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MODULE LEADER: Dr Aruna Palipana

MODULE CODE UFMFTA-15-3

MODULE TITLE THERMOFLUID SYSTEM

Original Exam Duration 3 hours

QUESTION 1

IMPORTANT: (1) You need a psychrometric chart for answering this question. Download it from the EXAMINATION area that you can access via the left hand side menu of Blackboard module page. (2) A scanned image or a photo of the completed psychrometric chart should be submitted with your solution to this question. 

Section A: 

(i) With the aid of sketches/diagrams, briefly explain two (02) methods of energy recovery from the exhaust air from a building air-conditioning system. [6 marks] 

(ii) Briefly describe two (02) methods of increasing the absolute humidity of the air in a room. [4 marks] 

(iii) Give two (02) reasons that could decrease the contact factor of a cooling coil used in a building air-conditioning system [2 marks] 

Section B: 

The primary outdoor air coming into a building air-conditioning system is at 34 °C dry bulb temperature (d.b.) and 10% relative humidity (RH). This primary air is then mixed with the secondary room return air, which is at 24 °C d.b. and 50% RH. The mass ratio of this air mixing, primary : secondary is 1:4. The mixed air then undergoes a sensible cooling process until the air is cooled to 18 °C d.b. and this cooled air is then used as the supply air into the conditioned space. The volume flow rate of outdoor air into the mixing unit is 0.6 m 3 /s. State any assumptions. Use the psychrometric chart and determine: 

(iv) the dry bulb temperature of the mixed air. [2 marks] 

(v) the relative humidity (RH%) of the supply air. [2 marks] 

(vi) the dew point of the room return air. [1 mark] 

(vii) the mass flow rate of the supply air leaving the cooler. [4 marks] 

(viii) the cooling coil load in Watts, if the cooler is 100% effective. [4 marks] [Total 25 marks

SOLUTION

ANSWER(purchase full paper to get all the solution)

SECTION A

1. By adding water vapor.
2. by changing the air temperature.

       iii)       

1. if the moisture content is high.
2. If there is an unclean air filter.

SECTION B

iv) dry bulb temperature = 34 – 24 =10⁰c   

v) relative humidity of the supply  = 50% - 10% = 40%RH

vi) dew point = 

vii) Q = mv

viii) p = mv

QUESTION 2

IMPORTANT: (1) You need the R134a pressure-enthalpy (p-h) chart for answering this question. Download it from the EXAMINATION area that you can access via the left hand side menu of Blackboard module page. (2) A scanned image or a photo of the completed R134a chart should be submitted with your solution to this question. 

Section A: 

(i) Sketch a schematic diagram of a vapour absorption refrigeration system clearly showing (a) the essential components, (b) flow directions of the refrigerant, absorbent and solution, (c) heat and work flow directions and (d) the high pressure and low pressure sides of the system. [4 marks] 

(ii) Briefly explain the trans-critical vapour compression refrigeration cycle that uses carbon dioxide (R744) as the refrigerant, sketching the pressure-enthalpy diagram for the cycle. [4 marks] 

(iii) Give two (02) reasons that could decrease the Coefficient of Performance of a refrigeration system. [2 marks] 

Section B: 

A two-stage cascaded vapour compression refrigeration system operates on R134a. Assume the compressors are isentropic and that throttling is isenthalpic. The refrigerant leaves each condenser at saturated liquid state and each evaporator at dry saturated vapour state. In the low temperature cycle, evaporation is at 90 kPa and condensation is at 5 OC. The refrigerant in the high temperature cycle evaporates at 0.3 MPa and condenses at 40 OC. The refrigerant mass flow rate in the high temperature cycle is 0.86 kg/s. Assume the heat transfer from the low temperature cycle to the high temperature cycle is through a heat exchanger that is 100% effective. Determine: 

(iv) the maximum temperature within this two stage cascaded system. [1 mark] 

(v) the quality of the refrigerant entering the low temperature evaporator. [1 mark] 

(vi) the refrigerant mass flow rate in the low temperature cycle. [3 marks] 

(vii) the cooling capacity (refrigeration effect) of the refrigeration system. [3 marks] 

(viii) the overall Coefficient of Performance of this two stage refrigeration cycle. [4 marks] 

(ix) the amount of heat available, in Watts, if this cascaded cycle is used as a heat pump. [3 marks] [Total 25 marks]

QUESTION 3

Section A: 

(i) Briefly discuss the significance of priming a centrifugal pump. [2 marks] 

(ii) Give sketches showing the overall pump curves (head vs flow rate) when two non-similar centrifugal pumps are connected (a) in series [3 marks] (b) in parallel [3 marks] 

(iii) State three (03) possible methods to reduce the risk of cavitation in a centrifugal pump, briefly explaining how each of those stated methods aid the reduction of the risk of cavitation. [6 marks] 

Section B: A centrifugal pump delivers 15 OC water at a rate of 300 m 3 /hr from a large, open reservoir. The water level of the reservoir remains constant at 3.5 m below the centre of the inlet of the pump. The suction line is 65 m long and has three (03) elbow joints. There is a foot valve with a strainer fitted at the reservoir end of the suction line. The inner diameter of the suction pipe is 200 mm. Take the Darcy friction factor for the pipe as 0.0035, the fitting factor for an elbow joint as 0.5 and the fitting factor for the combined strainer and the foot valve as 6.5. For water at 15 OC, take density as 1000 kg/m3 . The atmospheric pressure is 1.013 bar and the gravitational acceleration is 9.81 m/s2 . Vapour pressure for water at 15 OC is 0.01704 Bar. Ignore pipe entry and exit losses. State any assumptions. Determine: 

(iv) the dynamic head loss between the reservoir water surface and the pump inlet. [3 marks] 

(v) the total frictional head loss in the suction line and across the fittings. [4 marks] 

(vi) the available net positive suction head (NPSHA) at the pump inlet. [4 marks] [Total 25 marks]

QUSETION 4

Question 4: Rotary fluid machinery 

Section A: 

(i) Using a sketch of the relevant velocity diagram, discuss why backward curved vanes are the most efficient arrangement for a centrifugal pump compared with radial or forward curved vanes. [4 marks] 

(ii) For a centrifugal pump, briefly describe: 

(a) the onset of suction cavitation [3 marks] 

(b) possible causes of discharge cavitation [3 marks] 

Section B: 

A single stage (a rotor and a stator) axial flow compressor operates at 4670 rpm and has an isentropic efficiency of 86%. The inlet guide vanes deliver incoming air to the rotor blades with no pre-whirl at 32 m/s. This single stage compressor takes in air at 22 OC and 1 bar and delivers compressed air at 1.36 bar. Mid-blade radius is 0.16 m and the mid-blade performance is assumed to be indicative of overall performance. The mass flow rate of air is 3.6 kg/s and the compressor reaction is 50%. The axial component of the fluid absolute velocity remains constant throughout the compressor. For air compression, take adiabatic polytropic index ( = 1.4 and the specific heat at constant pressure Cp = 1005 J/kg(K. State any assumptions. 

(iii) Determine the rotor blade inlet angle, measured from the plane of rotation of the rotor, for shock free operation. [3 marks] 

(iv) Using the isentropic efficiency of the compressor and the compressor reaction given, calculate the actual power required for this single stage compressor. [6 marks] 

(v) Use the Euler’s theorem to determine the rotor blade outlet angle required. [6 marks] [Total 25 marks]

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Last updated: Jun 29, 2021 10:48 PM