BENG2 APPLIED THERMODYNAMICS
(THER 205)
TUTORIAL EXAMPLES IN REFRIGERATION
1. An ideal vapour compression
refrigeration system has an evaporating
temperature of -5ºC and a condensing temperature of 50ºC.
Compare the
condenser pressures; the evaporator pressures;
and the COP's when using: (a)
R12 , and (b) Ammonia, as the working
substance.
[
1.219 MPa, 0.261 MPa, 3.65; 2.033 MPa, 0.355 MPa, 3.85]
2. An
ideal vapour compression
refrigerator chills 10 kg/s of water from 25ºC to 5ºC. The plant uses R12 with an evaporating
temperature of -5ºC and a condensing temperature of 35ºC. Determine the power required by the
refrigerator.
If the
refrigerant was Ammonia, would more or less power be required? [ 150.8
kW; 4 kW less]
3. An ideal vapour compression heat
pump uses R12 and has a lake as a heat source.
The lake's minimum temperature is 4ºC. The hot water output from the heat pump is at
50ºC. Assuming 4K minimum
temperature differences across each heat-exchanger determine the volume flow
rate of R12 at entry to the compressor
and the COPhp for a
heating load of 10 kW. [ 0.00447 m³/s; 4.68 ]
4. An ideal vapour compression
freezing plant has an evaporating temperature of -40ºC and a
condensing temperature of 30ºC.
The plant uses R12 and freezes
1.6 tonnes/h of fish. The
reduction in specific enthalpy of the fish is 420 kJ/kg. Determine the COPref , the
power input to the plant, and the mass flow rate of R12. [ 2.43; 76.82 kW; 1.78
kg/s]
5. An ammonia refrigerator has a single
stage, single acting compressor of 127 mm bore & 152 mm stroke running at
240 RPM. The evaporator pressure is
0.1516 MPa and the condenser pressure is 1.350 MPa. The volumetric efficiency of the compressor
is 80% and its mechanical efficiency 90%.
The ammonia is dry saturated on leaving the evaporator and liquid leaves
the condenser at 32ºC.
Calculate the mass flow rate of refrigerant, the cooling capacity of
the plant, and the power input to the compressor. (NB: Actual Volume flow rate = Swept volume
flow rate x Volumetric efficiency) [0.00798 kg/s; 8.62 kW; 2.935 kW]
6. A heat pump using R12 takes energy from
the sea at an evaporating temperature of 0ºC and delivers it to an
hotel at a condenser pressure of 1.219 MPa.
The R12 is 20 degrees superheated at compressor entry and 20 degrees
sub-cooled at condenser exit.
The compression isentropic efficiency is 85%.
The heating
load is 30 kW.
(a) Determine
the COPhp of the cycle;
(b) If
the price of electricity is 5 p/kWh, determine the cost of running the plant continuously
for 180 days and compare this with burning solid fuel for the same period.
Assume solid fuels costs £65 per tonne,
burns with 70% efficiency and has a calorific value 32 MJ/kg. [ 5.3;
£1223; £131 cheaper]
7. The heating requirement for a house
sited beside the sea is 10kW. In
mid-winter the sea water temperature is 6ºC and the house internal
temperature is 20ºC. The
owner calculates that a heat exchanger (evaporator) put into the sea can
extract 0.5kW/m² for each degree of temperature difference between
sea and fluid in the heat exchanger.
Allowing for 5K temperature differences across both heat exchangers
(evaporator and condenser) calculate the area of the evaporator in the sea when
a heat pump using R12 is used to heat the house. Compare the running costs of the heat pump
powered by electricity at 8.2p/kWh with a house using gas heating costing
2.3p/kWh.
Assume the
refrigerant is dry saturated vapour at
evaporator exit, saturated liquid at condenser
exit; the isentropic efficiency of
compression is 60%; and the actual COPhp is 70% the
chart based coefficient. [3.44 m²; 16.4 p/h HP c.f. 23 p/h GAS]