Scroll Rotary. Dunham Bush. Screw Compressors. Used on large chilled water and refrigeration systems.
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HVACR 317 – Refrigeration Core System Components Compressors
Compressor Types • There are five types of compressors: • Screw • Centrifugal • Reciprocating • Scroll • Rotary
Screw Compressors • Used on large chilled water and refrigeration systems. • Available in sizes of 20 tons and up. • Have very little vibration. • Use a pair of special helical rotors (screws).
Screw Compressors • Screw compressors will operate smoothly when capacity is reduced as low as 10%. • Capacity control on screw compressors is accomplished by re-circulating refrigerant vapor inside the compressor.
Screw Compressors • The refrigerant vapor is drawn into the spaces between the lobes of the screws. • As the void (open area) between the screws get smaller, the gas is compressed and piped to the condenser.
Screw Compressors • Screw compressors come in various types: • Hermetic • Semi-Hermetic • Open (external drive) • Again, very little vibration as it has a continuous pumping action.
Centrifugal Compressors • Used in large commercial refrigeration systems. • Uses impellers or wheels • Designed to operate at high speeds • Vapor is moved rapidly in a circular path using centrifugal force.
Centrifugal Compressors • Operation: • Compression is caused by spinning the mass of vapor refrigerant at high speeds. • This outward spinning refrigerant is then caught in a channel • It is then compressed by the flow of refrigerant behind it.
Centrifugal Compressors • In the centrifugal compressor, there are no valves. • Refrigerant enters through the suction inlet, then through suction passages into the first wheel. • The refrigerant enters the wheel through openings near the shaft.
Centrifugal Compressors • Refrigerant is then slung outwards. • It is then is forced through the discharge outlet, then onto the compressor. • Lubrication is needed only at the end bearings; therefore, centrifugal compressors are mostly oil free.
Centrifugal Compressors • Do not need any valves or pistons. • Operate at very high speeds.
Reciprocating Compressors • Used in domestic and commercial refrigeration units. • They are a piston type compressor. • Normal RPM’s: • Older units – 1725 rpm • Newer unit – 3450 rpm
Reciprocating Compressors • These compressors are hermetically sealed. • Motor and compressor are sealed in a steal dome. • External switching must be provided.
Reciprocating Compressors • Dome is on the low side of the system. • Suction line ends at the steal dome. • Fills the dome with low temperature vapor.
Reciprocating Compressors • Optional oil cooler • Not present on all models. • Is an additional loop of pipe going through the crankshaft. • Removes heat from the oil. • If not used on your unit, then cap the tubes off.
Reciprocating Compressor • Main parts: • Cylinder • Pistons • Connecting Rods • Crankshaft • Cylinder Head • Valves
Reciprocating Compressors • The crankshaft and connecting rods change the rotating motion of the motor into a reciprocating motion. • This reciprocating motion causes an up and down motion on the pistons.
Reciprocating Compressor • When the piston is at the bottom: • The suction valve is open. • The cylinder fills with low temperature, low pressure vapor.
Reciprocating Compressors • As the piston travels up: • The suction valve closes • The cylinder pressure is greater than the suction pressure.
Reciprocating Compressor • When the piston is at the top • The temperature and pressure have increased. • A predetermined pressure opens the discharge valve. • High temperature, high pressure vapor flows to the condenser.
Reciprocating Compressors • When piston is at the top, cont’d: • There is dead space between the piston and the valve space. • This dead space, or clearance volume, is one of the reasons that reciprocating compressors are not 100% efficient.
Reciprocating Compressors • Reciprocating compressors are categorized by housing and by drive mechanisms. • Housing categories: • Hermetic • Semi-Hermetic • Open
Reciprocating Compressors • Hermetic (fully welded) • Motor and compressor are contained inside a single shell that is welded closed. • Sometimes called a tin can. • You cannot service hermetic compressors without cutting the shell open. These compressors are disposable. • They are cooled with suction vapor.
Reciprocating Compressors • Semi-Hermetic • Motor and compressor are contained inside a single shell that is bolted together. • They can be serviced by a technician by removing the bolts and opening.
Reciprocating Compressors • Semi-hermetic, cont’d • Generally use a splash type lubricating system in smaller compressors, or a pressure lubricating system in larger compressors (oil pump). • Often air cooled as well as vapor cooled. • Fins in the casting. • Fan mounted on top of compressor.
Reciprocating Compressors • Semi-Hermetic, cont’d • Sometimes are water cooled. • By use of a water jacket around the compressor.
