ON SYSTEM EFFICIENCY ON SYSTEM EFFICIENCY

In document Handbook - Refrigeration (Page 27-36)

ON SYSTEM EFFICIENCY 

Figure 16: Typical schematic of an air-to-air heat pump system. (source: CEMET) Figure 16: Typical schematic of an air-to-air heat pump system. (source: CEMET)

The

The following following major major components components are are required required in in is contained in is contained in a common a common gas-tight housing.gas-tight housing.

vvaappoouur r ccoommpprreessssiioon n rreeffrriiggeerraattiioon n ssyysstteemmss.. HHeerrmmeetitic c ccoommpprreessssoorrs s aarre e bbuuiillt t iinntto o a a wweellddeed d sshheellll,, and there is no access to the internal parts for  and there is no access to the internal parts for 

Refrigerant Refrigerant compressorscompressors.. servicing or repair. Semi-hermetic compressors areservicing or repair. Semi-hermetic compressors are

Evaporators.Evaporators. assembled with removable covers, usually sealed by assembled with removable covers, usually sealed by 

Throttling devices.Throttling devices. gaskets, enabling a limited amount of access gaskets, enabling a limited amount of access for on-for

on-

Condensers.Condensers. site maintenance.site maintenance.

Heat rejection equipment.Heat rejection equipment.

Both types of compressor are designed and built Both types of compressor are designed and built with specially selected motors. The motor's size with specially selected motors. The motor's size and type is matched to the motion work of the and type is matched to the motion work of the compressor for specific applications and compressor for specific applications and The purpose of the compressor in a refrigeration

The purpose of the compressor in a refrigeration refrigerants. To obtain the maximum efficiency therefrigerants. To obtain the maximum efficiency the system is to draw the low pressure refrigerant gas

system is to draw the low pressure refrigerant gas compressor must be closely matched to the systemcompressor must be closely matched to the system from the evaporator and compress it to a higher 

from the evaporator and compress it to a higher  duty.duty.

pressure. This enables the gas to be condensed pressure. This enables the gas to be condensed back into liquid by some convenient low cost

back into liquid by some convenient low cost Hermetic compressors and larger semi-hermeticHermetic compressors and larger semi-hermetic source of cooling, such as air or water.

source of cooling, such as air or water. compressors are usually suction-cooled, thecompressors are usually suction-cooled, the refrigerant passing over the motor windings before refrigerant passing over the motor windings before entering the compressor cylinders. This helps to entering the compressor cylinders. This helps to cool the motor windings, but reduces the

cool the motor windings, but reduces the capacity capacity  of the compressor. Externally cooled types, where of the compressor. Externally cooled types, where  the gas passes directly into t

 the gas passes directly into the cylinders, are usually he cylinders, are usually  Most compressors are driven by an

Most compressors are driven by an electric motor,electric motor, about 8% more efficient than the equivalentabout 8% more efficient than the equivalent sometimes built into a common casing. Other 

sometimes built into a common casing. Other  suction-cooled modelsuction-cooled models but are s but are only available up toonly available up to compressors have an external drive, the shaft

compressors have an external drive, the shaft a motor size of about 5 kW.a motor size of about 5 kW.

passing through a rotating gas seal where it exits passing through a rotating gas seal where it exits from the pressurised casing.

from the pressurised casing.

This type of compressor has an external

This type of compressor has an external drive shaftdrive shaft allowing a suitably sized motor to be

allowing a suitably sized motor to be selected andselected and connected to it, either with

connected to it, either with a direct coupling or viaa direct coupling or via These compressors have the motor directly 

These compressors have the motor directly  belts. It is important to size belts. It is important to size the motor accurately inthe motor accurately in attached to the main

attached to the main shaft, and the whole assembly shaft, and the whole assembly  relation to the relation to the compressor's duty. Running motorscompressor's duty. Running motors

3.1 COMPRESSORS 3.1 COMPRESSORS

3.1.1

3.1.1 TYPES TYPES OF OF COMPRESSOR COMPRESSOR HOUSING

HOUSING

3.1.3

3.1.3 OPEN OPEN COMPRESSORS COMPRESSORS 3.1.2

3.1.2 HERMETIC HERMETIC AND AND SEMI- SEMI-HERMETIC COMPRESSORS HERMETIC COMPRESSORS

3. EQUIPMENT

3. EQUIPMENT

at below their design duty reduces their power 

at below their design duty reduces their power  improved flow through valves:improved flow through valves:

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o rreedduucceed d pprreessssuurre e ddrroopp;;

 When comparing the input power requirements of   When comparing the input power requirements of  open and semi-hermetic compressors, the motor's

open and semi-hermetic compressors, the motor's minimised heat transfer from discharge tominimised heat transfer from discharge to efficiency

efficiency and and losses losses due due to to the the drive drive have have to to be be suction suction gas.gas.

 taken into account for

 taken into account for open drive machines.open drive machines.

Such modifications can improve efficiency by up to Such modifications can improve efficiency by up to   Where extended operation of the plant is

  Where extended operation of the plant is 20%, although in many cases the capital cost of 20%, although in many cases the capital cost of thethe envisaged it could prove viable to invest in an

envisaged it could prove viable to invest in an compressor will be higher because of the increasedcompressor will be higher because of the increased energy efficient (high efficiency) motor. At present

energy efficient (high efficiency) motor. At present complexity of manufacturing.complexity of manufacturing.

 the cost will be higher than a standard motor but  the cost will be higher than a standard motor but  this could change as the price differential between

 this could change as the price differential between It is critical to the reliability of reciprocatingIt is critical to the reliability of reciprocating standard and high efficiency motors is decreasing.

standard and high efficiency motors is decreasing. compressors that liquid refrigerant or largecompressors that liquid refrigerant or large The payback time, derived by

The payback time, derived by a simple cost analysis,a simple cost analysis, quantities of oil are not injected into the quantities of oil are not injected into the cylinders,cylinders, will usually be less than two years given the long

will usually be less than two years given the long as this will cause mechanical failure in theas this will cause mechanical failure in the running hours and may show a better return on

running hours and may show a better return on compressor.compressor.

investment.

investment.

Screw compressors are available for duties from Screw compressors are available for duties from about 50 kW up to

about 50 kW up to thousands of kilowatts and arethousands of kilowatts and are Reciprocating compressors are the most

Reciprocating compressors are the most commoncommon

generally used on medium to high temperature generally used on medium to high temperature  types of compressor and are available for a wide

 types of compressor and are available for a wide

applications. The geometry of the compressor  applications. The geometry of the compressor  range of applications.

range of applications.

determines its optimum pressure ratio.

determines its optimum pressure ratio. OperationOperation away from this ratio will significantly reduce its away from this ratio will significantly reduce its The design of a compressor is optimised for 

The design of a compressor is optimised for 

efficiency. For this reason manufacturers usually  efficiency. For this reason manufacturers usually  operation within a designated

operation within a designated application envelopeapplication envelope

produce a range of machines with different produce a range of machines with different with specified refrigerants. Operating a compressor 

with specified refrigerants. Operating a compressor 

operating characteristics.

operating characteristics.

at high temperature conditions with valves at high temperature conditions with valves designed for low temperature operation could designed for low temperature operation could

A large quantity of oil is injected into screw A large quantity of oil is injected into screw result in losses of up to 10% in the extraction rate.

result in losses of up to 10% in the extraction rate.

compressors to seal the running clearances compressors to seal the running clearances  With many compressors it could also result in t

 With many compressors it could also result in t hehe

between the rotors and the casing. The oil has to be between the rotors and the casing. The oil has to be motor being overloaded and tripping its protection

motor being overloaded and tripping its protection

removed from the refrigerant in a suitable sized compression is absorbed by the oil, which must be compression is absorbed by the oil, which must be Compressors have been developed with

Compressors have been developed with improvedimproved

removed by an oil cooler. It is preferable to cool the removed by an oil cooler. It is preferable to cool the efficiencies. The main areas of

efficiencies. The main areas of improvement are:improvement are:

oil by using a supply of

oil by using a supply of air or water. Using a supply of air or water. Using a supply of  refrigerant for cooling can reduce the system refrigerant for cooling can reduce the system

clearance volume reduction;clearance volume reduction;

3.1.5

3.1.5 SCREW SCREW COMPRESSORS COMPRESSORS 3.1.4 RECIPROCATING

3.1.4 RECIPROCATING

COMPRESSORS

COMPRESSORS

capacity

capacity by by up up to to 10%, 10%, with with a a corresponding corresponding loss loss of of compressors compressors that that the cothe correct rrect running running speed speed of of thethe e

effffiicciieennccyy.. ccoommpprreessssoor r hhaas s bbeeeen n uusseedd. . WWiitth h sseemmii--hheerrmmeettiicc compressors this speed is fixed by the design of the compressors this speed is fixed by the design of the built-in motor.

built-in motor.

The scroll type of

The scroll type of rotary compressor has been therotary compressor has been the subject of extensive development in recent years, subject of extensive development in recent years, as

as improved improved machining machining techniques techniques have have made made its its To To maintain maintain the the maximum maximum system system efficiency efficiency inin p

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Scroll compressors are being increasingly applied to multi-compressor system this can either bemulti-compressor system this can either be medium and small air-conditioning applications

medium and small air-conditioning applications achieved by switching a number of achieved by switching a number of compressors off compressors off  because of their quiet,

because of their quiet, low vibration operation andlow vibration operation and or by reducing their or by reducing their individual pumping capacities.individual pumping capacities.

good efficiency. Their efficiency advantage over 

good efficiency. Their efficiency advantage over  The best way to The best way to save energy is always to switch off save energy is always to switch off  reciprocating compressors at lower compression

reciprocating compressors at lower compression any unnecessary machines.any unnecessary machines.

ratios makes them ideal for high temperature ratios makes them ideal for high temperature refrigeration applications, such as beer cellar and refrigeration applications, such as beer cellar and milk tank cooling.

milk tank cooling.

Scroll compressors are also being developed for 

Scroll compressors are also being developed for  There are a number of There are a number of methods used to reduce themethods used to reduce the lower temperature

lower temperature applications.applications. capacity of capacity of compressors:compressors:

blocked suction gas,blocked suction gas,

suction valve lifting;suction valve lifting;

discharge gas recirculationdischarge gas recirculation The extraction rate and power input of a

The extraction rate and power input of a compressor depend principally on the

compressor depend principally on the evaporatingevaporating and condensing temperatures. Compressor  and condensing temperatures. Compressor  performance is usually presented in graphical (Fig) performance is usually presented in graphical (Fig) or in tabular format.

or in tabular format.

These data are presented at specific rating These data are presented at specific rating conditions, and corrections have to be made to conditions, and corrections have to be made to  take into account actual site operating conditions  take into account actual site operating conditions

for:

for:

suction gas temperature;suction gas temperature;

liquid subcooling.liquid subcooling.

Figure 18: Typical compressor performance Figure 18: Typical compressor performance Care must be taken with data for open

Care must be taken with data for open data. data. (sourc(source: e: ETSU)ETSU)

3.1.6

3.1.6 SCROLL SCROLL COMPRESSORS COMPRESSORS

3.

3. 1. 1.8 8 CA CAPA PA CI CITY TY CO CO NT NTRO ROL L

3.1.8.1 RECIPROCATING COMPRESSORS 3.1.8.1 RECIPROCATING COMPRESSORS

3.1.7

3.1.7 COMPRESSOR COMPRESSOR PERFORMANCE PERFORMANCE DATA

DATA

  When

  When selecting selecting a a compressor, compressor, it it is is important important to to about about 50% 50% capacity, capacity, but but below below this this it it falls falls off off very very  check

check the the manufacturer's manufacturer's data data to to ensure ensure that that the the quickly.quickly.

model chosen is of an energy efficient design. The model chosen is of an energy efficient design. The reduction in input power should match, as closely as reduction in input power should match, as closely as possible, the reduction in refrigeration duty.

possible, the reduction in refrigeration duty.

It is also worthwhile checking whether  It is also worthwhile checking whether 

There are two principal types

There are two principal types of evaporator:of evaporator:

supplementary compressor cooling is required supplementary compressor cooling is required

direct expansion (sometimes called "dry direct expansion (sometimes called "dry  while capacity control is in operation, as this will

while capacity control is in operation, as this will

expansion" or DX);

expansion" or DX);

need additional energy.

need additional energy.

flooded.flooded.

The number of stages of

The number of stages of capacity reduction that cancapacity reduction that can be obtained will depend on the design of the be obtained will depend on the design of the compressor, and is usually a function of the

compressor, and is usually a function of the number number  of cylinders. On suction cooled compressors the

of cylinders. On suction cooled compressors the These are commonly used to cool either air or aThese are commonly used to cool either air or a minimum capacity is often limited by the loss of 

minimum capacity is often limited by the loss of  liquid. The expansion device used with this type of liquid. The expansion device used with this type of  cooling of the motor.

cooling of the motor. evaporator is an expansion valve.evaporator is an expansion valve.

A direct expansion evaporator used for cooling air  A direct expansion evaporator used for cooling air  is shown in Figure 19. There are many different is shown in Figure 19. There are many different designs available using plain or finned tube, both designs available using plain or finned tube, both with and without forced circulation of air or some with and without forced circulation of air or some The capacity of a large screw compressor can be

The capacity of a large screw compressor can be

process fluid. Certain tube designs incorporate process fluid. Certain tube designs incorporate varied from 100% down to 10% by using a slide

varied from 100% down to 10% by using a slide

internal devices to maximise heat exchange and internal devices to maximise heat exchange and vane. The part load efficiency

vane. The part load efficiency is acceptable down tois acceptable down to

3.2 EVAPORATORS 3.2 EVAPORATORS

3.2.1

3.2.1 DIRECT DIRECT EXP EXPANSION ANSION

3.1.8.2 SCREW COMPRESSORS 3.1.8.2 SCREW COMPRESSORS

Figure 19: Liquid distribution on a direct expansion circuit. (source: ETSU) Figure 19: Liquid distribution on a direct expansion circuit. (source: ETSU)

  thus

  thus efficiency, efficiency, by by causing causing turbulence turbulence to to keep keep the the evaporated evaporated before before reaching reaching the the outlet.outlet.

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A typical evaporator will have a number of parallelparallel

important for the reliability of reciprocating important for the reliability of reciprocating circuits designed to:

circuits designed to:

machines, but less so for

machines, but less so for rotary compressors.rotary compressors.

maximise heat transfer;maximise heat transfer;

ensure good oil return;ensure good oil return;

minimise pressure drop.minimise pressure drop.

A distributor is used to ensure refrigerant flows A distributor is used to ensure refrigerant flows evenly between the different

evenly between the different parallel circuits.parallel circuits.

The efficiency of an evaporator can be affected by  The efficiency of an evaporator can be affected by  an uneven distribution of refrigerant, and hence an uneven distribution of refrigerant, and hence To enhance the heat

To enhance the heat transfer in air-cooled designs,transfer in air-cooled designs,

cooling, between the different circuits. maximise their surface the fins are

maximise their surface the fins are spaced as closely spaced as closely 

positioned - it should always be vertical so that positioned - it should always be vertical so that  together as possible without restricting the air flow.

 together as possible without restricting the air flow.

 there is an even feed through each outlet - or  there is an even feed through each outlet - or if oneif one On low temperature systems, where ice can form

On low temperature systems, where ice can form

distributor line becomes damaged.

distributor line becomes damaged.

on the fin surfaces, a wider spacing has to be used to on the fin surfaces, a wider spacing has to be used to

on the fin surfaces, a wider spacing has to be used to on the fin surfaces, a wider spacing has to be used to

In document Handbook - Refrigeration (Page 27-36)