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. Available in sizes of 20 tons and up. Have very little vibration. Use a pair of special helical rotors (screws).

Trane Chiller

Trane Compressor

Inside the Screw Compressor

Compression Diagram

Compression Diagram

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.

Screw Compressors

Centrifugal Chiller

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 Diagram

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.

Multi-Stage Compressor

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 Diagram

Reciprocating Compressors Diagram

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

Reciprocating Compressors

Reciprocating Compressors

Reciprocating Compressors

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.

Semi-Hermetic Compressors

Semi-Hermetic Compressors

Semi-Hermetic Compressors

Semi-Hermetic Compressors

Semi-Hermetic Compressors

Semi-Hermetic Compressors

Semi-Hermetic Compressors

Semi-Hermetic Compressors

Semi-Hermetic Compressors

Reciprocating Compressors Open Compressors The compressor and its motor are separate, connected by belts or special couplings. This compressor can be serviced and/or rebuilt in the field. Will most often develop leaks at the shaft seal.

Reciprocating Compressors Open Compressors (Shaft Seals) Shaft seals are used on Open Drive Compressors They are designed to prevent refrigerant leaks.

Reciprocating Compressors All seals use two rubbing surfaces One surface turns with the crankshaft and is sealed to the shaft with the O-Ring. Other surface is stationary and is mounted to the shaft housing.

Reciprocating Compressors The rubbing surface can be made from: Hardened steel and bronze Ceramic and carbon Teflon Graphite These surfaces must be lubricated.

Reciprocating Compressors There are four types of shaft seals: Packing Gland Stationary Unbalanced Diaphragm type Rotary

Reciprocating Compressors Drive Type (continued) Semi-Hermetic The compressor and motor are combined in one housing. This compressor can also be serviced in the field but service is limited.

Reciprocating Compressors Drive types (continued) Hermetic The compressor and motor are combined in one housing that is welded closed. This compressor is not serviceable at all.

Reciprocating Compressors Piston Quantity and Arrangement One to 16 pistons Arrangement can be: Vertical with 1-3 cylinders V Type with 2,4, or 6 cylinders W Type in multiples of 3 cylinders Radial with multiples of 5 cylinders X Type with multiples of 4 cylinders

Compressors – General Info The lifespan and efficiency of the compressor is based on the maintenance of the system. Dirty condenser coils, and improper charges can overheat compressors and raise pressures too high. This will affect both efficiency and life span.

Compressors, General Info Multiple compressors provide capacity control. In other words, when you have additional load you can use more than one compressor. If you have less load, you use fewer compressors.

Checking Valves Instructions for Checking Valves Install gauges on unit. Bleed air to service valves. Jump low pressure control. Start compressor. Turn low-side valve in two turns. Read low-side pressure

Checking Valves Instructions for Checking Valves, Cont’d Turn high-side valve in two turns. Read high-side pressure. Front seat low-side valve. Read low-side pressure. If 20” hg vacuum is reached in two minutes, low-side valves are good.

Checking Valves Instructions for Checking Valves, Cont’d. If 20” vacuum cannot be reached in two minutes, low-side valves are bad. Replace them. Next, stop compressor with low-side valves front seated. Read low-side pressure. If pressure remains lower than 20” vacuum, discharge valves are good.

Checking Valves Instructions for Checking Valves, Cont’d. If pressure remains lower than 20”, vacuum discharge valves are good. If pressure rises higher than 20” vacuum, then discharge valves are bad. Replace them. Never front seat discharge service valve while running. To finish up, back seat both valves and remove gauges.

Lubrication Systems There are three types of lube systems: Splash Type Forced Lubrication 3HP and Larger Systems

Lubrication Systems Splash Type: Use fingers on a shaft which splashes oil up onto the pistons and shaft. Used in older compressors.

Lubrication Systems Forced Lubrication: Small systems 3 HP and below use the crankshaft and piston movement to force oil to the bearings. Sometimes a combination of fingers on the shaft and the forced lubrication is used.

Lubrication Systems Larger Systems - 3HP and above A pump is mounted on the rear bearing housing. Oil is forced through a machined series of holes to the compressor bearings and connecting rods. Oil pump intake is connected to the compressor crankcase.

Lubrication Systems Oil pump intake pressure is always equal to the crankcase pressure (low side pressure). Oil pump outlet pressure is the sum of the crankcase pressure plus the pump pressure.

Lubrication Systems So, net oil pressure is equal to the suction pressure subtracted from the oil pump discharge pressure. Note: Oil pressure switch works off the net oil pressure. Net oil pressure will vary, normally between 30-40 psig.

Lubrication Systems A bypass valve can be added in cases of high oil pressure to keep the pressure under 60 psig. The bypass valve is not adjustable.

Refrigerant Migration Refrigerant Migration refers to the movement of liquid refrigerant to the compressor crankcase. The presence of this liquid refrigerant can affect the operation of the oil pump. Violent foaming on startup results in the loss of oil from the crankcase.

Refrigerant Migration The use of crankcase heaters and pump down controls can reduce and stop the refrigerant migration during the compressor “off cycle.”