Answer EKG Question 22
Q22. With reference to Refrigeration system onboard your vessel:
(ii) How these causes would be found and possible remedies.
(a) Sketch and describe high pressure cut-out in a refrigeration system.
(b) The refrigeration compressor has stopped due to operation of the h.p cut-out explain.
(i) The possible cause.
(ii) How these causes would be found and possible remedies.
(c) What steps are taken if the compressor “short-cycle” on low pressure cut-out?
Answer: (a) Sketch and describe high pressure cut-out in a refrigeration system.
when there is an over pressure on the condenser side of the compressor, the high pressure cut-out will cause the compressor to shut down. The device is required to be reset by hand which will allow to restart the compressor.
The bellows in the cut-out is connected by a small bore pipe between the compressor discharge and the condenser. The bellows tends to be expanded by the pressure and this movement is opposed by the spring. The adjustment screw is used to set the spring pressure.
when there is an over pressure on the condenser side of the compressor, the high pressure cut-out will cause the compressor to shut down. The device is required to be reset by hand which will allow to restart the compressor.
The bellows in the cut-out is connected by a small bore pipe between the compressor discharge and the condenser. The bellows tends to be expanded by the pressure and this movement is opposed by the spring. The adjustment screw is used to set the spring pressure.
During
normal running the switch arm is held by the switch arm catch and holds
the electrical circuit in place. Excessive pressure expands the bellows
and moves the switch arm catch around its pivot. The upper end slips to
the right of the step and releases the switch arm so breaking the
electrical contact and causing the compressor to cut out. The machine
cannot be restarted until the trouble has been remedied and the switch
reset by hand.
(b) The refrigeration compressor has stopped due to operation of the h.p cut-out.
(i) The possible cause:-
the H.P cut-out shall be activated if the delivery pressure is too high, as the H.P cut-out is fitted in delivery line. The delivery pressure should be confirmed at the delivery pressure gauge.
air in the system
condenser tubes dirty impaired cooling or partition wall corroded.
cooling water temperature is too high or water flow is short.
too much refrigerant in the system so the heat exchanging area is reduced.
delivery stop valve is not open wide enough.
gas suction pressure too high (this may be during start up only)
(ii) finding causes and possible remedies.
- frosting can be handled by de frosting
-low refrigerant if found, then recharge of same to be carried ou.
-if found trip setting incorrect then check and adjust them all.
-too much of cooling water in condenser if flowing, then adjust the same.
-air found in condenser then need to be purged.
-if TEV found partially blocked that the filters to be cleaned.
- check for leaking solenoid valve and overhaul the same.
-check for discharge valve malfunctioning and overhaul the same.
(c) What steps are taken if the compressor “short-cycle” on low pressure cut-out?
causes of LP cut-out and actions
suction stop valve in not open wide enough.
liquid refrigerant flow rate is limited. dryer filter is clogged.
less refrigerant in the system.
suction strainer choked.
expansion valve is clogged with ice/oil particle.
opening of expansion valve is small. degree of super-heat is high.
capacity of evaporator coil is lowed. (check fan, duct deposit on cooling coil, frosted coil).
the refrigerant leaks out of the feeler bulb of the expansion valve.
Regular leak tests and monitoring of parameters is important in keeping the plant running efficiently. Unless the charge is right, it will work poorly. When there isn't enough refrigerant, high and low sides pressures drop, which drops the system capacity. Over-charge will create excessive high pressures which also results in a drop in cooling efficiency. Before charging, it is essential to locate and fix all leakages. No matter how well the piping joints are made or how good the quality of materials used, expect to find leaks. The only way to ensure good performance is to stop those leaks first, which can cause endless head-aches to the operating engineer. To check for leaks, usually Nitrogen is charged in the system, and left to stand. Monitor the pressure gauge, which should remain steady, if the system is tight. When it is confirmed that there are no leaks, charge the refrigerant from the gas bottle, which is usually weighed to give a precise quantity of charge, which depends on the size of the system. First step in charging is to weigh the refrigerant and connect to the charging valve on the liquid line, using a good quality flexible tube which does not kink. Crack open the bottle valve to purge out air from the line.
Open compressor discharge and suction valves, condenser hot gas and liquid valves and liquid solenoid valves. Then open the charging valve. After charging is complete, close the liquid valve and check scale for weight of gas charged. The refrigerant level should show in the receiver glass. Keep cooling water circulation going in the condenser, to assist collecting gas.
Routine maintenance
The best way to avoid costly and annoying shut-downs is to test all the safeties are working, which will prevent damage to the machine. Adjust the low pressure switch so that it starts the compressor whenever the suction pressure rises above the desired setting. (Check this from the manual). Adjust the differential or cut-out points as low as possible, without having short-cycling. Check the high-pressure cut-off; which guards the system against cooling water failure. It is important to try out the interlocks — the compressor should be `locked out' if the lube oil pressure is lower than the set value.
Thermostatic expansion valve
This is probably the single most important component in the system, without which it would be impossible to carry out efficient cooling.
Check the thermostatic expansion valve for the correct amount of superheat. Put a thermometer on the suction line near the remote bulb of the thermostatic valve. Read the refrigerant temperature at a point as near the remote bulb as possible, but on the compressor side of the bulb. Read the suction pressure from the gauge. Difference between the thermometer reading on the suction line and the temperature calculated from the suction pressure gives the superheat.
The main job of the thermostatic expansion valve is to control the flow of refrigerant to the evaporator. If too little liquid enters your evaporator, it quickly flashes to a gas without absorbing much heat. When too much liquid enters the coil, not all of it is vapourised, with the result that liquid floods back to the compressor suction.
Either way you are in trouble. With insufficient liquid, cooling capacity falls off, while too much liquid returning can wreck your compressor valves and pistons, because the clearances are small, and liquid is incompressible. The superheat of the gas leaving the evaporator ensures that there is no liquid return. To control this, there is a thermal bulb placed at the exit, whose pressure is affected by the temperature of the gas leaving the evaporator. In effect, this is a means of sensing the outlet, which is used to control the inlet, i.e. the setting of the thermostatic expansion valve from the bulb. Ideally the liquid entering the evaporator coil should completely flash off to gas with a slight degree of superheat.
At some point in the coil, say A, saturated gas starts absorbing heat to become superheated at the point of exit. When the load falls off, there isn't enough heat absorbed between point A and the bulb to get the required degree of superheat — so the bulb cools, closing the expansion valve (due to the pressure difference) and reduces the liquid flow proportionately. In this way the required degree of superheat is maintained, and liquid is prevented from entering the compressor. Too high a superheat means that your cooling capacity is reduced (since more of the coil is used for superheat), while too low a superheat could lead to liquid flooding back to the compressor. The correct amount is specified on the valve and in the manual. Large pressure drop through the system low side cuts compressor capacity and can also reduce the expansion valve capacity. The pressure from the thermal bulb regulates the valve opening by acting against spring and evaporator pressure.
With large pressure drops through the evaporator coil, the saturation temperature of the gas near the outlet is lower (higher drop in pressure). With such low saturation temperatures, more superheating would be required to hold the expansion valve open, which would result in a drop in capacity. One way of avoiding this is by means of an external equaliser connection, so that the expansion valve is not affected by the pressure drop, and so responds only to the superheat of the gas. This is connecting the underside of the diaphragm to the evaporator outlet. Exact location of the equaliser depends on the layout, but is usually at the point of greatest pressure drop in the coil.
Maintenance planning
In carrying out routine maintenance, ensure that you follow the PMS on board. Given below are only general guide-lines :
Every three months :
a. Check for leakages.
b. Grease bearings, if required.
c. Check safety cut-outs.
d. Check belt tension.
Every six months :
a. Check all couplings.
b. Check oil filter and oil of compressor.
c. Clean strainers.
d. Check dryers.
Every twelve months :
a. Renew lube oil and filter.
b. Examine all valves.
c. Calibrate thermometers and gauges.
d. Weigh spare gas bottles.
e. Examine fans.
f. Recharge dryers.
Every twenty four months :
a. Over-haul reciprocating compressor (screw type run for longer periods).
b. Pistons, bearings and shaft to be examined.
c. Coolers to be checked and cleaned.
d. Pressure testing of heat exchangers.
(b) The refrigeration compressor has stopped due to operation of the h.p cut-out.
(i) The possible cause:-
the H.P cut-out shall be activated if the delivery pressure is too high, as the H.P cut-out is fitted in delivery line. The delivery pressure should be confirmed at the delivery pressure gauge.
air in the system
condenser tubes dirty impaired cooling or partition wall corroded.
cooling water temperature is too high or water flow is short.
too much refrigerant in the system so the heat exchanging area is reduced.
delivery stop valve is not open wide enough.
gas suction pressure too high (this may be during start up only)
(ii) finding causes and possible remedies.
- frosting can be handled by de frosting
-low refrigerant if found, then recharge of same to be carried ou.
-if found trip setting incorrect then check and adjust them all.
-too much of cooling water in condenser if flowing, then adjust the same.
-air found in condenser then need to be purged.
-if TEV found partially blocked that the filters to be cleaned.
- check for leaking solenoid valve and overhaul the same.
-check for discharge valve malfunctioning and overhaul the same.
(c) What steps are taken if the compressor “short-cycle” on low pressure cut-out?
causes of LP cut-out and actions
suction stop valve in not open wide enough.
liquid refrigerant flow rate is limited. dryer filter is clogged.
less refrigerant in the system.
suction strainer choked.
expansion valve is clogged with ice/oil particle.
opening of expansion valve is small. degree of super-heat is high.
capacity of evaporator coil is lowed. (check fan, duct deposit on cooling coil, frosted coil).
the refrigerant leaks out of the feeler bulb of the expansion valve.
Regular leak tests and monitoring of parameters is important in keeping the plant running efficiently. Unless the charge is right, it will work poorly. When there isn't enough refrigerant, high and low sides pressures drop, which drops the system capacity. Over-charge will create excessive high pressures which also results in a drop in cooling efficiency. Before charging, it is essential to locate and fix all leakages. No matter how well the piping joints are made or how good the quality of materials used, expect to find leaks. The only way to ensure good performance is to stop those leaks first, which can cause endless head-aches to the operating engineer. To check for leaks, usually Nitrogen is charged in the system, and left to stand. Monitor the pressure gauge, which should remain steady, if the system is tight. When it is confirmed that there are no leaks, charge the refrigerant from the gas bottle, which is usually weighed to give a precise quantity of charge, which depends on the size of the system. First step in charging is to weigh the refrigerant and connect to the charging valve on the liquid line, using a good quality flexible tube which does not kink. Crack open the bottle valve to purge out air from the line.
Open compressor discharge and suction valves, condenser hot gas and liquid valves and liquid solenoid valves. Then open the charging valve. After charging is complete, close the liquid valve and check scale for weight of gas charged. The refrigerant level should show in the receiver glass. Keep cooling water circulation going in the condenser, to assist collecting gas.
Routine maintenance
The best way to avoid costly and annoying shut-downs is to test all the safeties are working, which will prevent damage to the machine. Adjust the low pressure switch so that it starts the compressor whenever the suction pressure rises above the desired setting. (Check this from the manual). Adjust the differential or cut-out points as low as possible, without having short-cycling. Check the high-pressure cut-off; which guards the system against cooling water failure. It is important to try out the interlocks — the compressor should be `locked out' if the lube oil pressure is lower than the set value.
Thermostatic expansion valve
This is probably the single most important component in the system, without which it would be impossible to carry out efficient cooling.
Check the thermostatic expansion valve for the correct amount of superheat. Put a thermometer on the suction line near the remote bulb of the thermostatic valve. Read the refrigerant temperature at a point as near the remote bulb as possible, but on the compressor side of the bulb. Read the suction pressure from the gauge. Difference between the thermometer reading on the suction line and the temperature calculated from the suction pressure gives the superheat.
The main job of the thermostatic expansion valve is to control the flow of refrigerant to the evaporator. If too little liquid enters your evaporator, it quickly flashes to a gas without absorbing much heat. When too much liquid enters the coil, not all of it is vapourised, with the result that liquid floods back to the compressor suction.
Either way you are in trouble. With insufficient liquid, cooling capacity falls off, while too much liquid returning can wreck your compressor valves and pistons, because the clearances are small, and liquid is incompressible. The superheat of the gas leaving the evaporator ensures that there is no liquid return. To control this, there is a thermal bulb placed at the exit, whose pressure is affected by the temperature of the gas leaving the evaporator. In effect, this is a means of sensing the outlet, which is used to control the inlet, i.e. the setting of the thermostatic expansion valve from the bulb. Ideally the liquid entering the evaporator coil should completely flash off to gas with a slight degree of superheat.
At some point in the coil, say A, saturated gas starts absorbing heat to become superheated at the point of exit. When the load falls off, there isn't enough heat absorbed between point A and the bulb to get the required degree of superheat — so the bulb cools, closing the expansion valve (due to the pressure difference) and reduces the liquid flow proportionately. In this way the required degree of superheat is maintained, and liquid is prevented from entering the compressor. Too high a superheat means that your cooling capacity is reduced (since more of the coil is used for superheat), while too low a superheat could lead to liquid flooding back to the compressor. The correct amount is specified on the valve and in the manual. Large pressure drop through the system low side cuts compressor capacity and can also reduce the expansion valve capacity. The pressure from the thermal bulb regulates the valve opening by acting against spring and evaporator pressure.
With large pressure drops through the evaporator coil, the saturation temperature of the gas near the outlet is lower (higher drop in pressure). With such low saturation temperatures, more superheating would be required to hold the expansion valve open, which would result in a drop in capacity. One way of avoiding this is by means of an external equaliser connection, so that the expansion valve is not affected by the pressure drop, and so responds only to the superheat of the gas. This is connecting the underside of the diaphragm to the evaporator outlet. Exact location of the equaliser depends on the layout, but is usually at the point of greatest pressure drop in the coil.
Maintenance planning
In carrying out routine maintenance, ensure that you follow the PMS on board. Given below are only general guide-lines :
Every three months :
a. Check for leakages.
b. Grease bearings, if required.
c. Check safety cut-outs.
d. Check belt tension.
Every six months :
a. Check all couplings.
b. Check oil filter and oil of compressor.
c. Clean strainers.
d. Check dryers.
Every twelve months :
a. Renew lube oil and filter.
b. Examine all valves.
c. Calibrate thermometers and gauges.
d. Weigh spare gas bottles.
e. Examine fans.
f. Recharge dryers.
Every twenty four months :
a. Over-haul reciprocating compressor (screw type run for longer periods).
b. Pistons, bearings and shaft to be examined.
c. Coolers to be checked and cleaned.
d. Pressure testing of heat exchangers.
Helium gas, a noble gas, is inert, non-toxic, and has low boiling and melting points. Its lightness makes it ideal for lifting balloons and as a coolant in various applications, including MRI machines and cooling nuclear reactors. However, its limited availability and rising prices pose challenges.
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