ASEN 3113
Thermodynamics and Heat Transfer
Fall 2005
ASEN 3113
Experimental Laboratory 1
Basic Refrigerant Training Unit (BRTU)
OBJECTIVES

Learn the components involved in the vapor-compression refrigeration cycle

Apply first and second law thermodynamic concepts to evaluate the efficiency of an actual vapor-compression
refrigeration cycle.

Develop an awareness of sources of instrument error and equipment limitations.

Develop the skills to deliver a clear and concise report of the experiment, your findings and your interpretation.
REQUIRED DELIVERABLES

Attendance at every lab period is required to run the experiment, record data, analyze results, and prepare your
report. Instructions for weekly tasks and the individual report will be presented during the scheduled lab time.

Prepare a written report of your laboratory experiment with results, analysis, discussions and conclusions.
Guidelines for report content and format will be provided in a future lab session.
Theory
1. Review of Basic Refrigeration Principles
1.1 Background
Refrigeration is the process of extracting heat from a space or a material and rejecting that heat typically-to the surroundings. The devices that produce refrigeration are refrigerators and heat
pumps. The most common refrigeration cycle is the vapor compression refrigeration cycle in
which the refrigerant is vaporized, condensed and compressed.
It is well known that heat flows from the direction of decreasing temperatures. This heat transfer
process occurs in nature without the need of any device. The reverse process, however, cannot
take place without energy input. The transfer of heat from a low temperature space to a higher
temperature one requires the use of refrigeration devices.
Both refrigerators and heat pumps are devices that transfer heat from low to high temperature
medium. They are basically the same devices, but differ in their function. The refrigerator is used
to maintain the refrigerated space at low temperatures by absorbing heat, and discharge that heat
into a higher temperature environment. The role of the heat pump, on the other hand, is to
maintain a heated space at higher temperatures. This objective is accomplished by absorbing heat
from a low temperature source such as the outside environment in the winter and supplying this
heat to a warmer medium such as a house. Schematics of a refrigerator and a heat pump are
shown in Figures 1(a) and (b).
7/31/2017
1
ASEN 3113
Thermodynamics and Heat Transfer
Fall 2005
Figure 1: Basic operation of (a) a Refrigerator, and (b) a Heat Pump.
The coefficient of performance (COP) is a measure of the efficiency of refrigerators and heat
pumps. The definition of the coefficient of performance is the desired output over the required
input. It is defined as:
For a refrigerator
COPrefrigerator = QL / Wnet,in
For a heat pump
COPheat pump = QH / Wnet,in
where QL is the cooling effect or the desired output for the refrigerator ,QH is the heating effect
or the desired output for the heat pump, and Wnet,in is the work input or the required input.
7/31/2017
2
ASEN 3113
Thermodynamics and Heat Transfer
Fall 2005
1.2 Reversed Carnot Cycle
The Carnot cycle is a totally reversible cycle and consists of two reversible isothermal and two
isentropic processes. The cycle is an ideal cycle and has the maximum thermal efficiency for
given low and high temperature limits. This cycle serves as a reference cycle to evaluate the
relative efficiency for actual power cycles. Carnot cycles can also be used to analyze the
coefficient of performance of refrigeration cycles. When all the four processes that comprise the
Carnot cycle are reversed, the Carnot cycle is called the reversed Carnot Cycle. Reversing the
cycle will also reverse the direction of any heat and work interaction. A refrigerator, which
operates in a reversed Carnot cycle, is called a Carnot refrigerator.
Figure 2 shows a schematic of the processes involved in a Carnot refrigerator. The refrigerant
absorbs heat isothermally from a low-temperature source at TL in the amount of QL in process (12). The refrigerant is compressed isentropically to state 3, and its temperature rises to T3. Then,
the heat is rejected isothermally to a high-temperature sink at TH in the amount of QH in process
(3-4). Finally the refrigerant expands isentropically to state 1, where the temperature drops to
TL.
Figure 2: Basic processes for a Carnot refrigerator.
The coefficient of performance of reversed Carnot refrigerator and heat pump is a function of the
heat source and heat sink absolute temperatures and can be expressed as follows:
For a Carnot refrigerator:
COPrefrigerator = 1 / (TH/TL-l)
For a Carnot heat pump:
COPheat pump = l / (1-TL/TH)
7/31/2017
3
ASEN 3113
1.3
Thermodynamics and Heat Transfer
Fall 2005
Ideal Vapor-Compression Refrigeration Cycle
There is some impracticality associated with the reversed Carnot cycle that can be eliminated by
vaporizing the refrigerant completely before it is compressed, and by replacing the turbine with a
throttling device. Throttling devices are flow restricting devices such as an expansion valve or
capillary tube. The result is a cycle that is called the ideal vapor-compression refrigeration cycle.
Figure 3(a) shows the schematic of the ideal vapor-compression refrigeration cycle. The
processes of the ideal refrigeration cycle are shown in Figure 3(b).
Figure 3: Ideal Vapor Compressor Refrigeration Cycle.
(1-2) Isentropic compression in a compressor.
(2-3) P = constant heat rejection in a condenser.
(3-4) Throttling is an expansion device.
(4-1) P = constant heat absorption in an evaporator.
7/31/2017
4
ASEN 3113
Thermodynamics and Heat Transfer
Fall 2005
1.4 Actual Vapor-Compression Refrigeration cycle
Any refrigeration cycle consists of four main components as shown in Figure 3(a). The
components shown in Figure 3(a) are a compressor, a condenser, an expansion valve, and an
evaporator. For a refrigeration mode, the evaporator is located in the conditioned space, while
the condenser is located in the environment. The location of the condenser and evaporator is
reversed in the case of heat pump mode.
Due to irreversibilities that occur in various components in the refrigeration cycle, actual vaporcompression cycle differs from an ideal cycle. Two common sources of irreversibilities are fluid
friction that causes pressure drop, and heat transfer from or to the surrounding. The saturation
line of an actual vapor-compression refrigeration cycle is shown in Figure 4.
Figure 4: Actual Vapor-Compression Refrigeration Cycle.
7/31/2017
5
ASEN 3113
Thermodynamics and Heat Transfer
Fall 2005
2. General Test Setup and Operation Procedure
2.1 Equipment Setup
Figure 5 depicts the Basic Refrigeration and Air Conditioning Training Unit and figure 6 is a
photograph of the unit in the ITLL.
The following list identifies the various parts of the equipment unit. Note that the thermocouples
are added to measure temperature at appropriate locations.
(1) Compressor
(2) Condenser
(3) Evaporator
(4) Thermal Expansion Valve
(5) Automatic Expansion Valve
(6) Capillary Tube
(7) Accumulator
(8) Receiver
(9) Pressure gages
(10) Valves
(11) Thermocouple wires
7/31/2017
6
ASEN 3113
Thermodynamics and Heat Transfer
Fall 2005
Figure 5. Schematic of Basic Refrigeration Training Unit (BRTU)
7/31/2017
7
ASEN 3113
Thermodynamics and Heat Transfer
Fall 2005
Figure 6. Photograph of the Basic Refrigeration Training Unit (BRTU)
7/31/2017
8