Refrigeration Tutor

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Objective of this mechanical project is to perform test on the refrigeration tutor to determine different COPs and other performance parameters.

Experimental Setup : All the components of refrigeration bench are displayed on a portable sun mica stand. The set up consists of –
  • An air cooled fin type condenser
  • Hermetically sealed compressor
  • Evaporator with calorimeter and heater 
  • Two expansion devices-capillary and thermostatic expansion valve
  • Electrically operated solenoid valve
  • High pressure and low pressure cut-outs for safety of the compressor
  • Two energy meter for measuring energy input to compressor and calorimeter heater 
  • Rotameter is used for measuring the flow rate of liquid refrigerant (R-12) in the refrigeration cycle

Working Principle : The refrigerating tutor is based upon the vapour compression refrigeration cycle(VCRC). It consists of four basic processes (see fig 1 and 2 below):

  • Reversible adiabatic(isentropic) compression in compressor(s1=s2)
  • Constant pressure heat rejection in condenser
  • Isentropic ( h3 = h4 ) expansion in capillary tube/ Expansion device
  • Constant pressure heat addition in evaporator
VCRC Curves

Alternative arrangement of passing the liquid refrigerant through a capillary and a thermostatic expansion valve is provided with suitable shut off valves where the liquid is throttled to low pressure and temperature and sent to evaporator where it boils. A drier and sight glass is present in the way to throttling device. A heater provided at the bottom of the calorimeter offers heat load which is balanced by refrigerating effect produced.

The calorimeter consists of 25 liters of water as medium, the temperature of which could be observed with a dial thermometer. Separate pressure gauges are provided to measure condenser and evaporator pressures and four suitable thermometers are supplied to measure temperatures at various locations. HP LP cutout and thermostat is provided for safety and voltmeter and ammeters to measure voltage and current of the compressor, and heater input to balance the refrigerating effect produced by the evaporator. Main switch and switches for heater, compressor, fan and solenoid valve to stop the liquid refrigerant flooding the evaporator should the supply fail while operation on thermostatic expansion valve are provided.

Calculations

Enthalpy values of specimen trial(from Table R-12):
Enthalpy of condenser inlet H1 = 394.802 kJ/kg
Enthalpy of condenser outlet H2 = 240.574 kJ/kg
Enthalpy of evaporator inlet H3 = 240.574 kJ/kg
Enthalpy of evaporator outlet H4 = 362.439 kJ/kg

  • COPth = Nth/Wth = (H4 – H3)/(H1 – H4) = 3.766 
  • COPact = Nact/Wact Nact = 5/132 * 3 kW
    • = 0.114 kW Wact = 3/108 * 3 kW
    • = 0.138 kW COPact = 0.820 1 rev/sec
    • = 3 kW(1200 rev = 1 KWh)
  • COPrel = COPact/COPth = 0.820/3.766 = 0.189
  • COPcarnot = T(l) [T(h) – T(l)] = 265/341-265 = 3.487
  • Efficiency of the plant on Carnot cycle basis = COPact/COPcarnot = .235
  • Actual capacity of the plant = Nact/3.489 ton = 0.114/3.487 = 0.033 ton
  • Mass flow rate of the refrigerant
    • m = [Rotameter reading(f) × 0.001 m3] / [sp. Vol of liquid at Pc(from table)]
    • = 12 × 10-3 / 0.803×10-3 = 14.94 kg/hr = .004 kg/s
  • Actual volumetric efficiency = m × sp volume of suction vapor(from table) / piston displacement per hr. = 14.94×0.0254/2.15 = 0.177 = 17.7 %
  • Dimensionless ratios
  • Condenser to evaporator heat transfer = (Hci – Hco)/(Heo – Hei) = 1.266
  • Power consumption ratio = Wact/Wth = 0.139/0.487 = 0.285
  • Compression ratio = Compressor discharge pr/ Compressor suction pr = 15.513/4.013 = 3.866

Result

CycleCOP
Actual 0.82
Theoretical 3.766
Ideal ( Carnot) 3.487

Precautions

  • Do not forget to put on solenoid valve switch while running the plant on thermostatic expansion valve. If not opened, condenser pressure (greater than 215 psi) will shoot up and HP cutout will stop the compressor.
  • The evaporator pressure should not decrease below 20 psi otherwise LP cutout will switch off the compressor.
  • Start the system on capillary first. Starting the system initially on thermostatic expansion valve will demand the compressor works against heavy condenser pressure.
  • Do not open charging valve unless required for charging.
  • Observe condenser inlet temperature, which should lie in between 70 to 75 °C. If it exceeds 75°C, stop the plant immediately. It means the condenser cooling air temp is high. Cooling table fan adjustment to the compressor is advised.
  • Calorimeter bar temperature should not decrease below 15°C otherwise thermostat will switch off the compressor.

Discussion

COP obtained in these calculations is quite low. This may be due to non-uniform heating by the heater. It may also be due to the time provided for heating the liquid is not sufficient and while taking the readings, steady state was yet to be achieved. Also the rotameter observations are liable to be wrong as accurate reading from the rotameter is very difficult to get.

Questions

1. What is the function of solenoid valve?

It is a shut off valve that is actuated by an electro magnetic coil so designed that when the coil is energized. Its function is more of a safety device than of an expansion valve because while running the plant on thermostatic expansion valve, if solenoid valve is not put on, condenser pressure will shoot up and HP cutout will stop the compressor

2. What is the difference between actual and standard VCRC?

There is a difference between actual vapor compression cycle and the standard vapor compression cycle(see fig below). In the standard vapor compression cycle, the pressure drops due to friction in the condenser and the evaporator are not taken into consideration. Sub cooling in the condenser and super heating in the evaporator are also not considered in the standard vapor compression cycle. In the fig A’B’C’D’ represents the standard vapor compression cycle whereas ABCD is the actual vapor compression cycle. CC’ indicated sub cooling in the condenser whereas AA’ indicates the super heating in the evaporator.

Actual Standard Cycle

3. Name all safety & control devices used and function of each in one line.

  • High pressure cutout to switch off compressor if condenser pressure exceeds 2.15 to 2.20 lb/inch2 (1.514 kPa to 1.549 kPa)
  • Low pressure switch to switch off compressor if evaporator pressure falls below 20lb/inch (140.86 kPa)
  • Thermostat to switch off compressor if the calorimeter bath temp falls below 15°C (set).
  • Solenoid valve to prevent flooding of evaporator if power fails.

4. Which refrigerant is used in refrigerating tutor and why?

In our experiment, we use Freon-12.Its chemical name is dichlorodifluoro Methane(CCl2F2).It is commonly used for all refrigeration purposes. It is colourless and odourless liquid. It is non-toxic, non-flammable, non-explosive and non-corrosive. It condenses at moderate pressure under normal atmospheric conditions and boils at – 29.5 deg cel at atm. Pr. This property makes it suitable for all purpose refrigeration and it can be used with all three types of compressors.

5. What type of condenser is being used?

we have used forced convection air- cooled condenser. Heat transfer takes place by air convection around the condenser surface. Air is circulated by means of a fan or blower.

6. How pressure drop takes place in capillary tube?

The required pressure drop is caused due to heavy frictional resistance offered by a small diameter. Tube. Resistance is directly proportional to the length and inversely proportional to the diameter.

Attachments:
Download this file (Refrigeration-Tutor-Report.pdf)Refrigeration Tutor[Project Report]2539 Kb