Mid Semester Examination

ME340A: Introduction to Refrigeration and Air Conditioning

Instructor: Prof. Sameer Khandekar

Time 120 minutes Marks: 120

Question 1: A solar powered mobile refrigeration system needs to be designed for making ice-candies/’Kulfi’ mounted on a vending cart (‘Thela’) utilizing brine (aqueous solution of ethylene glycol) as the secondary refrigerant, primary refrigerant being HFC-134A.

Each candy/kulfi is made in a 50 ml thin metallic conical mold of base diameter 30 mm (neglect the volume of candy mold itself), and the capacity of the refrigeration system should be such that 256 such candies. The kulfi is made from water, cream and other milk derivatives + stabilizers + sweeteners (constituent properties are given in the table; see back of this page).

The shape of the refrigerator box should be square parallelepiped with its total volume (including everything) not exceeding ~ 130 liters. The lowest expected surface temperature of the candy mold surface is ̶ 8°C and the ambient temperature in summer can reach + 42°C (A minimum temperature differential of 5°C between the brine and evaporator, and 8°C between condenser fluid and the ambient, respectively, is required for local heat transfer). Natural convection (with h = 35 W/mK) occurs on the outside surface of the refrigerator box with the ambient (except the top cover, which is completely adiabatic). Natural convection between brine and kulfi-molds as well as the evaporator surface can be taken as 120 W/m2K. Radiation can be neglected in the entire system. Each single candy can be taken as a thermally lumped system.

The fabrication of the brine container as well as the outside body of the refrigerator can be done by thin galvanized iron sheet metal (Gage 22, equivalent to 0.8 mm thick sheet). You can use glass fiber insulation of 25 mm thickness having k = 0.03 W/mK and ρ = 50 kg/m3 to insulate the brine container.

(07 marks each) Using this data and constraints, you are expected to undertake first design iteration of the refrigerator system. Your answers should contain enough details so as to communicate your design properly.

(i) Determine and summarize the appropriate dimensions of your proposed design. Draw good quality design views showing possible arrangement of placing the kulfi molds in the brine container.

(ii) Show clear schematic view(s) of the vending cart with the installed refrigerator showing the layout of the entire system components, i.e. candy molds, brine container, evaporator coil, insulation material, compressor, expansion device, condenser, connecting tubing and the PV solar unit.

(iii) Make a resistance network model of all possible modes of heat transfer in the refrigeration system, as soon as it starts from the ambient condition (the entire system is at 42°C when compressor starts to operate). Use suitable assumptions to keep the model simple and explain your model in words.

(iv) Estimate the ‘instantaneous peak load’ of the compressor at the time when the machine starts to operate from initial ambient conditions.

(v) Estimate the ‘nominal load’ of the designed refrigeration system when steady-state is reached.

(vi) Draw an ideal VCR cycle on the P-h diagram and find out the mass flow rate of the refrigerant corresponding to ‘instantaneous peak load’ and ‘nominal steady-state load’ operation.

(vii) How much power is required, only to freeze the 256 candies from + 42°C to ̶ 8°C in 4 hours.

(viii) What is the outer wall temperature of the refrigerator during its steady-state operation?

(ix) If the solar PV is estimated to have an overall electrical conversion efficiency of 15%, and its size cannot exceed 5m2, can it cater to peak loads? Do you suggest a battery backup? The average solar insolation is expected to be 380 W/m2.

(x) Suggest three more suitable insulation materials which can be utilized in the refrigerator, along with their typical thermal conductivity values.

(xi) What more data is required to design the condenser of the system? Explain briefly the design and analysis procedure, if forced air cooling is to be proposed.

(xii) Write all important assumptions which you have invoked during the design process. Suggest some methods to improve the modeling.

Properties Kulfi/Ice Candy

% volume

Freezing Point

Specific heat kJ/kgK

Latent heat of fusion

kJ/kg

Average Density kg/m3 Above

freezing Below

Freezing

Water + Cream 60 - 5.6 °C 2.95 1.63 210 995

Other milk derivatives + stabilizers

40 - 0.6 °C 3.79 1.95 294 1035

Brine properties

k = 0.45 W/mK, ρ = 1060 kg/m3, Cp =3.6 kJ/kgK, μ = 0.01 Pa-s, Pr = 70, β = 0.35 x 10-3 per K

Question #2 (30 marks): Write ‘True’ or ‘False’ in the space provided.

Note: +2 for the correct answer and -1 for the wrong answer.

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Question 3 (6 marks): Draw a properly labelled P-h diagram of an ‘REAL’ vapor compression refrigeration cycle

with two-stage compression process having an intercooler strong enough to bring the vapor back to saturation

condition after the first stage of compression. Write all deviations from ideal cycle.

P-h diagram:

Deviations from actual cycle:

# Question True/False

1. One of the largest refrigeration plant, located in Qatar (Pearl of Qatar), has an installed capacity of about 1250000 Tons of refrigeration.

2. A thermodynamically closed system is one which does not allow energy and mass to pass through its boundaries.

3. Change in volume of a fixed amount of gas under isobaric conditions is possible only via a ‘work’ interaction on/by it.

4. A constant volume gas thermometer measures temperature which is very close to the Kelvin Temperature Scale.

5. Under steady operation, shaft work and electrical work are the only forms of work a simple compressible system may involve.

6. ‘Triple point’ of a pure substance may involve two solid phases and one liquid phase.

7. One ton of refrigeration is equal to 3.86 kW if we use a short ton (907 kg of water).

8. All other parameters remaining the same, a reversed Brayton cycle is as good as a reverse Carnot cycle.

9. Latent heat of the refrigerant plays an important role in the operation of a Bell-Coleman refrigeration cycle.

10. A compact heat exchanger is characterized by a surface area typically exceeding 1000 m2 per cubic meter of the heat exchanger volume.

11. The ‘rating problems’ deal with the determination of the heat transfer rate for an existing heat exchange system at a specified temperature difference.

12. Typical thermal conductivity values of common gases (N2, He, CO2 etc.) is of the order of 1 W/mK.

13. Turbulent flow velocity profiles are flatter with high gradients near the wall, leading to much smaller shear stress as compared to laminar flows.

14. The effectiveness of a heat exchanger is independent of the capacity ratio c for NTU values of less than about 0.3.

15. The effectiveness of a heat exchanger is independent of the flow arrangement.