Unit 23 Compressors Slides (CRF)-Mr.Graham

Section 5: Commercial Refrigeration

Unit 23: Compressors

Objectives (1)

• After studying this unit, you should be able to:

– Explain the function of the compressor in a

refrigeration system

– Discuss compression ratio
– Describe four different methods of compression

Objectives (2)

– State specific conditions under which a compressor

is expected to operate

– Explain the difference between a hermetic

compressor and a semi-hermetic compressor

– Describe the various working parts of reciprocating

and rotary compressors

The Function of the Compressor

• Considered the heart of the refrigeration systems

– Compressors are vapor pumps
– Increases suction pressure level to the discharge

pressure level

– Suction gas from the evaporator enters the

compressor

– Refrigerant is discharged to the condenser

The Function of the Compressor:
Compression Ratio

• Compression ratio compares pumping conditions

– Defined as the high side absolute pressure (psia)

divided by the low side pressure (psia)

– High compression ratio can lead to overheated

compressor oil and reduced refrigerant flow through
the system

– Reduced refrigerant flow reduces system capacity

The Function of the Compressor:
Two-Stage Compression

• Two-stage compression lowers the ratio

– Utilizes two compressors

• One discharges into suction of the other

– Also referred to as compound compression
– Often used when the compression ratio of a single

compressor system exceeds 10:1

– Often used in low-temperature commercial and

industrial storage applications

The Function of the Compressor:
Two-Stage Compression Illustrated

Figure 23–3 Two-stage compression. Notice that the second stage of the
compressor is smaller than the first stage

Types of Compressors

• Reciprocating
• Screw compressors
• Rotary compressors
• Scroll compressors
• Centrifugal compressors

Reciprocating Compressors:
Fully Welded Hermetic

• Reciprocating compressors

– Categorized by housing/drive mechanisms

• Open and hermetic

• Fully welded hermetic compressors

– Motor and compressor welded in a shell
– Cannot be field serviced; typically a “throw-away”

compressor

– Cooled by suction gas from the evaporator
– Lubricated by the splash method

Reciprocating Compressors:
Serviceable Hermetic

• Serviceable hermetic compressors

– Bolted together; can be field serviced
– Has a horizontal crankshaft
– Smaller compressors are splash lubricated; larger

compressors use pressure lubrication

– Often air cooled
– Piston heads are located at the top of the

compressor

Reciprocating Compressors:
Open-Drive, Belt-Driven, and Direct-Drive

• Open-drive compressors

– Can be direct drive or belt-driven compressors
– Must have a shaft seal to prevent leakage
– Bolted together; can be field serviced

• Belt-driven compressors

– Have the compressor and motor shafts parallel to

each other

• Direct-drive compressors

– Compressor and motor shafts placed end to end

The Rotary Screw Compressor

• The rotary screw compressor

– Open motor design; used in larger installations
– Uses two matching, tapered, machined, and screw-

type gears that squeeze the refrigerant vapor from
the inlet to the outlet

Reciprocating Compressor Components
(1)

• The crankshaft

– Transfers motor motion to the piston
– Creates back and forth motion of the piston

• Connecting rods

– Connects the crankshaft to the pistons

• The piston

– Slide up and down in the cylinder; compress/expand

the refrigerant

Reciprocating Compressor Components
(2)

• Refrigerant cylinder valves

– Durable, flexible steel
– Two styles: the ring valve and the flapper (reed)

valve

– Serve both the suction and the discharge ports of the

compressor

• The valve plate

– Holds the suction and discharge flapper valves

Reciprocating Compressor Components
(3)

• The head of the compressor

– Holds the top of the cylinder and its components

together

– Contains both high/low pressure refrigerant

• Mufflers

– Designed to reduce compressor noise

• The compressor housing

– Encases compressor; sometimes motor

Reciprocating Compressor Components
(4)

• Compressor motors in a refrigerant atmosphere

– Motors in a refrigerant atmosphere
– Motor electrical terminals
– Internal motor protection devices
– The serviceable hermetic compressor
– Open-drive compressor

• The shaft seal

– Designed to keep the refrigerant and the atmosphere

separated

Belt-Drive Mechanism Characteristics

• Motor pulley called the drive pulley; compressor

pulley called the driven pulley
– Pulleys can be adjusted to change compressor

speed

• Drive size x Drive rpm = Driven size x Driven rpm

– Shafts must be properly aligned
– Pulleys with multiple grooves must be used matched

sets of belts

Direct-Drive Compressor Characteristics

• Direct drive compressors turn at the same speed as

the motor used

• Motor shaft and compressor shaft must be in very

close alignment end to end

• Motor shaft and compressor shafts are joined with a

flexible coupling

Reciprocating Compressor Efficiency

• Determined by initial compressor design
• Four processes take place during the compression

process
– Expansion (re-expansion)
– Suction (Intake)
– Compression
– Discharge

• Clearance volume

Reciprocating Compressor Efficiency:
Piston Starts Down

Figure 23–42 An illustration
of what happens inside the
reciprocating compressor
while it is pumping. When
the piston starts down, a low
pressure is formed under the
suction reed valve. When
this pressure becomes less
than the suction pressure
and the valve spring tension,
the cylinder will begin to fill.
Gas will rush into the
cylinder through the suction
reed valve

Reciprocating Compressor Efficiency:
Bottom Dead-Center

Figure 23–43 When the piston gets
near the bottom of the stroke, the
cylinder is nearly as full as it is
going to get. There is a short time
lag as the crankshaft circles
through bottom dead-center, during
which a small amount of gas can
still flow into the cylinder

Reciprocating Compressor Efficiency:
Piston Starts Up

Figure 23–44 When the piston starts
back up and gets just off the bottom
of the cylinder, the suction valve will
have closed, and pressure will begin
to build in the cylinder. When the
piston gets close to the top of the
cylinder, the pressure will start to
approach the pressure in the
discharge line. When the pressure
inside the cylinder is greater than the
pressure on the top side of the
discharge reed valve, the valve will
open, and the discharge gas will
empty out into the high side of the
system

Reciprocating Compressor Efficiency: Top
Dead-Center

Figure 23–45 A
reciprocating compressor
cylinder cannot completely
empty because of the
clearance volume at the top
of the cylinder. The
manufacturers try to keep
this clearance volume to a
minimum but cannot
completely do away with it

Discus Valve Design

• Allows a closer tolerance inside the compressor

cylinder at top dead-center

• Gives the compressor more efficiency because of

less clearance volume

• Has a larger bore and allows more gas through within

a short period of time

New Technology in Compressors:
Discus Compressor Technology

• Discus compressor technology

– Offers onboard diagnostics
– Monitors discharge temperature
– Contactor protection
– Remote diagnosis
– Integrates system electronics
– Reduced number of brazed joints
– Consistent field installation

New Technology in Compressors:
The Scroll Compressor

• The Scroll Compressor

– Scroll Compressor Operation

• Two spiral shaped scrolls fit inside one another

– Scroll Compressor Advantages
– Two-Step Capacity Control
– Digital Capacity Control

• Electronic variable frequency drives (VFDs)

– Scroll Compressor Protection

New Technology in Compressors:
Centrifugal Compressors

• Centrifugal compressors

– Used for air conditioning in large structures
– Rotor shaft and impellers the only moving parts
– The shaft acts as the rotor for the permanent-magnet

synchronous motor

Liquid in the Compression Cylinder

• If liquid enters cylinder, damage will occur

– Liquids cannot be compressed
– Liquid slugging can cause immediate damage to the

compressor components

– Common causes of liquid slugging include an

overfeeding metering device, poor evaporator air
circulation, low heat load, defective evaporator fan
motor and a frosted evaporator coil

System Maintenance and Compressor
Efficiency

• High suction pressures and low discharge pressures

keep the compression ratio low
– Dirty evaporators cause suction pressure drop
– Low suction reduces compressor pumping capacity
– Dirty condensers increase head pressure
– Compression ratio is increased by dirty or blocked

condenser and evaporator coils

Summary (1)

• The discharge gas can be quite hot because the heat

contained in the cool suction gas is concentrated
when compressed in the compressor.

• Three types of compressors are usually used to

achieve compression in commercial refrigeration: the
reciprocating, rotary screw, and scroll compressor.

• Hermetic and open-drive are two types of

reciprocating compressors.

Summary (2)

• Two types of hermetic compressors are welded

hermetic and serviceable hermetic or semihermetic.

• The shell of the serviceable hermetic or semihermetic

compressor is bolted together and can be
disassembled in the field.

• Most reciprocating compressor motors are cooled by

suction gas. Some are air-cooled.

Summary (3)

• The motors of all hermetic compressors operate in a

refrigerant and oil atmosphere and special
precautions must be taken in their manufacture and
servicing.

• Special internal overload devices are used on

hermetic motors that operate inside a refrigerant
atmosphere.

Summary (4)

• One type of internal overload protection device

interrupts the actual line current. If this overload
device does not close when it should, the compressor
is defective.

• Another internal overload device is a pilot-duty type

that interrupts the control voltage.

Summary (5)

• Some compressors use thermistors in or near the

motor windings to sense motor heat. The thermistors
are then wired to a solid-state electronic compressor
protection module, which will decide when the
compressor should be cycled off because of
excessive heat.

• Reciprocating compressors are positive displacement

pumps, meaning that when they have a cylinder full
of gas or liquid, the cylinder must be emptied or
damage will occur.

Summary (6)

• Open compressors have a motor on the outside of

the refrigeration or air-conditioning system.

• The motor can be mounted beside a compressor, the

compressor and motor shafts may be side by side, or
the motor may be mounted at the end of the
compressor shaft with a flexible coupling between
them.

• Shaft-to-motor alignment is very important.

Summary (7)

• Belts for belt-drive applications come in different

types.

• Discus valve design provides for smaller clearance

volume, a greater area through which the gas can
flow, and consequently greater efficiency.