UNIT-II AIR COMPRESSORS REFRIGERATION AND AIR CONDITIONING

Part-A (2 Marks)

• What is meant by single acting compressor?
• What is meant by double acting compressor?
• What is meant by single stage compressor?
• What is meant hy multistage compressor?
• Define isentropic efficiency
• Define mean effective pressure. How is it related to in power of an I.C engine.
• What is meant by free air delivered?
• Explain how flow of air is controlled in a reciprocating compressor?
• What factors limit the delivery pressure in reciprocating compressor?
• Name the methods adopted for increasing isothermal efficiency of reciprocating air compressor.
• Why clearance is necessary and what is its effect on the performance of reciprocating compressor?
• What is compression ratio?
• What is meant by inter cooler?
• Name four important properties of a good refrigerant
• What is the difference between air conditioning and refrigeration?
• What is the function of the throttling valve in vapour compression refrigeration system?
• In a vapour compression refrigeration system, where the highest temperature will occur?
• The vapour absorption system can use low-grade heat energy in the generator. Is true of false?
• Name any four commonly used refrigerants.
• Explain unit of Refrigeration.
• Why throttle valve is used in place of expansion cylinder for vapour compression refrigerant machine.
• What are the effect pf super heat and sub-cooling on the vapour compression cycle?

Part – B (16 Marks)

• Draw neat sketch of simple vapour compression refrigeration system and explain.

• Explain with sketch the working principle of aqua Ammonia refrigeration system.

• Explain with sketch the working principle of water-Lithium bromide refrigeration system.

• A refrigeration system of 10.5 tonnes capacity at an evaporator temperature of -12°C and a condenser temperature of 27°C is needed in a food storage locker. The refrigerant Ammonia is sub cooled by 6°C before entering the expansion valve. The compression in the compressor is of adiabatic type. Find 1. Condition of vapor at outlet of the compressor.2. Condition of vapor at the entrance of the Evaporator 3.COP &power required.

• A single stage single acting air compressor is used to compress air from 1 bar and 22°C to 6 bar according to the law PV1 .25 = C. The compressor runs at 125 rpm and the ratio of stroke length to bore of a cylinder is

1.5. If the power required by the compressor is 20 kW, determine the size of the cylinder.

• A single stage single acting air compressor is used to compress air from 1.013 bar and 25°C to 7 bar according to law PV 1.3 = C.The bore and stroke of a cylinder are 120mm and 150mm respectively. The

compressor runs at 250 rpm .If clearance volume of the cylinder is 5% of stroke volume and the mechanical
efficiency of the compressor is 85%, determine volumetric efficiency, power, and mass of air delivered per minute.

• A two stage singe acting air compressor compresses 2m3 airs from 1 bar and 20°C to 15 bar. The air from the low pressure compressor is cooled to 25°C in the intercooler. Calculate the minimum power required t run the compressor if the compression follows PV1.25=C and the compressor runs at 400 rpm.

UNIT – III CONDUCTION PART – A

• What is Fourier's Law of heat conduction?
• What is temperature gradient?
• What is coefficient of Thermal conductivity?
• Give some examples of heat transfer in engineering.
• Define Temperature field.
• Define heat flux.
• Define thermal Diffusivity.
• What is Lap lace equation for heat flow?
• What is Poisson's equation for heat flow?
• What critical radius of insulation;
• Give examples for initial'&; boundary conditions.
• What is a Fin?
• Define efficiency of the fin ..
• Define effectiveness of the fin.
• Give examples of use of fins in various engineering applications.
• What is meant by Transient heat conduction?
• Give governing differential equation for the one dimensional transient heat flow.
• What is Biot number?
• What is Newtonian heating or cooling process?
• Give examples for Transient heat transfer.
• What is meant by thermal resistance?
• What is meant by periodic heat transfer?
• What are Heisler chart?
• What is the function of insulating materials?

Part- B (16Marks)

• A pipe consists of 100 mm internal diameter and 8 mm thickness carries steam at 170°C. The convective heat transfer coefficient on the inner surface of pipe is 75 W/m2C. The pipe is insulated by two layers of insulation. The first layer of insulation is 46 mm in thickness having thermal conductivity of 0.14 W/m°C. The second layer of insulation is also 46 mm in thickness having thermal conductivity of 0.46 W/mC. Ambient air temperature = 33°C. The convective heat transfer coefficient from the outer surface of pipe = 12 W/m2C. Thermal conductivity of steam pipe = 46 W/m°C. Calculate the heat loss per unit length of pipe and determine the interface temperatures. Suggest the materials used for insulation.

• A long rod is exposed to air at 298°C. It is heated at one end. At steady state conditions, the temperature at two points along the rod separated by 120 mm are found to be 130°C and 110°C respectively. The diameter of the rod is25mmOD and its thermal conductivity is 116 W/m°C. Calculate the heat transfer coefficient at the surface of the rod and also the heat transfer rate.

• (i) A furnace wall consists of three layers. The inner layer of 10 cm thickness is made of firebrick (k =1.04 W/mK). The intermediate layer of 25 cm thickness is made of masonry brick (k = 0.69 W/mK) followed by a 5 cm thick concrete wall (k = 1.37 W/mK). When the furnace is in continuous operation the inner surface of the furnace is at 800°C while the outer concrete surface is at 50°C. Calculate the rate of heat loss per unit area of the wall, the temperature at the interface of the firebrick and masonry brick and the temperature at the interface of the masonry brick and concrete.

(ii) An electrical wire of 10 m length and 1 mm diameter dissipates 200 W in air at 25°C.
The convection heat transfer coefficient between the wire surface and air is 15 W/m2K. Calculate the critical radius of insulation and also determine the temperature of the wire if it is insulated to the critical thickness of insulation.

• (i) An aluminium rod (K =204 W/mK) 2 cm in diameter and 20 cm long protrudes from a wall which is maintained at 300°C. The end of the rod is insulated and the surface of the rod is exposed to air at 30°C. The heat transfer coefficient between the rod's surface and air is 10 Vl/m2K. Calculate the heat lost by the rod and the temperature of the rod at a distance

• (i) Define the Biot and Fourier numbers.                                                                                            (4)

(ii) What is meant lumped capacity? What are the physical assumptions necessary for a lumped capacity unsteady state analysis to apply?                                                                                             (4) (iii)A slab of Aluminum 5 cm thick initially at 200°C is suddenly immersed in a liquid at 70°C for which the convection heat transfer co-efficient is 525 W/m2K. Determine the
temperature at a depth of 12.5 mm from one of the faces 1 minute after the immersion. Also calculate the energy removed per unit area from the plate during 1 minute of immersion. Take P = 2700 bar, Cp
= 0.9 kJlkg. OK, k=215W/mK, ά = 8.4X 10-5 m2/s.(8)

• A composite wall is formed of a 2.5 cm copper plate (k = 355 W/m.K), a 3.2 mm layer of asbestos (k = 0.110 W/m.K) and a 5 cm layer of fiber plate (k = 0.049 W/m.K). The wall is subjected to an overall temperature difference of 560°C (560°C on the Cu plate side and O°C on the fiber plate side). Estimate the heat flux through this composite ~all and the interface

temperature between asbestos and fiber plate.                                                                                            (16)

• A steel tube k=43.26 W/mK of 5.08 cm 10 and 7.62 cm is covered with 2.54 cm of asbestos Insulation k=0.208 W/mK The inside surface of the tube receives heat by convection from a hot gas at a - temperature of 316°C with heat transfer coefficient ha=284 W/m2K while the outer surface of Insulation is exposed to atmosphere air at 38°C with heat transfer coefficient of 17 W/m2K Calculate heat loss to atmosphere for 3 m length of the tube and temperature drop across

UNIT – IV CONVECTION PART – A

• What is Convective heat transfer?
• Sketch formation of boundary layer and show laminar, transition & turbulent flow.
• Write down differential equation for Continuity of fluid flow.
• State Newton's law of cooling.
• Differentiate between Natural & Forced convection.
• State Buckingham's 1t'theorem.
• What is meant by Dimensional analysis?" ,
• Sketch boundary layer development in a circular pipe.
• What is Reynolds analogy?
• What is Colburn analogy?
• Define the Bulk temperature.
• Define velocity boundary layer thickness.
• Define thermal boundary layer thickness.
• Distinguish between laminar & turbulent flow.

• What is meant by critical Reynolds number?
• Define skin friction coefficient.
• Give examples for free convection.
• Define Grashof number.
• Sketch, temperature and velocity profiles in free convection on a vertical wall.
• Define momentum thickness.
• Define Displacement thickness.
• List the dimensionless numbers.
• What are the uses of dimensional analysis?
• Explain the term Dimensional homogeneity.
• What are the limitations of Dimensional analysis?

PART – B

01. Air at 200 kPa and 200°C is heated as it flows through a tube with a diameter of 25 mm at a velocity of 10 m./sec. The wall temperature is maintained constant and is 20°C above the

(ii) Atmospheric air at 275 K and a free stream velocity of 20 m/s flows over a flat plate
1.5 m long that is maintained at a uniform temperature of 325 K. Calculate the average heat transfer coefficient over the region where the boundary layer is laminar, the average heat transfer coefficient over the entire length of the plate and the total heat transfer rate from the plate to the air over the length 1.5 m and width 1 m. Assume transition occurs at Ree = 2xl05

(ii) Air at 25°C flows over 1 m x 3 m (3 m long) horizontal plate maintained at 200°C at 10 mls.
Calculate the average heat transfer coefficients for both laminar and turbulent

(ii) A steam pipe 10 cm outside diameter runs horizontally in a room at 23°C. Take the outside surface temperature of pipe as 165°C. Determine the heat loss per unit length of the pipe. (10)

• (i) Explain for fluid flow along a flat plate:
• Velocity distribution in hydrodynamic boundary layer
• Temperature distribution in thermal boundary layer
• Variation of local heat transfer co-efficient along the flow.                                                             (8)

(ii) The water is heated in a tank by dipping a plate of 20 cm X 40 cm in size. The

07.  Air at 400 K and 1 atm pressure flows at a speed of 1.5 m/s over a flat plate of 2 m long.

09. A circular disc heater 0.2m in diameter is exposed to ambient air at 25°C. One surface of the disc is insulated at 130°C. Calculate the amount of heat transferred from the disc when it is.

10. (i) Distinguish between free and forced convection giving examples. (4)

• A steam pipe 10 cm OD runs horizontally in a room at 23° C. Take outside temperature of pipe as 165 ° C. Determine the heat loss per unit length of the pipe. Pipe surface temperature reduces to 80° C with 1.5 cm insulation. What is the reduction in heat loss?(12)

UNIT – V RADIATION PART – A

• Define Radiation heat transfer.
• What is Stefan's Bolts Mann law?
• What is Intensity of radiation?
• Define Shape factor.
• What is Radiation Shield?
• Define Quantum theory.
• Define Emissive power of a black surface.
• Defme concept of Black body.
• Define Planck's distribution law.
• Define Wien's distribution law.
• Define Emissivity of a surface.
• What is meant by Kirchhoff's law?
• Define Irradiation.
• Define Radiosity.

• Distinguish between Absorptivity & Transmittivity of radiation.
• What are the gases, which radiate heat?
• What is mean beam length in Gas Radiation?
• What is the equation for radiation between two gray bodies?
• Distinguish between Reflectivity & Transmittivity.
• Differentiate Opaque body & perfectly transparent surface.
• Write down the Wien's formula.
• Write down the heat transfer equation for Radiant exchange between infinite
• parallel gray planes.

PART – B

• Liquid Helium at 4.2 K is stored in a dewar flask of inner diameter = 0.48 m and outer diameter = 0.5 m. The dewar flask can be treated as a spherical vessel. The outer surface of the inner vessel and the inner surface of the outer vessel are well polished and the emissivity of these surfaces is 0.05. The space between the two vessels is thoroughly evacuated. The inner surface of the dewar flask is at 4.2 K while the outer surface is at 300 K. Estimate the rate of heat transfer between the surfaces. (16)

• A thin aluminium sheet with an emissivity of 0.1 on both sides is placed between two very large parallel plates that are maintained at uniform temperatures Tl = 800 K and T2 = 500 K and have emissivities £"1

= 0.2 and £"2 = 0.7 respectively. Determine the net rate of radiation heat transfer between the two plates per unit surface area of the plates and

Determine the percentage reduction in the heat transfer rate if a polished aluminium radiation

06. (i) State and prove the following laws:

• Kirchoffs law of radiation
• Stefan - Boltzmann law                                                       (8)