AKRAMANUJAN.ORG5th JULY 2019 Function 4 CONTINUED FROM BLOG 110

INSTRUMENTATION

(171) show by sketch how the electric resistance thermometers measuring exhaust temperatures of the main engine are connected to a common display unit.

Answer:- Fig shows a multipoint display unit incorporating double pole switches. By using this type instead of the single pole switch type earth faults are eliminated.. During assembly compensating resistances are fitted into individual circuits where required in order to balance the effects of the leads.

(172) What is a torductor and how does it measure shaft power:-

Answer:-The torductor is, as the name implies, a torque inductor, it is a stress transducer that is eminently suited to the measuring of torque in rotating shaft. It gives a high power output and requires no slip rings or other shaft attachments since it operates without any contact. Fig. 19 shows a ring torductor. It consists of one primary ring which carries four poles, marked N,S, that is supplied with 50Hz alternating current. Two outer secondary rings have four poles each, arranged at 450 to the primaries, all of which are connected in series with mutually reversed windings.

No contact exists between the poles and the shaft there being a 2 to 3 mm air gap provided to ensure this.

When no torque is applied to the shaft there are no stresses in the shaft and the magnetic fields between NS poles induced in the shaft will be symmetrical, the equipotential lines are then situated symmetrically under the secondary poles s1, s2, as shown and secondary flux and voltage will then be zero.

When a torque is being transmitted the equipotential lines form an asymmetrical pattern, as shown, due to the mutually per-perpendicular unlike stresses, acting at 450 to the shaft axis, causing increased permeability in one direction and decreased permeability at 900 in the other direction. This causes the s1 pole to become magnetically slightly positive and the s2 pole slightly negative.

The output from the secondaries of the ring torductor is of the order of a few milliwatts, which is large enough to be used without any amplification. If this signal is now married to a speed signal, from a tachogenerator perhaps, then the power

being developed could be displayed direct onto one dial. It is possible to obtain the specific fuel consumption, i.e. the fuel consumption in kg/kW h, direct onto a single gauge.

Advantages of the ring torductor include robustness, versatility, reliability and accuracy.

(173) Describe an oil in water sensor.

Answer:-The Diagrammatic arrangement shown in figure is an oil trace in water sensor syatem that uses ultra violet light as the sensor. The oil trace in water mixture from a ballast tank is supplied to the system conditioner which delivers a near homogeneous mixture to the flow control valve, through the sensor unit to waste. As the fluid passes through the glass, long wavelength light from the ultra violet lamp causes fluorescence if oil particles are present. This now visible light can be detected by a photo cell unit which produces a signal that can be readily amplified and then used for alarm and control. The amount of fluorescence is dependent upon the amount of oil-water mixture and this affects the amount of visible light detected by the photo cell.

Purge air is supplied to the glass to avoid fouling of the glass and incorrect signals.

For calibration an injection pump can be used to deliver to the sensor a known oil-water mixture.

Since this system generally designed to be used on tankers, all electrical connections, etc., would be situated on the safe side of a pump room bulkhead, pneumatics for shut down of ballast discharge, etc., can be arranged in the pump room. Advantages of the system are measurement is direct and the response is instaneous, it is safe, rugged and accurate (better than 3% and can detect 1 to 10 ppm of oil in water).

(174) How is accuracy of measurement enhanced by instrumentation?

Answer:- In all measuring devices the accuracy is enhanced by magnifying the displacement of the sensor. A very common example is the difference in measurement of small lengths using a vernier caliper and a micrometer. In the Vernier caliper the least count is .0.1 mm, whereas in the micrometer it is .001 and it is further magnified to a micron in the dial gauge by combining the measuring device with suitable gearing . But these mechanical devices of magnification have their inbuilt inaccuracies caused by lost motion (slippage) as well as friction of

parts in contact. To overcome this inaccuracy if the magnification is converted to equivalent elecrtrical quantity the required magnification and accuracy are obtained.

Consider a weighing machine using a conventional coiled springs as the resisting force against weight and measurement recorded by a pointer operated by a suitable rack and pinion gearing.

The same arrangement can be recorded by the movement of two capacitor plates connected to a DC voltage which will provide a more accurate and magnified reading. If the weighing scale is meant to weigh 0 to 100 kg, the conventional scale may give readings in 1kg divisions , whereas the electronic scale will give readings with divisions of 100 gms.

(175) Describe how a distance boiler water level sensor works.

Answer:-the boiler water level sensor is shown in sketch depicted below. Steam condenses in the reservoir maintaining a constant level therein. The pressure difference h act on some form of differential pressure device (it could be a diaphragm type ) Which varies an air pressure signal which in turn would be used in a control system. A balance leg is provided in order to avoid extreme pressure differences on the differential pressure device in the event of the terminal value on the boiler being closed.

REFRIGERATION AND AIR CONDITIONING.

(176) With reference to a domestic refrigeration system, state two causes of each ofthe following conditions, stating how each cause may be rectified:a) Excessive discharge pressure b) Low pressure at evaporator suction

c) Compressor cycling d) Compressor running for long periods but not lowering the temperature in the cold rooms ,e) Compressor operating noisily

Answer:- (a)Causes for excessive discharge pressure are:(1) Air in the system.(2) Condenser cooling not effective

Remedy:- (1) Air to be purged out from the gas. All the gas is collected in the receiver after condensing . compressor is stopped and air is purged out from the purging cock on the receiver. (2)Condenser tubes to be de-scaled and cleaned. (b) causes for Low pressure at evaporator suction. (1) ice formation on evaporator coils. Hence heat transfer to the refrigerant is retarded. (2) Expansion valve defective , more liquid going into the evaporator causing reduction in initial dryness fraction Remedy:- (1) Defrosting the coils by hot gas circulation or other means. (2)Changing the expansion valve if defective or renewing the thermostat element if defective. (c)Causes for compressor cycling. (1) gas undercharged. (2)Moisture in the gas so effective cooling by gas reduced Remedy:- (1) charge the system with extra gas. (2)Remove moisture from the gas by changing the drier element(silica gel) (d)Causes of Long hours of running in cold room. (1)Defective or damaged insulation in cold room. (2) heavy ice formation on evaporator coils. Remedy:- (1) Repair the insulation.

(3) Defrost the cold room by hot gas circulation or other means.

(e)Causes of compressor operating noisily.

(1) Worn bearings (2)Slack belt drive. Remedy:-

(1) Compressor to be overhauled.Tighten the belts or renew belts if they are worn heavily.

(177) With reference to the lubrication of refrigeration compressors:a) State the advantages of using synthetic oils b) Explain why oil may be carried over from the compressor c) Describe a device which returns oil from the compressor discharge to the compressor sump d) State two reasons why an accumulation of oil in the evaporator is undesirable.

Answer:-

(a)Lubricants perform several functions in a compressor system. Of course, they must be able to lubricate the machine. Compressor lubricants are often a specialized blend of additives and base oils in order to provide the necessary lubricating properties while still being compatible with the refrigerant. Any incompatibility of the base oil and the refrigerant could have disastrous results for the equipment.The majority of compressor lubricants are synthetic. This allows them to have a longer service life and handle the rigors of the system better than mineral-based fluids. Although the compatibility between the refrigerant and the lubricant is perhaps the most pressing issue in terms of lubrication, there are many others as well. For instance, moisture contamination can be very detrimental to some synthetic base oils that are

hydrolytically unstable. Moisture reacts with the base oil to form acids, change the viscosity and impair the oil’s lubricating properties. This can lead to premature compressor failure as well as improper system cooling.Lubricant problems are common in any system. One way to avoid having issues with the gases being compressed is to simply remove the lubricant from the equation. This is a frequent occurrence with the “dry” compressors becoming more widely used. “Dry” refers to the lack of oil in the compression chamber. If the lubricant is not in the compression chamber, it is much less likely to mix with the refrigerant and cause problems. However, in wet or flooded compressors, the oil is present in the compression chamber and mixes intimately with the refrigerant. In these systems, the lubricant’s compatibility with the refrigerant is paramount.(b) In a refrigeration compressor of the reciprocating type, the sump lub-oil is also used for the cylinder lubrication, and the oil sticking to the cylinder walls is carried over with the pressurized refrigerant. Although efficient separators are fitted on the discharge line , yet some oil gets carried along with the refrigerant.(C)

Oil separators: Oil separators as sketched above is of the impingement type , fitted in hot gas discharge lines from the compressor. The type shown, is a closed vessel fitted with a series of baffles or a knitted wire mesh through which the oil-laden vapour passes.The reduction in velocity of the vapour as it enters the larger area of the separator allows the oil particles,which have greater momentum, to impinge on the baffles. The oil then drains by gravity to the bottom of the vessel where a float valve controls flow to the compressor crankcase. The chief disadvantage of separators placed in the hot gas discharge line is the possibility of liquid refrigerant passing from the separator to the compressor crankcase when the compressor is stopped. In order to minimize this risk, the separators should be placed in the warmest position available. It is good practice to drain the oil from the separator into a receiver containing heating elements, where the liquid refrigerant boils off to the compressor crankcase through a float valve.

(d) (1) Oil is a bad conductor of heat and if it sticks to the walls of the evaporator, heat flow to the refrigerant is impaired.

(2)If heat flow is impaired the refrigerant does not evaporate completely and can be a wet vapour at the evaporator end or at the compressor suction, which is bad for the compressor.

(178) (a) With reference to a vapour compression refrigeration plant, explain whyeach of the following conditions are desirable:(i) Superheating at the compressor suction; (ii) Undercooling at the condenser outlet.

b) Describe with the aid of a Pressure- Enthalpy diagram, how the evaporator cooling load is affected by the conditions stated in Q6(a).

Answer :- (a) 1 Superheating at the compressor suction is desirable because it ensures that the vapour at the end of the evaporator coil and entering the compressor is completely dry so that liquid hammer at the cylinder head is avoided, when the piston is on the upward stroke. (2) Under cooling at the condenser end reduces the temperature of the liquid, so that when throttling at the expansion valve occurs , its heat absorption capacity is increased , thereby more heat is extracted from the mass to be cooled and results in lower temperature of the body.(b)The thermodynamic effect of superheat at the suction results in extra work done by compressor to extract the heat taken in by superheating the gas at suction , but this is necessary to safeguard the compressor.

(179) a) Sketch a diagrammatic arrangement of a fully automatic direct expansion domestic refrigeration system which supplies a number of cold rooms. b) State with reasons, Five desirable thermodynamic properties of a refrigerant

Answer:-

(b) Desirable Properties:-

1. Moderate condensing pressure, obviating the need for heavily constructed compressors, condensers and high pressure piping.

2. High critical temperature, as it is impossible to condense at a temperature above the critical, no matter how much the pressure is increased.

3. Low specific heat of the liquid. This is desirable as throttling at the expansion valve causes liquid refrigerant to be cooled at the expense of partial evaporation.

4. High latent heat of vaporization, so that less refrigerant may be circulated to perform a given duty.

TO VEG ROOM

TO MEAT ROOM

FROM VEG ROOM

FROM MEAT ROOM

5. The refrigerant should be non-corrosive to all material used in the construction of the refrigerating machinery and system and It should be stable chemically.

(180) With reference to ships air conditioning plant:a) Define ·the term comfort zone; b) State the objective of maintaining the conditioned air within the comfort zone; c) State with reasons, FIVE areas from which conditioned air must not be recirculated.

Answer:- (a) The comfort zone is the area indicated in the psychrometry chart, bounded by the upper and lower limits of the RH 60% and RH 40% respectively (relative humidity) and the higher and lower limits of temperature 280C and 230C respectively. This is an average demarcation and can vary from person to person. (b) The objective of air conditioning is to maintain the temperature and relative humidity of air within an enclosed space to conform and be within the limits of the comfort zone. And this is done as follows: Summer and in Tropics: (1) The air entering the AC plant has an

RH OF 70% and a temperature of 400C. The air is cooled down to about 260C and this results in condensation of the humidity from the air as its temperature is reduced. The air entering in is now at about 270C and 50% RH.

(2)Air enters at 400C and RH 30%: The air is cooled down to 260C and water sprayed in to increase humidity. This results in a RH 50% and temperature 270C.

(3) Winter : Air enters at 00C and RH 70%. By heating the air to 400 C and cooling down to 250C,the humidity in the air is reduced to about 50%. The air entering the space is at about 260C and RH 45—50%

(c) The five areas where recirculation is not done are :

(1) toilets and (2) wash rooms from where bacterial infection can spread. (3) dining rooms and(4) galleys for the same reasons. (5) hospital to prevent and spread bacterial activity.(181) With reference to a ship's air conditioning system:a) state the effects of each of the following faults:i) corroded return air trunkings ii) wasted condenser division plate iii) blocked evaporator drains; iv) defective capacity control b) State the main health danger that may arise in the air conditioning plant and the measures taken to prevent it occurring

Answer:- (a) (1) Corroded air return trunkings may be holed , whereby the suction from these trunkings is reduced and more fresh air is taken in the blower causing a load increase on the AC compressor with loss of cooling effect. In addition it is also a health hazard by fungi from corrosion entering the air. (2)Wasted condenser division plate will result in loss of cooling effect of the condenser and the vapour may not be completely liquefied resulting in poor cooling effect and ultimately short cycling of the machine. (3) If evaporator drains are blocked water accumulates in the evaporator causing and results in increase of humidity in the room, and can also be a health hazard (4) Defective capacity control is caused by a defective thermostat. This results in short cycling of the machine and cooling not properly controlled.(b)The main health hazard in the AC plant is the breeding of the legionella bacteria which causes pneumonia to persons inhabiting such areas where the AC machines are

not properly cared for. Legionella bacteria breeding can be prevented by regular periodical cleaning and blowing through the evaporator filters and ensuring that the drains are not blocked .Finally periodical disinfecting the filter elements.

(182) With reference to air conditioning systems:a) Explain the conditions that will allow Legionella bacterium to proliferate b) Explain the three methods that will enable Legionella to enter the human body c) State four symptoms that may indicate a person has contacted Legionnaires disease d) State the conditions that may render chlorination to be ineffective or lessefficient in the treatment of legionella in air conditioning systems

Answer:- Legionella bacterium will proliferate in damp surroundings with stagnant water accumulation at ambient temperature . The evaporator coil casings and the air filters are the ideal breeding spaces for this bacterium.(b) The three methods by which legionella bacteria can enter the human body are: (1) by consuming water infected by legionella. (2) By breathing damp air infested with this bacteria. (3)Hand to mouth contact after touching objects and surfaces infested by this bacteria.

(c) The four symptoms are : (1) high temperature. (2) cough. (3) joints aching. (4) cold.(d) Chlorination at 50ppm normally carried out in water storage tanks is effective , but the source of breeding

legionella is the AC system where treatment by chlorine is not possible .For chlorination to be effective the water has to be stored in a tank , whereas the evaporator casing is not a water storage space , on the contrary it is to be maintained dry. Hence chlorination will not help

(183) With reference to refrigeration systems define each of the following termsa) latent heat(enthalpy) b) vapour pressure c) saturation d) superheat e)sub cooling f) zeotropic blend g) azeotropic blend h) temperature glide

Answer:- The answers given below are in reference to Refrigeration and Air conditioning

(a) Latent heat is the heat required to change the phase from liquid to vapour at a given pressure. Enthalpy is the total heat at a given state of the liquid or vapour. (b)Vapour pressure is the pressure exerted by the vapour at a given temperature. (c) With reference to Air conditioning the word saturation means the maximum limit for the air at a given temperature and pressure to absorb water vapour. This point is also called 100% relative humidity. If atmosphere contains 100% relative humidityat agiven pressure and temperature, and the temperature drops down, rain fall occurs because the air now rejects the excess water vapour in relation to the new temperature. (d)Superheat is the temperature above the boiling point of a liquid at a given pressure, when all the liquid is converted to vapour.

e)Subcooling is the temperature below the boiling point at which state the liquid after change of phase from vapour is existing.

(f)Azeotropic blend is a mixture of two or more different vapours having a common boiling point which is different from each of the vapours forming the blend .

g) A zeotrope is a blend of two, or more, gases which when mixed compose a blend that has two, or more, boiling temperatures (depending upon the number of substances). When boiling the more volatile substance evaporates first, modifying theblend composition locallyh) Glide is the temperature difference of the boiling temperatures of blendedsubstances in a zeotropic blend

(184) With reference to reciprocating compressors for refrigeration duties:a) Describe the relief and unloading arrangements; (2)b) Explain why. relief and unloading arrangements are provided; (4)c) A refrigerating compressor controlled on stop/start is observed to be running excessively. On inspection the cut out is observed to be in order and the system is not short of gas and there is no air in the system. State four reasons for the excessive running of the machine. (4)

Answer:- (a) The valve assembly is show in figure in more detail. The delivery valve is held in place by a safety spring which is fitted to allow the completed valve to lift in the event of liquid carry over to the compressor. The delivery valve is an annular plate with its inside edge seated on the mushroom section and its outside edge on the suction valve housing.

The suction valve passes gas from the suction space around the cylinder .

The control system include a high pressure cut-out which will cut off the compressor if the delivery pressure exceeds the set value. A safety bursting disc is also fitted between the compressor discharge and the suction. This may be of nickel with a thickness of 0.05 mm. A ruptured disc is indicated by suction and discharge pressure equalizing.

(b) Reasons:

(1) the gas is not sufficiently cooled , due to highly scaled condenser tubes.

(2) Air mixed with gas in the system.

(3) Moisture in the gas causing blockage of expansion valve by ice formation..

(4) Ice formation on evaporator coils preventing heat transfer to the gas passing through the evaporator pipes.

(185)a) From an interpretation of the instrumentation, explain why a high pressure cut out in a vapour compression refrigeration system may operate under each of the following conditions:(i) Overcharge; (2)(ii) Air in the system; (2)(iii) Lack of coolant; (2)(iv) Incorrect refrigerant. (2)b) If the high pressure cut out has been tampered with and the settings altered, describe how it may be re- adjusted when no instruction manuals are available.

Answer:- (a) (1)If the gas is overcharged , the space in the receiver for the gas is reduced which leads to gas accumulation and subsequent rise in pressure causing the HP cut-out to operate. (2)Air is an additional gas in the constant volume space and as per Daltons law of partial pressures it will contribute its pressure which becomes the total pressure in the system causing an increase in pressure which will operate the HP cut-out. (3) Lack of coolant will result in higher vapour temperature without condensing resulting in higher pressure causing the HP cut – out to operate. (4) The incorrect refrigerant may have a higher temperature when leaving the condenser and hence cooling may be insufficient causing a higher resultant pressure, which will operate the HP cut-out.

(b) Find out the highest expected sea temperature likely to be, allow a margin ofsay 8°C, this would give a condensing temperature. Use this to find the correspondingpressure of the gas. Allow a safety margin of say 10% and set the cut out at that.Contact manufacturers as soon as possible to obtain correct figure.

(186) With reference to indirect expansion refrigeration systems:a) Explain what is meant by a secondary refrigerant; b) State a suitable fluid for a secondary refrigerant; c) Explain the possible consequences if the density of the secondary refrigerant is:(i) too low; (ii) too high. d) State the reason why different streams of a secondary refrigerant may be included in the system.

Answer:- In direct expansion systems the cooling is conveyed through circulated air and that is sufficient for moderately sized cooling chambers. In large chambers especially refrigerated cargo carriers a secondary medium for conveying the cooling effect is necessary for better and more effective cooling.

(b) The common secondary medium is brine which is a solution of calcium chloride in fresh water and its speciality is that it will not freeze at low temperatures. it is cheap and hence economic (c)(1)If the density of the secondary refrigerant is too low it may freeze. (2)If it is too high it may precipitate and cause blockage of the coolant flow. It can also be corrosive if the density is high. (c) Different streams of secondary refrigerants are needed when different substances are maintained at different temperature levels and hence the secondary streams of

the refrigerants are to be maintained at their corresponding temperatures.

(187) With reference to the lubrication of a reciprocating Freon refrigeration compressor:a) State with reasons, the characteristics required by the lubricating oil; b) Discuss each of the following with respect to cause and remedy:

(i) Copper plating; (ii) Freon cloud point; (iii) Freon floc point.

Answer:- (a) The characteristics required for a good lubricant is as follows: (1)It should have a good viscosity. (2)It should have a very low pour point (3) it should be chemically and thermally stable (4) It should have dispersive quality.Reasons ;- High viscosity is essential for any lubricant to reduce friction and carry away the heat of friction. The oil comes in contact with the refrigerant both in the liquid and vapour state and it should not chemically react with the lubricant. Hence chemical stability is required. Thermal stability is essential since the lubricant temperature varies from very low to high temperature and during these transformations the lubricant should not undergo any chemical change. The lubricant must not dissolve impurities in it as that will change the quality of the lubricant.(b)(i) If moisture is introduced into refrigeration systems using Freon refrigerants, it can cause complex chemical reactions between the oil, gas and water resulting in the gradual displacement of copper from copper surfaces. These are

deposited on the liner and bearing surfaces at high temperatures as a copper coating.,To prevent this, the oil must be of high chemical stability and moisture must be prevented from entering the system. Driers should be employed to remove any moisture that does enter.(ii) Some of the refrigerant will dissolve in the lubricant and change its properties. The Freon Cloud Point, is the temperature at which waxy particles solidify to give a cloudy appearance to oil in refrigerant. A low cloud point is desirable for refrigeration lubrication, in which waxy particles might otherwise settle out. Thus the oil chosen must be compatible with the Freon to give an acceptable cloud point(Hi) With refrigerating oils used with Freon an additional test is carried out,This is called the floc point test. This test shows how the oil flows at low temperatures in the presence of arefrigerant . If the floc point is too high oil iscarried over with the gas as wax and deposit on cooling coils.