In short heat can’t flow from low temp to high temp ... · Bimetallic strip is in contact for normal working due to excessive current, it gets heated bends, results in opening the

MAHARASHTRA STATE BOARAD OF TECHNICAL EDUCATIOD(Autonomous)

(ISO/IEC-27001-2005 Certified)

Summer – 12 ExaminationsSubject Code : 12131 Model Answer Page No : 1 /25

Q. 1 a) (i) Write statement of Second law of Thermodynamics ( Marks Allotted-4 Marks)

Answer – It is stated by Clausius and Kelvin Clausius statement

No process is possible whose sole result is the transfer of heat from a body of lower

temperature to a body of higher temperature.

Spontaneously, heat cannot flow from cold regions to hot regions without external work being

performed on the system, which is evident from ordinary experience of refrigeration, for example.

In a refrigerator, heat flows from cold to hot, but only when forced by an external agent, a

compressor.

Kelvin statement

No process is possible in which the sole result is the absorption of heat from a reservoir and

its complete conversion into work.

This means it is impossible to extract energy by heat from a high-temperature energy source

and then convert all of the energy into work. At least some of the energy must be passed on to heat a

low-temperature energy sink. Thus, a heat engine with 100% efficiency is thermodynamically

impossible. This also means that it is impossible to build solar panels that generate electricity solely

from the infrared band of the electromagnetic spectrum without consideration of the temperature on

the other side of the panel

In short heat can’t flow from low temp to high temp, without any external source.

Note – Each statement –02 marks. Any two statements are required.

Q. 1 a) (ii) Sketch diagram of Practical Ammonia – Water Vapour absorption system. Label it

Vapor Absorption Refrigeration System:











compressor.

Kelvin statement























compressor.

Kelvin statement
















or similar Fig

Note – For diagram 2 marks and for labeling the parts 2 marks

Q. 1 a) (iii) what is "Eco-friendly refrigerants”?

Eco-friendly refrigerants, means which helps to save the environment while keeping cool.Running your home's AC unit this summer will not only keep you comfortable, but it can also help savethe environment if it contains the latest friendly refrigerant. The Freon family refrigerants areresponsible for chain reaction, which leads to destroy ozone layer.

Most air conditioners use refrigerant R-22, which adds to the problem of global climate change.In order to avoid this problem, outdated air conditioning unit may be replaced by using eco-friendly R-410A , R-134a refrigerant.

Note – Explanation Eco-friendly refrigerants – 3 marks and Name of Eco-friendly refrigerants – 1Mark.




or similar Fig









or similar Fig









Q. 1 a) (iv) Sketch and explain construction and working of solenoid valve in Refrigeration

or similar Fig

A solenoid valve is an electronically operated device. It is used to control the flow of liquids orgases in a positive, fully-closed or fully-open mode. The valve is commonly used to replace a manualvalve or where remote control is desirable. A solenoid is operated by opening and closing an orifice ina valve body that permits or prevents flow through the valve. The orifice is opened or closed throughthe use of a plunger that is raised or lowered within a sleeve tube by energizing the coil. The bottom ofthe plunger contains a compatible sealing material, which closes off the orifice in the body, stoppingflow through the valve.

The solenoid assembly consists of a coil, plunger, and sleeve assembly. In a normally closedvalve, a plunger return spring holds the plunger against the orifice, preventing flow through the valve.When the coil is energized, a magnetic field is produced, raising the plunger and allowing flow throughthe valve. In a normally open valve, when the coil is energized, the plunger seals off the orifice,stopping flow through the valve.

Note – Sketch and labeling of parts – 2 marks and construction and working 1 mark each.

Q.1 (b) (i) Compare Central Air-conditioning direct expansion type system with package aircondition system. (Note: 01 Marks to each point)

S.NO Direct Expansion (DX) Type CentralAir. Condition System

Package Air Condition System

1 Cooling Space is directly chilled byrefrigerant

Cooling Space is indirectly chilled byrefrigerant





or similar Fig













or similar Fig









2 Efficiency of this system is higher Efficiency of this system is less3 Not feasible to carry refrigerant to large

distanceUnit can be placed directly near thecooling area



Summer – 12 ExaminationsSubject Code: 12235 Model Answer Page No : 4 /25

S.NO Direct Expansion (DX) Type Central Air.Condition System


4 Used for cooling the small buildings or the

rooms on the single floor.

Used for cooling hall or big areas.

5 Higher installation Cost. Lower installation cost.

6 Maintenance is costly. Maintenance is less.

7 Assembled at site. Assembled in factory.

8 Higher capacity Lower capacity

Q.1 (b) (ii) (Marks Allotted: 6 Marks)

Yes (2 Marks)

It humidifies the air without the use of ordinary wick filters. Inside the air washer, specialhumidifier discs rotate through a water bath. Humidification is performed using the self-regulating coldevaporation principle.

In an absorption dehumidifier moisture is extracted in a sorbent material. sorbent materialcontains a vast number of microscopic pores in which water is absorbed

Advantages as humidifier – (2 Marks)

1. Air is washed and cleaned from dust, pollen and household odors.2. Air is cleaned naturally, safely and quietly.3. Humidification is performed using the self-regulating cold evaporation principle.

Advantages as dehumidifier – (2 Marks)

1. The air is dehumidifies by removal of moisture by heating.2. The temperature of water is lower than the dew point of the air flow.3. In dehumidifier the airflow passes a cooled surface.
















Yes (2 Marks)






















Yes (2 Marks)










Q.2 a) Flooded type Evaporator :- (Marks Allotted: 4 Marks)

or similar Fig (2 Marks)

It consists with float control valve, float chamber, float valve & Accumlator. (2 Marks)

Level of liquid refrigerant is controlled (constant) in accumulator & in float chamber. Only liquid refrigerant supplied to evaporator by gravity. At the end of Evaporator, there is mixture of refrigerant (liq+vapour).It strikes on

baffle plates and there is separation of liquid & vapour refrigerant, due to densitydifference

Only vapours are supplied to compressor.

Advantages of Flooded Type Evaporator:

Evaporating coil is always full with liquid refrigerant, irrespective of load condition. Henceheat transfer rate is good leads to better cop.

Q.2 b) (Marks Allotted: 4 Marks)

Working of Overload Protector :- (2 Marks)

To protect compressor motor winding from damage to excessive current due to any fault incircuit or overload.





























Bimetallic strip is in contact for normal working due to excessive current, it gets heated bends,results in opening the circuit--- gives protection to motor winding.




Sketch of Over load Protector :- (2 Marks)

or similar Fig

Q.2 c) (Marks Allotted: 4 Marks)

RSHF = Room Sensible Heat / Room total Heat (2 Marks)

= RSM / ( RSH + RLH )

GSHF = Total Sensible Heat / Grand total Heat (2 Marks)

= TSH / GTH = TSH / TSH + TLH

= ( RSH + OASH) / ( RSH + OASH) + (RLH + OALH)






or similar Fig












or similar Fig










Q.2 d) (Marks Allotted: 4 Marks)

Air Extraction System of Ventilation :- (2 Marks)

Supply of Fresh air in the room in sufficient rate & removing of used hot air to outside theroom.

It depends on : (2 Marks)

Extraction rate – e.g. ranges 30 L/s to 150 L/s Fans – polluted & hot air extracted. Duct work- Inside or outside the room. Extract Location – Near ceiling. Deleted heat transfer system - Insulated

Q.2 e) (Marks Allotted: 4 Marks)

Air Refrigeration System :-

or similar Fig. (2 Marks)

Methods: i) Simple air cooling system ii) Simple air evaporative cooling system etc.

























Working of Simple air cooling system :- (2 Marks)

Used for Air craft. Main Components- Compressor (driven by gas turbine),Heat exchanger, Cooling turbine

cooling air Fan




Air required for refrigeration system is bled off from main compressor. This high pressure andTempt air cooled in HE where ram air is used for cooling.

It is further cooled in Gas T due to expansion process. Work of turbine is used to drive coolingfan which draws cooling air through heat exchanger.

System is good for ground surface cooling and for low flight speed.

Q.2 f) (Marks Allotted: 4 Marks)

Industrial a/c System: Functions: (2 Marks)

i) To provide comfort to workers.ii) To Control Tempt & RH.

iii) To preserve food storage & transport.iv) To maintain clinic atmosphere.

In Industry a/c tempt & RH is very important. eg

S.No. Industry Temp. RHA Baking processes :

i) Cake Icingii) Mixing

2124

5055-70

B Electrical :i) Insulation Windingii) Covered wires

4015-26

560-70

C Foundries Moulding 15-21 -D Paint drying 15-32 25-50

e) In MQC lab for precision measurement specific tempt is required.

f) In textile industry RH is very important etc

(any Two application - 2 Marks)



cooling air Fan














2124

5055-70


4015-26

560-70







cooling air Fan














2124

5055-70


4015-26

560-70







Summer – 12 ExaminationsSubject Code: 12235 Model Answer Page No: 9 /25

Q-3 (a) (Marks Allotted: 4 Marks)

i) Crankcase electric heater: (2 Marks)

This is provided to keep the compressor oil warm during the idle period. The oil has to be keptwarm as it absorbs gas and rate of absorption increases as temperature of the oil falls. The absorption ofgas dilutes the oil, resulting in poor lubrication and premature wear of bearing and journals. This willadversely affects the volumetric efficiency of the compressor. So before starting the compressor itshould be ensured that crankcase electric heater is working.

(ii) Foaming of oil : (1 Marks)

If crankcase pressure falls below that of the evaporator, refrigerant vapour from the evaporatormigrates to the oil in the crankcase during idle period. On the start up of compressor, crankcasepressure suddenly reduces. Hence some liquid refrigerant flashes and this violent boiling of the liquidcauses foaming and thinning of oil.

Effects: (Any Two effect - 1 Marks)

1. Foaming reduces the oil viscosity and hence its lubricating capability.

2. Foaming on entering the oil pump blocks the entry of adequate oil necessary to buid-up the requiredoil pressure.

3. Foaming shows misleading level of oil on the oil sight glass.

4. Foaming creates slugging, resulting in damage to the components of the compressor.

Q-3 (b) (one mark for each point 1x4= 04 marks)

S.No Open type Compressors Hermetically sealed type1 ) Construction: (Any One)































1 Crankshaft extends through thecompressor i. e. simple

Compressor and motor have acommon shaft i. e. Complicated

2 Housing and is connected to the drivingmotor.

And are enclosed in one housing.

3 Unit is bulky. Unit is Compact.




S.No Open type Compressors Hermetically sealed type2 ) Sealing :

1 Around the shaft of compressor, shaftseal is Necessary to prevent leakage ofrefrigerant

No need to use a crankshaft seal.

3 ) Cost :1 High initial cost Low initial cost.

4 Noise level1 Noise levels are high. Noise levels are low. Quit in

operation.

Q-3(c) Representaion of psychrometric priocess (Marks Allotted: 4 Marks)

i) Latent heating- It is a process of humidification. (1 mark)

or similar Fig.

ii) Heating with humidification (1 mark)














operation.



or similar Fig.















operation.



or similar Fig.


or similar Fig.




iii) Cooling with dehumidification :- (1 mark)

or similar Fig.

iv) Adiabatic chemical dehumidification:- (1 mark)

or similar Fig.





or similar Fig.


or similar Fig.





or similar Fig.


or similar Fig.

Q-3(d) (Marks Allotted: 4 Marks)

Hand expansion valve :- (1 mark)

In hand expansion valve there is an orifice or valve port which gives resistance to the liquidflow thereby producing necessary pressure drop. The conical shaped needle valve extends down intothe valve port and resists the flow area through the port. Valve rests on its conical seat when it isclosed.




or similar Fig. (1 mark)

Advantages: - It is most simple . (1 mark)

or similar Fig.









or similar Fig.









Disadvantages: - Requires an operator to regulate the flow of refrigerant . (1 mark)

Suitable only where the load is constant.

Uses:- Used in ice making plants and cold storage.

Q-3(e) (Marks Allotted: 4 Marks)

Properties of insulating material: - each point one mark 1 x 4 = 4 marks (at least four points)

1. Should have low thermal conductivity.

2. Should be resistive to moisture

3. Should have good mechanical strength.

4. Should be fire proof

5. Should be easiness in lying.




Q-4 (a) i) (Marks Allotted: 4 Marks)

1) Oil safety switch :- (1 mark)

or similar Fig.
















or similar Fig.
















or similar Fig.

It is provided to stop the compressor when the net oil pressure of the compressorlubrication system falls below the safety limit to protect the bearing. This is used in the compressorswith forced feed lubrication. Time delay device is incorporated in the switch. (1 mark)

2) Pressure equalizer : (2 mark)

or similar Fig.




Q. 4) a) ii) (Marks Allotted: 4 Marks)

1. Product load : (2 mark)

The heat emitted from the products to be stored is product load. The product includes foodstuff, medicine etc. Product load includes the heat removal from the product at the time of initialloading and also the heat produced by product during storage. Heat to be removed depends upon themass of product, its mean specific heat, entering product temperature, final product temperature desiredand storage time.

2 Miscellaneous load :- (2 mark)

This includes-

Duct heat gain-ducts running to the air conditioned room adds sensible heat .



or similar Fig.








This includes-




or similar Fig.








This includes-


Fan load -power consumed by the fan is converted into heat and is given to the air.

Moisture gain-water vapour will enter in to the conditioned space through walls and ceiling insummer, as vapour pressure of outdoor air is greater than that of conditioned space. This adds latentheat into the room.

iii) Domestic refrigerator : - (Marks Allotted: 4 Marks)Sketch : (2 mark)

or similar Fig.




OR

or similar Fig.




or similar Fig.




OR

or similar Fig.




or similar Fig.




OR

or similar Fig.

Explanation : (2 mark)

Domestic refrigerator provides food storage space maintained at low temperature and formsice cubes for domestic consumption..Capacity is usually specified by the internal gross volume anddeep freezer volume in liters.

Refrigerant used - R-12 or R-134a.

Expansion device - capillary tube

Condenser - Natural convection air cooled type.

iv) Vapour compression cycle : (Marks Allotted: 4 Marks)

On P-H diagram :- (2 mark)



Summer – 12 ExaminationsSubject Code : 12235 Model Answer Page No : 16 /26On T-S Diagram :- (2 mark)

1-1 = suction superheat3-3 = subcooling























Q-4b) (Marks Allotted: 6 Marks)

i) Compare:- (min. six points each one mark 1x 6 = 06 marks)

S.No Vapour compression cycle Vapour absorption cycle

1 Compressor is used Compressor is not used

2 Energy supplied is low Energy supplied is high.

3 Requires high grade energy i.e Mechanical or

electrical

Requires low grade energy i.e. heat as Input

energy.

4 System has more wear, tear and noise due To

moving parts like compressor.

System has no wear, tears and is quiet in

Operation as no compressor.

5 Poor performance at partial loads. Performance is not affected by load Variation

6 Traces of liquid in the suction line Damages

valves and other compressor Parts.

No such problem.

7 The system is used when no cheap source of

Energy is available.

It is used at spots where waste heat is

Available in large.

8 Cop is higher Cop is lower




ii) Adantages :- (min. six points each one mark 1 x 6 = 06 marks)

S.No Water cooled condenser Air cooled condenser

1 Lower power requirement per TR Higher power requirement per TR.

2 Are used for large capacity plants Used for low capacity plants.

3 Have high heat transfer capacity Low heat transfer capacity as air has lowThermal conductivity.

4 There is no fan noise, as no fan. There is fan noise

5 Complicated construction, initial cost is High, Simple in construction, low initial cost,







electrical


energy.








No such problem.




Available in large.


















electrical


energy.








No such problem.




Available in large.












maintenance cost is high low Maintenance cost.

6 Difficult to handle No handling problem.

7 Corrosion and fouling effect is high. No corrosion and fouling.

8 Heat transfer rate is high Heat transfer rate is low

Q-5 a) (Marks Allotted: 4 Marks)

i) Heat pump :- (1mark)

or Similar Fig.

It is work absorbing device operating in a cycle removes heat from low temperature body anddelivers it to high temperature body. It is used for heating in winter. Its performance is expressed interms E.P.R., energy performance ratio.

COP of heat pump = 1+COP of refrigerator




ii) Refrigerator:- ( 1mark )







or Similar Fig.













or Similar Fig.







It is a device which operating in a cycle maintains a body at a temperature lower than that ofits surroundings. It is used for cooling in summer. Its performance is expressed in terms of COP.

iii) COP :- (1 marks)

COP = RE / WS Legand – RE – Refrigerating effect WS – Work supplird.

iii) Tonnes of refrigeration :- (1 marks)

1 TR = 210 KJ / Min or 3.5 KJ/sec.

b) Properties of Refrigerant :- (min. four points each one mark =4 mark)

1. Should have low boiling and freezing point.

2. Should be non-toxic, non- flammable and non-corrosive.

3. Should be chemically stable, and should not have bad effects on stored products.

4. Should have low viscosity and specific volume.

5. Should have high thermal conductivity.

6. Should have high latent heat.




c) (Marks Allotted: 4 Marks)

Secondary refrigerants:- (2 marks)

Are the refrigerants which do not directly take part in the refrigeration system. They are firstcooled with the help of primary refrigerant and are further used for cooling purpose.Commenly usedsecondary refrigerants are water, brines and glycols.





































Uses: - (2 marks)

1. In all big and commercial and industrial refrigeration plants.

2. Used in the places where coils and pipes containing toxic refrigerant should be kept away fromthe load place.

3. When the temperature requirement of the different parts in a same building are to be varied.

d) Sling Psychrometer :- (Marks Allotted: 4 Marks)

Sketch:- (2 marks)

or Similar Fig.




Explanation :- (2 marks)

The sling psychrometer consists of the dry bulb and wet bulb thermometers fixed side by sideon a wooden or metal plate. This plate is attached to a wooden handle through a swivel connection sothat the complete assembly can be easily rotated or whirled. The wet bulb thermometer is fixed on aplate at a lower level than the dry bulb thermometer and its bulb is covered by a wet wick.

To find RH :-

Uses: - (2 marks)





Sketch:- (2 marks)

or Similar Fig.






To find RH :-

Uses: - (2 marks)





Sketch:- (2 marks)

or Similar Fig.






To find RH :-

To find relative humidity of air, psychrometer is whrilled round rapidly for about a minute untilthe reading of wet bulb becomes steady. Now note the DBT and WBT. By using psychrometric chart,locate the point where DBT and WBT lines intersects. Read now the value of relative humidity curvethat passes through this point.

or similar fig.

It can also be calculated from the chart, by knowing DBT and WBT.

e) Various types of grills and resisters:- (Marks Allotted: 4 Marks)

Grills :- (2 marks)

or similar fig.



Summer – 12 ExaminationsSubject Code : 12235 Model Answer Page No : 21/25

Grills : - A decorative covering for the duct outlet is known as grill. The grill is rectangular outletopening consisting of vertical and horizontal perforations, ie vanes or louvers which may be fixed oradjustable. Grill can be located in floor, high side of wall or in the ceiling.

Resistors :- (2 marks)

When grill is provided with damper then it is called resister. The damper fitted controls the massflow rate of air. It can be located in floor, high side of wall or in the ceiling.


or similar fig.



Grills :- (2 marks)

or similar fig.








or similar fig.



Grills :- (2 marks)

or similar fig.







Fixed type resister is a resister with stationary louvers that are factory set to direct the air in aspecifig unchangeable pattern.

or Similar fig.

(Any one2 marks)

Adjustable resister are provided with movable bars or louvers which are horizontal orvertical or both. Direction change or upward or downward air flow is possible.

or Similar fig.




f) (Marks Allotted: 4 Marks)

List various types of condensers:- ( 3 Marks)

1. Air cooled condensers-i) Natural convection type

ii) Forced convection type


or Similar fig.

(Any one2 marks)


or Similar fig.









or Similar fig.

(Any one2 marks)


or Similar fig.








2. Water cooled condensers- i) Tube-in-tube type

ii) Shell-and-coil type

iii) Shell-and-tube type.

3. Evaporative condensers.

Air cooled tube type condenser is used in the household refrigerator: ( 3 Marks)

1. This condenser is cheaper.

2. Wire welded to condenser tubes increases heat transfer area, hence effective condensation.

3. This type of condenser requires very little maintenance.

4. Condenser is mechanically strong as wire provides mechanical strength.

Q.6 a) (Marks Allotted: 4 Marks)

or similar Fig. ( 2 Marks)




H.P Cut off :- Used as safety control. When discharge pressure exceeds, the power supply to motor isoff. Hence compressor is in off condition. ( 1 Marks)

L.P Cut off :- Used as safety control. & AS tempt control .When suction pressure reduces belowpressure determined value power supply becomes off. Hence motor & compressor is in off condition.

( 1 Marks)

















( 1 Marks)

















( 1 Marks)

b) (Marks Allotted: 4 Marks)

It is observed that from window AC water (wonder safe) drips because tempt. of Evaporatingcoil is less than dew point Tempt of surrounding air. ( 1Marks)

When surrounding air flows over it water particles in air gets condensed, collected in tray &drips to outer side of room. ( 1 Marks)

Flow rate changes as per season. ( 1 Marks) In summer RH is higher ,while in winter it less. Hence rate of condensation charges. ( 1 Marks)


i) Natural Ventilation :- ( 1 Marks)

The process of supplying fresh air in the room removing hot used air to out side the room,without using mechanical system.

There are a Two Following Methods :- ( 1 Marks)

i) Wind driven Method :- Difference in Static Pressure creates wind flow due to global &(thermal) tempt difference.

ii) Dynamic Pressure :- When wind comes into contact with hill or building.

ii) Buoyancy –driven :- ( 1 Marks)

Occurs due to density difference between inside air & outside air.

Warmer air Lower density moves in upward direction from top must be exhaustedto atmosphere.




ii) Buoyancy – driven advantages:- ( 1 Marks)

Needed in summer. Stable air flow. Greater control in choosing area of air intake. Sustainable method.






































Factors to be considered for selection of insulating material for air ducts and ceiling.

Thermal conductivity must be low. (Any Four 4 Marks) Strength must be good. Permanence (disintegration of material due to chemical reaction when exposed to

atmosphere) must be good. Must be light weight. Water repellent : (moisture absorption increases thermal conductivity) must be low. Odorless. Fire-proof. Low cost.


i) Refrigerant used in system :- R12 or R13 ( 2 Marks)

or Similar Fig.




ii) Capacity of System : Q = Mw Cpw ( Ti - To) ( 1 Marks)

Where Mw – Rate of water consumption.

Cpw – 4.187 = Space heat of water.

Ti – Inlet Tempt of water.







or Similar Fig.














or Similar Fig.








To - Outlet tempt of water.

iii) Type of evaporator :- (Bare type coil) plate type Evaporator ( 1 Marks)

iii) Condenser :- Forced air cooled finned type condenser

_________________________

Documents

In short heat can’t flow from low temp to high temp ... · Bimetallic strip is in contact for normal working due to excessive current, it gets heated bends, results in opening the