2 compressor
TRANSCRIPT
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
c
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efficiency tric x volumecompressor of ratent Displaceme
trefrigeran of rate flow Volume
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esseffectiven of measurecommon a is (This
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efficiency Volumetric
Mass flow rate
ratent displaceme compressor D where
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trefrigeran of rate flow Mass
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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