Q) Give refrigerant properties.
ANS.)1.Non toxic, non flammable, non explosive, non corrosive
2.economical, easy leak detection, easily available, environment friendly
3.low sp. heat, low boiling pt., low sp. vol.
4. high latent heat, high critical temp., moderate working pressures
5.miscible with lubricants without reacting chemically
Q)Give refrigerant and ref l.o. most imp. property, only one? what is the significance of critical temp and condensing press.?
ANS.) Most important property of l.o of refrigerant system is its floc point as it happens only in the presence of refrigerant.
Critical temp is the temp. above which a gas can not be liquified no matter how much you high the pressure is. clearly refrigerant should have a high critical temp.
Condensing pressure is the pressure at which the gas starts to liquify at a certain temp.; so gas should be pressurised enough so that it can be condensed easily inside condenser
Q)Why secondary refrigeration system is adopted in ref. cargo vessels? Why not only primary refrigerant be used in the system?
Ans.) 1. No environmental destruction.
2. much larger reserve of cold than in primary system.
3. less amount of refrigerant is used.
4. easy leak detection as system is small.
5. cheaper
Q)Define dry bulb, wet bulb temp and dew point.?
Ans.) Dry bulb temp is the temperature of air measured by thermometer freely exposed to air but shileded by radiation and moisture.
wet bulb temp is the temp when bulb is subjected to water (cotton soaked with water) and air is blown thru, water evaporates taking latent heat from bulb, thus reducing the temp. and it depends on humidity in air
At 100% saturation, wet bulb temp=dry bulb temp
dew point is temp at which water vapor present in air starts condensing at that pressure, i.e. temp at which air is saturated.
Q)Why is COP of AC plant higher than ref. plant?
Ans.) COP=Ref. effect/ heat of compression
the area under the curve T6 T7 represent that of AC and that of curve T1 T2 represent that of refregerator because more heat is to be removed from refrigeartion system to drop the temperature below -20 degree celcius in comparison to 20 degree celcius in AC. hence COP OF AC IS MORE THAN REFERIGERATION SYSTEM as heat of compression is low in AC.
Q)How to increase COP of refrigeration system?
Ans.)By subcooling
Q)How to improve ref. compressor efficiency?
Ans.) Refer comp effeciency can be incresed by superheating and subcooling.
Q)Dryness fraction in refrig. system?
Ans.) Ratio of mass of gaseous ref to the total mass of ref(liquid+gas) mixture..
Q)What is the purpose of subcooling line in ref.?
Ans.)With subcooling, flashing off in expansion valve gives lower dryness fraction, which is better so that more heat can be extracted in evaporator to attain superheat
Q)Why is equalizing line given in ref. system, what is its effect on TEV? Why not provided in some referigeration systems?
Ans.) It is used in the installations where a large drop in pressure occurs in the evaporator (more than 0.3 bar).In the expansion valve, the pressure acting outside (top) of the bellow corresponds to the saturation pressure plus the degree of superheat of the refrigerant leaving the evaporator. This pressure is trying to open the valve against the spring force from below the diaphragm. The external equalizer connection has a saturation pressure of the refrigerant leaving evaporator, to act below the diaphragm. Therefore the saturation pressure from the external equalizer connection balances the saturation pressure of the sensing bulb leaving only the pressure due to degree of super heat only to oppose the spring force. Therefore, this degree of superheat is supposed to open the expansion valve.By this method, we are getting a control over only the degree of superheat of the refrigerant leaving the evaporator coil. This is important to ensure proper utilization of the refrigerant and that no liquid refrigerant reaches the suction of the compressor.
Q)How and where to check for leak in ref. system?
Ans) 1. Soap solution test of flanges, valve glands, pr. switches, dryer covers, safety v/v at condenser, TEV
2.Electronic leak detector:-sample of air passed over heated Ptelement, if halogens present, positive ions increase, which are sensed in a circuit
3. Halide leak detector :-flame colour changes in presence of refr. gas. when gas passes over glowing Cu, CuCl2 is formed which changes the colour of flame from pale blue to bright red. It cannot be used for hydrocarbon refrigerants
Q)What is defrosting? Explain the various methods?
Ans.)Off cycle Evaporator fans are left running with the compressor not operating using room air to defrost the evaporator coil block.
Electric Electric heater elements are fitted into the evaporator coil block to raise coil block temperature and melt the frost and ice.
Hot gas Discharge gas from the compressor is diverted into the evaporator and used for defrosting,
Reverse cycle The purpose of the evaporator coil block and condenser are reversed to warm the evaporator to achieve defrosting.
Secondary Refrigerant Installations The coil block contains a fluid in the form of a secondary refrigerant. These systems either use electric elements in the coil block or circulate a warm fluid though the coil block.
Water defrosting Water is fed through sparge pipes on the top of the coil to cascade over the coil block surfaces to melt the ice.
Q)How to know the presence of air in ref. system and then how to purge it?
Ans.)1.High condenser pressure Small bubbles in the sight glass
2.Compressor discharge pressure high/running hot More superheat
3.Pressure fluctuations Inefficient working
Purging procedure:
In the normal operation, measure the liquid refrigerant pressure, temp. after condenser
Check the corresponding saturation temp. from the PT chart or compound gauge
Compare the measured temp. with the determined saturation temp. for any subcooling and adjust the flow of the cooling water through the condenser to achieve near saturation condition inside the condenser.Now close the condenser outlet valve, start the compressor and pump down the liquid to the condenser, checking the pressure in the suction line. If this pressure is allowed to drop down below the atmospheric pressure then there could be chances of air ingress into the system.Circulate the cooling water till the outlet and inlet temp. become equal, a check to ensure complete pump down operation.Check the condenser sea water outlet temp., check the corresponding refrigerant pressure from the PT chart, due to the presence of air they won’t match, current pr. will be higher.Now release air from the condenser, through the vent cock until the pressure of the refrigerant in the condenser corresponds to the saturation pressure at the sea water outlet temperature. As per regulation never purge the air and gas in open atmosphere, always collect it into gas recovery cylinder.
Q)What are the alarms, trips and safeties in refer compressor?
Ans.) 1.HP cut out
2.Differential lube oil pressure cut out
3.LP cutout
4.Cylinder head relief valve
5.Unloaders /capacity controllers
6.Relief valve in the condenser
7.Cooling water low flow/high temperature alarm
8.Oil separator Dryer
9.Non return shut off valves
Q)Explain refer gas charging procedure, both HP and LP side? What checks to be done while charging? What tools are given to avoid refrigerent release in atmosphere?
Ans.)
- Connect gas bottle or charging cylinder, vacuum pump and charging point in the reefer system to the valve block.
- The discharge of the vacuum pump is to be connected in the empty recovery bottle.
- First open the valve between vacuum pump and charging bottle located in the valve block without opening the main valve of the charging cylinder. This will remove all the air inside the pipe. Once vacuum is reached, close the valve of charge cylinder in the valve block.
- Now open the valve of the charging point pipe in the valve block and run the vacuum pump until the vacuum is reached. This will remove the trapped air from this pipe. Then shut the valve in the valve block
- Now keep the system idle for 5 minutes to check there is no pressure drop. This will ensure there are no leakages in the system
- Now open charging bottle pipe valve and the charging point pipe valve located in the valve block. This will set the line for charging. Ensure that the vacuum pump valve is shut.
- Now open the main valves in the charging cylinder and charging point of the reefer system
- Do not overfill the system. Make sure the receiver has 5 % space for expansion
Q)Define absolute humidity, what is the unit?
Ans.)Absolute humidity It is the amount of water vapor present in 1m3 of dry air (g/m3) at 1atm Specific humidity It is the amount of water vapor present in 1kg of dry air (g/kg) (no unit)
Q) Define relative humidity? Give an alternative definition?
Ans.) Rel Humidity is the gms of water vap. present in air/ gms of water vapor reqd. for saturation at that temp.
Q) Explain about legionella bacteria, where it can be found?
Ans.) Legionella bacteria is mostly found in stagnant water present in ac blower closet. this bacteria is a reason for pneumonia onboard the ship. it is mainly found in blower filters, humidifier and drains. hence filters should be renewed monthly and humidifier to be drained and cleaned with hypochlorinated solution and drain should always be free and to be flushed with hypochlorinated solution.