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77 Cards in this Set

  • Front
  • Back

The function of a compressor is to

draw the refrigerant from the evaporator where it has absorbed heat by changing from a liquid to a vapour.

the compressor must then raise the pressure of the refrigerant vapour sufficiently that the condensing temperature of the vapour will be higher than that of the cooling medium.

What happens to the refrigerant in the evaporator

it changes from a liquid to a vapour as it absorbs heat.

How high must the compressor raise the pressure of the refrigerant vapour?

enough so that the temperature of the vapour is higher than that of the cooling medium.

Compressors can be divided into 3 main classifactions:

- recriprocating

- rotary

- centrifugal

Reciprocating compressors are

in which a piston travels back and forth inside of a cylinder, thus drawing in and compressing the vapour.

Rotary Compressors are

those that use helical rotors (screws) or an eccentric rotor with vanes to compress the vapour

Centrifugal compressors are

those that use rapidly revolving impellers to draw in the vapour and discharge it at high velocity (using centrifugal force)

High velocity/low pressure vapour is converted into low velocity/high pressure vapour before it leaves the compressor.

In a centrifugal compressor ______ velocity/_____ pressure vapour is converted into _____ velocity/_____ pressure vapour before it leaves the compressor.

High/Low, Low/High

In a reciprocating compressor the piston is forced up and down in the cylinder by means of a _______________ attached to a _______________ driven by an ____________.

connecting rod, rotating crankshaft, electric motor

_______ and _______ valves are mounted on the cylinder head of a reciprocating compressor.

The valves operate automatically

suction, discharge

The operating cycle of a reciprocating compressor consists of two strokes:

Suction and Discharge.

the cycle is repeated during each revolution of the compressor

3 types of reciprocating compressors

- open

- hermetic

- serviceable (semi-hermetic)

a reciprocating compressor is called open because

one end of the crankshaft extends outside the crankcase.

a sealing arrangement is required to prevent refrigerant from escaping along the shaft.

Open type reciprocating compressor is suitable for what types of drives:

- electric motor

- turbine

- internal combustion engine

the driver can be directly coupled to the crankshaft or it can drive the compressor by means of pulley's and V-belts.

A hermetic reciprocating compressor is driven by

an electric motor directly connected to the crankshaft.

the need for shaft sealing is eleminated on a hermetic type reciprocating compressor because

the compressor and motor are mounted within the same housing.

servicing of a hermetic type reciprocating compressor is not possible because

the housing is sealed by welding.

on a hermetic type reciprocating compressor the low temperature vapour drawn from the evaporator is often passed around the motor windings before entering the suction of the compressor. why?

to provide cooling for the motor windings.

what designs use a hermetic type reciprocating compressor?

low-capacity compressors used in applicances such as self-contained refrigerators, freezers, and AC units

serviceable or semi-hermetic type reciprocating compressors are

similar to the hermetic type however the cylinder heads, end plates, and bottom plate can be removed for servicing of the internal mechanism.

reciprocating compressors can also be identified by the number of cylinders. such as

single stage is usually adequate

sometimes two stage is used

* quite often the single stage compressors have two or more cylinders acting in parallel

Methods of classifying reciprocating compressors

- stages

- by arrangement (i.e. vertical, horizontal, 45° inclined, V-type, W-type, and radial)

- single acting (usually vertical) or double acting (usually horizontal)

modern reciprocating compressors are build for __________ operation and are therefore quite _________.

high speed operation, compact

Main parts of a reciprocating compressor:

- body

- crankshaft

- pistons and connecting rods

- suction and discharge valves

- safety head

- compressor bearings

The compressor body of a reciprocating compressor may be either one piece or two piece. describe each.

one piece - has crank case and cylinders in one block

two piece - has these parts cast separately then bolted together with a gasket in between.

cylinders in the compressor body of a reciprocating compressor are made with ________________ for air cooling. If water is used for cooling the cylinder must be made with a __________.

integral fins, water jacket

* cylinders of larger machines may be provided with replaceable cylinder liners

the main connecting rod bearing surfaces of crankshafts are __________, usually ________, and ________ .

perfectly round, case hardened, highly polished.

when forced lubrication is used on a reciprocating compressor crankshaft:

the shafts have drilled oil passages

small capacity compressors are often equipped with a __________ type of crankshaft, while larger compressors are equipped with the ___________ type crankshaft.

eccentric, conventional

In a rotary compressor the compression of the low-pressure vapour drawn from the evaporator is produced by

rotary motion.

Stationary single blade rotary compressor is widely used for _________________ and features ____________.

low capacity refrigeration applications, a rotor or roller within a cylinder.

the rotor or roller of a stationary single blade rotary compressor is eccentric to the drive shaft so that

as the roller rotates, one point on its circumference is always in contact with the cylinder.

*the vapour to be compressed is pushed around ahead of the roller.

how does the the single blade move in a single blade stationary rotary compressor.

- spring loaded into a slot in the cylinder wall.

- as the rotor turns the blade moves back and forth in the slot (maintaining firm contact with the rotor at all times)

what is the purpose of the blade in a stationary single blade rotary compressor?

to provide separation between the suction outlet and the discharge outlet of the cylinder.

the discharge port of a stationary single blade rotary compressor is equipped with a _______ to prevent __________ when the compressor is not running.

check valve or reed valve prevents back flow of the high-pressure vapour via the compressor into the low-pressure side

Rotating sliding vane rotary compressor is used

in low and medium capacity compressors

Rotating sliding vane rotary compressor consists of

a cylindrical rotor having radial slots into which sliding vanes fit.

Rotating sliding vane rotary compressor is mounted __________ in the cylinder so that _________.

eccentrically so that it nearly touches the wall between the suction and discharge ports.

*heads and endplates close the cylinder off and hold the rotor shaft.

the path of the vapour in a Rotating sliding vane rotary compressor is

- refrigerant vapour is drawn into the compressor through suction slots in the cylinder wall in a section of the cylinder where the volume of space between the vanes, rotor, and wall is increasing (large).

- the vapour is trapped between the vanes and pushed towards the section with decreasing volume (less space for same amount of vapour) as a result the vapour is compressed

- this compressed vapour is forced through the discharge ports in the cylinder wall into the discharge line

- vapour is prevented from returning into the compressor due to a check or reed valve

Helical rotor (screw) rotary compressor is used in

medium and high capacity refrigeration

Helical rotor (screw) rotary compressor consists of

two specially shaped helically grooved rotors or screw that fit together in twin cylinders inside a double casing.

capacity is controlled on a Helical rotor (screw) rotary compressor by

a hydraulically operated slide valve in the casing along with the inlet and outlet channels

in a Helical rotor (screw) rotary compressor the male _______ rotor usually has four _______ along the length of the rotor which mesh with the corresponding usually six ___________ on the female ________ rotor.

driven - helical lobes

helical grooves - driven

rotors in a Helical rotor (screw) rotary compressor revolve at very high speed, up to _______ rpm

3500 rpm

Due to the high speed of operation the flow of vapour through a Helical rotor (screw) rotary compressor is practically continuous. The screw compressor a

positive displacement unit.

when used a refrigeration system the Helical rotor (screw) rotary compressor is usually driven

directly by an electric motor

*the motor and compressor are often combined in a single casing to form a hermetic unit.

the ___________ in rotary compressors is critical.

direction of rotation

screw compressors are always always equipped with a ______________ which ensures proper rotation.

phase sequence control

*operation in the reverse direction will cause immediate damage to a screw type compressor design.

in both reciprocating and rotary compressors the vapour is compressed by

the direct action of pistons, vanes, rollers, or gears which push the vapour into a decreasing space thus compressing it.

*positive displacement

in a centrifugal compressor the vapour

- is given high velocity by means of a rapidly rotating impeller.

- this high velocity vapour travels through passages of increasing cross-sectional area (bigger opening) where this velocity is converted into pressure

the energy transformation in a centrifugal compressor: __________ of the low pressure/high velocity vapour is transformed into _________ in the high pressure/low velocity vapour.

kinetic energy to potential energy

the advantages of the centrifugal compressor over other types is

- its simplicity of operation

- maintenance is minimal

the only moving parts in a centrifugal compressor are

the impeller and the shaft

there are no _____, _______, or ______ in a centrifugal compressor

valves, pistons, cylinders

the pumping efficiency of a centrifugal compressor _____________ with speed. (increases or decreses)


*since it operates at high speed it can be directly coupled to an electric motor or other type of high speed driver.

centrifugal compressor are designed as either open type or hermetic type.

define open type and hermetic type.

Open type - shaft extends through (outside of) the housing

hermetic type - compressor and motor are sealed inside one housing

the main disadvantage of a centrifugal compressor with a single impeller (single stage) is

it is usually unable to boost the pressure of the vapour dawn from the evaporator up the required condenser pressure. the vapour condenses back into a liquid.

*generally the compressor must be equipped with two or more impellers

an application in which a single stage centrifugal compressor is suitable is

where the pressure difference between the evaporator and condenser is quite small

how is the pressure increased in a multi-stage (more than one impeller) centrifugal compressor

pressure is boosted in stages.

after pressure is raised a certain amount in one impeller it is discharged into the inlet of the next one which will raise the pressure an equal amount.

*there can several stages depending on the number of impellers

shaft seals used on what type of compressors

- used on open type compressors

*when the compressor and motor are connected together outside of the compressor crankcase

shaft seals prevent

- refrigerant and oil leakage along the shaft whether the compressor is operating or at rest.

- moisture and air from passing into the compressor

seals can be classed as ________ or ________.

stationary or rotating

packing gland seals are used mainly on

large capacity ammonia compressors operating at low speeds

packing gland seals are made of

semi-metallic material impregnated with graphite.

how is a packing gland seal lubricated

- packing is mounted on the shaft that extends through the compressor crank case

- a hollow perforated ring called a lantern ring is placed between two sets of packing

- lubricating oil is pumped into the lantern ring to provide pressurized oil lubrication for the shaft and packing

a lantern ring is

a hollow, perforated ring designed to allow for pressurized oil lubrication of packing and shaft.

a bellows seal is

- metallic bellows soldered at one end to a seal nose (ring) around the shaft.

- the other end is fastened to a flange that is mounted to a cover plate. a seal gasket prevents leakage

- the shaft rotates freely

diaphragm seal

- a rotating type seal

- a diaphragm is soldered to a seal nose between the compressor body and cover plate.

- the seal collar rotates with the shaft rubbing against the seal nose

- diaphragm maintains constant pressure between the seal nose and the rotating collar.

- lubricating oil reduces friction between the seal nose and collar.

Advantages of rotating mechanical seals over packing glands

- require less maintenance

- reduced wear on rotating shafts

- leakage is reduced to a minimum

all types of mechanical seals consist of two flat sealing surfaces at right anges to the compressor shaft. they are:

the sealing ring - is held in position by a spring

the mating ring - which contacts the sealing ring

in a rotating mechanical seal what how do the sealing and mating rings move?

- the sealing ring rotates with the shaft (the ring and spring are held in place by a shell fastened to the compressor shaft with a set screw)

- the mating ring is held stationary within the compressor casing

*the sealing ring is forced against the mating ring by it's spring, thus preventing leakage between the two faces

O-ring gaskets prevent leakage where on a rotating mechanical seal?

- between the casing and the mating ring

- between the shaft and the sealing ring

read compressor capacity on pg 116-15

its too confusing to make a card out of right now

piston displacement is

the volume of refrigerant that can be circulated through all the cylinders of a reciprocating compresser per unit of time.

*most manufacturers rate it in terms of cubic meters per minute or hour

(reciprocating compressor)

volume displaced per minute or hour depends on:

- cylinder bore

- length of stroke

- number of cylinders

- rpm

- volumetric efficiency

the size of compressor required for a particular application depends on

the volume of refrigerant to be compressed

piston displacement can be calculated by the formula:

V= A x L x N x R


V= Volume (cubic meter per minute)

A= cross sectional Area (meters squared)

L= Length of stroke (meters)

N= Number of cylinders

R= rpm