Chapter 116 - Definitions - Refrigeration Compressors

Front 1

The function of a compressor is to

Back 1

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.

Front 2

What happens to the refrigerant in the evaporator

Back 2

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

Front 3

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

Back 3

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

Front 4

Compressors can be divided into 3 main classifactions:

Back 4

- recriprocating

- rotary

- centrifugal

Front 5

Reciprocating compressors are

Back 5

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

Front 6

Rotary Compressors are

Back 6

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

Front 7

Centrifugal compressors are

Back 7

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.

Front 8

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

Back 8

High/Low, Low/High

Front 9

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

Back 9

connecting rod, rotating crankshaft, electric motor

Front 10

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

The valves operate automatically

Back 10

suction, discharge

Front 11

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

Back 11

Suction and Discharge.

the cycle is repeated during each revolution of the compressor

Front 12

3 types of reciprocating compressors

Back 12

- open

- hermetic

- serviceable (semi-hermetic)

Front 13

a reciprocating compressor is called open because

Back 13

one end of the crankshaft extends outside the crankcase.

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

Front 14

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

Back 14

- 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.

Front 15

A hermetic reciprocating compressor is driven by

Back 15

an electric motor directly connected to the crankshaft.

Front 16

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

Back 16

the compressor and motor are mounted within the same housing.

Front 17

servicing of a hermetic type reciprocating compressor is not possible because

Back 17

the housing is sealed by welding.

Front 18

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?

Back 18

to provide cooling for the motor windings.

Front 19

what designs use a hermetic type reciprocating compressor?

Back 19

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

Front 20

serviceable or semi-hermetic type reciprocating compressors are

Back 20

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

Front 21

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

Back 21

single stage is usually adequate

sometimes two stage is used

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

Front 22

Methods of classifying reciprocating compressors

Back 22

- stages

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

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

Front 23

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

Back 23

high speed operation, compact

Front 24

Main parts of a reciprocating compressor:

Back 24

- body

- crankshaft

- pistons and connecting rods

- suction and discharge valves

- safety head

- compressor bearings

Front 25

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

Back 25

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.

Front 26

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 __________.

Back 26

integral fins, water jacket

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

Front 27

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

Back 27

perfectly round, case hardened, highly polished.

Front 28

when forced lubrication is used on a reciprocating compressor crankshaft:

Back 28

the shafts have drilled oil passages

Front 29

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

Back 29

eccentric, conventional

Front 30

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

Back 30

rotary motion.

Front 31

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

Back 31

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

Front 32

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

Back 32

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.

Front 33

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

Back 33

- 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)

Front 34

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

Back 34

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

Front 35

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

Back 35

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

Front 36

Rotating sliding vane rotary compressor is used

Back 36

in low and medium capacity compressors

Front 37

Rotating sliding vane rotary compressor consists of

Back 37

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

Front 38

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

Back 38

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.

Front 39

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

Back 39

- 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

Front 40

Helical rotor (screw) rotary compressor is used in

Back 40

medium and high capacity refrigeration

Front 41

Helical rotor (screw) rotary compressor consists of

Back 41

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

Front 42

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

Back 42

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

Front 43

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.

Back 43

driven - helical lobes

helical grooves - driven

Front 44

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

Back 44

3500 rpm

Front 45

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

Back 45

positive displacement unit.

Front 46

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

Back 46

directly by an electric motor

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

Front 47

the ___________ in rotary compressors is critical.

Back 47

direction of rotation

Front 48

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

Back 48

phase sequence control

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

Front 49

in both reciprocating and rotary compressors the vapour is compressed by

Back 49

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

*positive displacement

Front 50

in a centrifugal compressor the vapour

Back 50

- 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

Front 51

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

Back 51

kinetic energy to potential energy

Front 52

the advantages of the centrifugal compressor over other types is

Back 52

- its simplicity of operation

- maintenance is minimal

Front 53

the only moving parts in a centrifugal compressor are

Back 53

the impeller and the shaft

Front 54

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

Back 54

valves, pistons, cylinders

Front 55

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

Back 55

increases

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

Front 56

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

define open type and hermetic type.

Back 56

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

hermetic type - compressor and motor are sealed inside one housing

Front 57

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

Back 57

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

Front 58

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

Back 58

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

Front 59

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

Back 59

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

Front 60

shaft seals used on what type of compressors

Back 60

- used on open type compressors

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

Front 61

shaft seals prevent

Back 61

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

- moisture and air from passing into the compressor

Front 62

seals can be classed as ________ or ________.

Back 62

stationary or rotating

Front 63

packing gland seals are used mainly on

Back 63

large capacity ammonia compressors operating at low speeds

Front 64

packing gland seals are made of

Back 64

semi-metallic material impregnated with graphite.

Front 65

how is a packing gland seal lubricated

Back 65

- 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

Front 66

a lantern ring is

Back 66

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

Front 67

a bellows seal is

Back 67

- 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

Front 68

diaphragm seal

Back 68

- 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.

Front 69

Advantages of rotating mechanical seals over packing glands

Back 69

- require less maintenance

- reduced wear on rotating shafts

- leakage is reduced to a minimum

Front 70

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

Back 70

the sealing ring - is held in position by a spring

the mating ring - which contacts the sealing ring

Front 71

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

Back 71

- 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

Front 72

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

Back 72

- between the casing and the mating ring

- between the shaft and the sealing ring

Front 73

read compressor capacity on pg 116-15

Back 73

its too confusing to make a card out of right now

Front 74

piston displacement is

Back 74

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

Front 75

(reciprocating compressor)

volume displaced per minute or hour depends on:

Back 75

- cylinder bore

- length of stroke

- number of cylinders

- rpm

- volumetric efficiency

Front 76

the size of compressor required for a particular application depends on

Back 76

the volume of refrigerant to be compressed

Front 77

piston displacement can be calculated by the formula:

Back 77

V= A x L x N x R

Where:

V= Volume (cubic meter per minute)

A= cross sectional Area (meters squared)

L= Length of stroke (meters)

N= Number of cylinders

R= rpm