Refrigeration

Compressors

Content

• Types of compressors

• Performance of reciprocating compressor

• Some construction details

• Safety features

Function of compressor

Main function is to move refrigerant in the system by:

• Remove gas from evaporator• Raise the pressure of the gas• Discharge gas to the condenser coil for

heat rejection

Low-side pressureHigh-side pressure

Compressor Types

• Positive displacement- no reversal of flow

- transfer work by means of hydrostatic force

- e.g. reciprocating, screw, vane

• Non-positive displacement- reversal of flow is possible

- transfer work by means of hydrodynamic force

- e.g. centrifugal

Types of compressors

Size range

Reciprocating compressor

• Advantage– Low cost

– Established technology

– Direct or belt drive

– Good part load and full load efficiency

– Simple control

• Disadvantage– Many moving parts

– Small flow rate

Reciprocating Compressors: Hermetic

• Hermetic- motor-compressor unit is total sealed, therefore non-serviceable

- motor cooled by incoming suction vapor

- use mainly for residential appliance

Reciprocating Compressors: Semi-hermetic

• Semi-hermetic- Partially sealed, therefore accessible for service

- motor cooled by incoming suction vapor

- larger size range (5-100kW)

Reciprocating compressor- open

• Open-External drive; motor separate from compressor element- burnt motor can be

easily replaced- do not rely on suction

for cooling- can incorporate morecylinders and higheroutput

Semi-hermetic Reciprocating Compressor

This is the common compressor used offshore.

• No problem with shaft seal leakage

• Runs cooler than open type ( higher service factor)

• Accessible for repair• Compact• No pulleys, belts or coupling• Efficient at part-load and full

load• Simple control, operation and

maintenance• Less noise

Carlyle 6D/E Model

Semi-hermetic Compressor

Source: Carlyle 06 Model Catalog

Work in Reciprocating Compressor

Work in Reciprocating Compressor

• Clearance volume is the volume between TDC piston position and the cylinder head

• Expansion of this volume occurs initially during the suction stroke

Clearance Volume Reexpansion

Effect of Clearance Volume

1-4 Re-expansion 4-3 Suction3-2 Compression1-2 Discharge

V c = Clearance volume

Effect of Increasing Clearance

Effect of Suction Pressure

Decreasing Suction PressureP1>P2>P3 (i.e. T1>T2>T3)𝛈1> 𝛈2> 𝛈3 (i.e. efficiency drops)

Effect of Discharge Pressure

Increasing Discharge PressureP2>P1 (i.e. T2>T1)𝛈2< 𝛈1(i.e. efficiency drops)

Compressor Volumetric Efficiency

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Refrigeration Capacity

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Power Required

• The power requirement of a compressor is= Mass flow rate x work of compression per unit mass

= m ( ∆ h c)

))((D)(

compressor ofPower

s

vch

Performance of Reciprocating Compressors

Two important operating variables are:

• Evaporating temperature (pressure)

• Condensing temperature (pressure)

Two important performance characteristics are:

• Refrigeration capacity

• Power requirements

Effect of Evaporating temperature

As evaporating temperature decreases•Volumetric efficiency drops•Mass flow rate also drops

Effect of Evaporating Temperature

The curve of power has a zero value at 2 points.

•When mass flow is zero•When ∆h c is zero

(most systems operate on the left side of the peak power)

Effect of Evaporating Temperature

Refrigerating capacity increases significantly with increasing evaporator temperature

Effect of condensing temperature

As condensing temperature increases,•Volumetric efficiency drops•Mass flow rate also drops

Effect of condensing temperature

Power required by the compressor increases with increasing condensing temperature.A low condensing temperature is desirable.

Effect of condensing temperature

Refrigerating capacity always decreases with increasing condensing temperature. A low condensing temperature is desirable.

manufacturer’s data

Performance Map

Compressor Performance

•For any one compressor, volumetric efficiency is not a constant. It changes with operating conditions of the system•The refrigerant used also has an effect on efficiency value.

•Due to piston and valve leakage and heat loss, actual efficiency drops by another 10-15%

• Approximately, actual compressor efficiency=actual volumetric efficiency

Actual compressor efficiency

Source: Refrigeration & Air Conditioning by WF Stoecker

Compressor Safety Control

• Low/high pressure switches

• Low and high temperature switches

• Low oil pressure switch

• Pump-down control

• Crankcase heater

• Motor overload protection

Pressure Switches

• Low

– When suction pressure drops below a preset value, compressor is stopped

• High

– When discharge pressure approaches a high level, the cutoff pressure, compressor is stopped

Temperature switches

• Low temperature at suction can result in frosting

• High discharge temperature can result in

– Oil breakdown

– Excessive wear

– Reduce life of discharge valves

• Most compressor has limit on discharge temperature (265 F for Carlyle compressors)

Oil Pressure

• Compressors need lubricant for the following reasons• Lubrication of bearings

• Remove heat

• Assist with gas sealing

• Low oil pressure switch will stop the compressor if oil pressure fails to reach a predetermined level within a certain time period

Crankcase Heater

•To reduce the migration of refrigerant to the crankcase•Heaters are commonly fitted to crankcase•Remains in operation whenever the compressor is idle

Pumpdown Control

Pump-down control helps to prevent liquid slugging during startup.

• When the system is shut down, the suction solenoid valve is closed first.

• Compressor continues to pump the refrigerant to the condenser.

• As the suction pressure drops below the cut-in pressure, the compressor stops.

Compressor Capacity Control

When the system load on the cooling coil doesn’t match the full load capacity of reciprocating compressor, the capacity modulation is:

On/off control

Cylinder unloading

Suction cutoff unloading (holding the valve open)

Hot gas bypass

Variable speed drive

Multiple Compressor ON/OFF

Cylinder Unload De-energized

Cylinder Unload Energized

Cylinder Unload

Suction Cut-off Unloading

Hot Gas Bypass

Hot gas bypass valve

Evaporator Condenser

Bypass into evaporator inlet

Bypass into suction line

Variable Speed Compressor

• Vary the speed to match precisely match the load

• For reciprocating compressor, the lowest speed shall not affect the oil lubrication system

Why Compressor Failed?

Compressor Failure: Causes

•Is system design/

installation/ operation

OK?

•Are components

properly matched?

•Which component

malfunction?

Causes of Compressor Failure

Mechanical issues

1. Liquid floodback

2. Dirt (acid or moisture)

3. Overheating

4. Lack of lubrication

Electrical unbalanced

1. Voltage

2. frequency

Refrigeration: What can go wrong?

Refrigeration

(mechanical)

issues

Contamination Overheating

High gas

return

temperatur

e

High

discharge

temperatur

e

Dirt

Moisture

Non-condensable Liquid

slugging

Loss of

Lubrication

Low load

Oversized

TXV

Low oil

pressure

Poor oil

return

Electrical: What can go Wrong?

Electrical

Problem

Unbalanced

voltage

High

Amperage

Winding overheat

Motor failure

Low dialectic

resistance

Overheat

Low dialectic

resistance

Motor failure

Solving the Problem

If the compressor is replaced without attending to the root cause, surely the new compressor will also fail.

1. Identify the root causeLubricating oil analysisTear-down analysis System diagnosis

2. Repair & Replace3. Clean the system before restart

Seizure of bearing surfacesAt connecting rod and crankshaft

Broken fragments