Experiment 1 - Pressure/Temperature Relation

Objectives:
At the end of this experiment session, you will be able to:

 Explain the pressure temperature relation.

 Explain the p h (Molliere) diagram.
 Explain the p h (Molliere) diagram terms.
 Explain the p h (Molliere) diagram process.
 Explain the Theoretical cooling cycle
 Explain the Practical cooling cycle
 Explain the Pressure losses in the evaporator and condenser
 Explain the Enthalpy changes in the metering process
 Explain the Enthalpy changes in the metering and compression process
 Understand the pressure temperature relation graph.

Equipment Required:
 TPS-3950
 TPS-3952

Discussion:
1.2.1 Pressure/temperature relation
The refrigeration system is a closed system composed as follows:

Capillary Tube

Evaporator Condenser

Low Pressure Side High Pressure Side

Compressor

Figure 1-12
The system contains coolant of the compressor in high pressure on one side of the system, which
spreads in low pressure up to transferring to gas state on the other side of the system.

Compressing gas causes the pressure to rise and its temperature to rise at the same ratio.

The compressor in the system sucks the gas from the evaporator and compresses it in the
direction and inside the condenser. In the condenser, which is built as a radiator, a lot of heat is
created, which is cleared by the fan that draws air from the surroundings through the condenser
pipes.

The high-pressured gas causes another action. It raises the boiling temperature of the gas. The
boiling temperature is the temperature where liquid turns to gas and vice versa.

Lowering the high pressure and temperature of the gas in the condenser causes it to turn into
liquid.

The compressed liquid is drawn by the compressor and flows through the Receiver/Drier (called
also the Accumulator), which dries the water of the liquid. From the receiver/drier the gas keeps
flowing to the evaporator through the evaporator fix orifice. The evaporator fix orifice is a
narrow passage in the A/C pipes. It causes high gas and liquid pressure at one side (the
condenser side of the system) and a very low pressure at the other side (the evaporator side).

The liquid flow pace through it is limited and is determined by the pressure difference in the
system.

The small amount of coolant passing through the evaporator fix orifice is drawn to the
evaporator and from there spreads at once.

The liquid spreading causes pressure descent of the liquid. This pressure descent causes
temperature descent of the liquid to 0oC (32oF) of the evaporator pipes. The system is designed to
this temperature in order to achieve maximal cooling efficiency. Lower temperature will cause
the water steam around the evaporator to freeze, which will heat the air flow through the cooling
walls of the evaporator.

Pressure descent also causes the dropping of the boiling point of the coolant to 0 oC (32oF).
Transferring air by the compressor through the evaporator pipes and from there to the
passenger's compartment causes the air to cool and the coolant in the evaporator to get extra heat.

The extra heat for the liquid is not enough to change its temperature (which stays at 0 oC (32oF)),
but manages to turn it to gas.
In the gas state, it arrives to the compressor and compressed back to the condenser direction.

As mentioned before, the A/C system is a close system, which changes the temperature by
pressure change in two areas of the system.
As we saw in the previous experiment, the system includes two pressure sensors, which stop the
compressor operation (by disconnecting the clutch) in too high pressure in the system or too low
pressure (which indicates among other things lower temperature).

The A/C system includes two connection points, where two pressure meters are connected.
These connection points are located at the two sided of the compressor and allow measuring
Discharge (compress pressure) and Suction (suction pressure).

The pressures in the system are influenced by the air temperature transferred by the fan through
the evaporator.

This air comes from outside the vehicle or from the passenger's compartment (in the recirculation
state) to an area from which the compressor pulls the air. The pushed air temperature (called
Shop Air) influences the A/C system's pressures as we shall see during the experiment.

1.2.3 Basic cooling cycle in the Molliere (ph) diagram

Molliere Diagram – An enthalpy-entropy or enthalpy-pressure chart showing the
thermodynamic properties of the refrigerant.

The Mollier diagram is the European version of the Anglo-American Psychrometric Chart. They
are identical in content but not in appearance.

The cooling material thermodynamic conditions at every point of the cooling cycle are described
in the Molliere diagram. Each cooling material has its own Molliere diagram. The R134a
Molliere diagram is described in section 1.2.7.

P
Saturated
Sub-cooled Vapor Line
Region
Pressure PSIA

Saturated
Liquid Line
Condensation
3 2
Superheated
Throttling Region
Compression
4 1
Evaporation
Wet Region
h
Enthalpy, BTU per
LB.
Figure 1-14

The diagram is constructed from three regions separated by a curved line shaped as a bell, thus
the name.
1) The region where the refrigerant is in a liquid state is described on the bell's left side. This
region in also called the Compressed Liquid Region or the Sub-Cooled Region.
2) The region where the refrigerant is in two states (liquid and gas) is described inside the
bell. This region is called the Wet Region, and it describes the substance evaporation or
condensation processes under given temperature and pressure. This is a saturated state.
3) The region where the cooling material is in the gas state is described on the bell's right
side. This region is also called the Superheated Vapors Region or the Superheated Region.

1.2.3.1 The use of psychrometric Molliere diagram

1) Absolute pressure (P) – Choosing an arbitrary point on the diagram, and then moving
from this point to the right or to the left horizontally to the end, and read the absolute
pressure in PSI units.

2) Enthalpy (h) – Choosing an arbitrary point on the diagram, and then moving from this
point up or down vertically to the end, and read the Enthalpy in BTU units per Libra.

The Enthalpy is zeroed at -40oF temperature in the Molliere diagram when the substance
on the bell's left line is in a liquid state. The Enthalpy change between two given points in
order to calculate the heat amount added or detracted from the cooling material is
important in the Molliere diagram.

3) Temperature (T) – The temperature is a value read (in Fahrenheit) differently in each of
the diagram's regions. The temperature's values surround the bell's line where the high
value is at its vertex, and the low values are at its bottom (at each side).

Reading the temperature at the liquid region is done by a vertical descent to the left side of
the bell.

Reading the temperature inside the bell is done by a horizontal movement (right or left)
towards the bell's line.

Reading the temperature at the gas region is done by moving on a very steep curved line
(almost vertical) from an arbitrary point towards the right side of the bell.

4) Specific Volume (V) – Appears only in the gas region and us described by a moderate and
very curved lines. The specific volume units in the Molliere diagram are cubic feet per
Libra.

5) Entropy (S) – A thermodynamic characteristic presenting the relation between the

Enthalpy and the absolute temperature of the substance in Kelvin or Rankin degrees. A
more important use is the change in the Entropy presenting the relation between the
amount of heat added or detracted from the substance to the absolute temperature where
the heat transfer occurs.
 The Entropy described in the Molliere diagram is specific Entropy, which means Entropy
per mass unit.

The general Entropy units are:

 Joule per Kelvin degree (international system).

 Kilocalorie per Kelvin degree (technical system).
 BTU per Rankin degree (British system).
The specific Entropy units are:

 Joule per kilogram per Kelvin degree (international system).

 Kilocalorie per kilogram per Kelvin degree (technical system).
 BTU per Libra per Rankin degree (British system).

The Entropy appears only in the gas region, and it is described by curved lines, steeper
than the specific volume lines but more moderate than the temperature line in this region.

6) Dryness coefficient (quality) (X) – A thermodynamic characteristic existing only inside

the bell when the substance changes its state, and actually is in liquid and gas states. This
characteristic defines how much of the substance mass percentage is in gas state.

M ¿ gas
X= ¿¿ ¿
M ¿ gas+M ¿ liquid
As much as the point moves horizontally towards the bell's right side, the dryness
coefficient rises, aspires to the gas state. The substance on the right line is in a saturated
vapor state, and its dryness coefficient is 1 (there is no substance percentage in liquid
state). In this state, the substance finishes the evaporation process or starts the condensation
process.

As much as the point moves horizontally towards the bell's left side, the dryness coefficient
decreases, aspires to the liquid state. The substance on the left line is in a saturated liquid
state, and its dryness coefficient is 0 (there is no substance percentage in gas state). In this
state, the substance finishes the condensation process or starts the evaporation process.

The dryness coefficient in the Molliere diagram is given in percentages by nine steep lines
inside the bell (10 to 90). The zero line is the bell's left line and the 100 line is the bell's
right line.

1.2.3.2 Theoretical cooling cycle

Point 1 describes the cooling material condition at the compressor entrance. The cooling material
is situated on the saturated vapor line in low temperature and pressure. The compressor rises the
gas temperature and pressure, thus at point 2 (the compressor's output), the cooling material is in
a gas state with high temperature and pressure.
The theoretical compression process is defined as a process where heat does not pass the
system's boundaries, and there is no energy passage to the environment, thus there is no change
in the Entropy, and the process occurs along a unified Entropy line.

The electrical energy used by the compressor's operation is defined as an external operation,
which must be invested in order to produce the cooling process. As a result of this operation
there is change in the Enthalpy between points 1 and 2. The Enthalpy at point 2 is higher than the
Enthalpy at point 1 and the difference between them is the energy needed for the compressor's
operation.

Compressor h = h2 – h1

In the gas region, the substance is situated at point 2 (far from the bell), and in this state it enters
the condenser. The compressor raises the substance's temperature, and it enters the condenser in
a higher temperature than the environment temperature (outside the condenser).

The temperature difference between the cooling material and the environment causes heat
transfer from the substance to the environment. Because the substance state is superheated gas
(far from the bell), the heat reduced from it is expressed by temperature decrease (sensible heat),
and moving closer to the bell.

The cooling material gradually losses temperature until it reaches the condensation temperature
at the existing pressure (saturated vapor). This state is described by point 2'.

Additional heat loss puts the substance into the bell, and the condensation process starts with no
change in the substance's temperature (latent heat). The cooling material delivers heat to the
environment, and gradually turns from gas to liquid.

At point 3, the cooling material finishes its path in the bell only in a liquid state at high
temperature and pressure (saturated liquid).

In process 2-3, the substance losses energy to the environment and its Enthalpy drops. This loss
value is calculated according to the Enthalpies difference between points 2 and 3.

Condenser h = h2 – h3

For recycling the process, the cooling material should be subtracted from point 3. The metering
device installed between points 3 and 4 performs this function.

Explanation about metering devices appears in section 1.3.7.

The cooling material enters the metering device as hot liquid in high pressure, and exits at point
4 as a mixture of liquid and gas (relatively low dryness coefficient) in low temperature and
pressure.

The theoretical metering process is defined as a process where there is no heat transfer to the
environment, and no external work is invested in it, thus through the process the Enthalpy
remains unchanged.

The temperature and pressure of the cooling material drop and supposedly transfers sensible heat
to the environment, but on the other hand, already starts its boiling process at the metering
device, and a small part of it turns to gas (due to the temperature and pressure drop, it is also easy
for the substance to expand and boil in low temperature). The cooling material boiling in the
metering device is called Flash Gas. The required heat for the substance to boil is latent heat.

In a theoretical metering process, the sensible heat decrease equals the latent heat rise, thus there
is no change in the substance's Enthalpy. This is why process 3-4 is described as a vertical line
starting in high pressure on the saturated liquid line, and finishing in low pressure inside the bell.

h3 = h4

At point 4, the cooling material enters the evaporator as a mixture of liquid and a small
percentage of gas in low temperature and pressure. The metering device is designed to lower the
evaporation temperature below the environment temperature (outside the evaporator).

The temperature difference between the cooling material and the evaporator's environment
causes heat transfer from the environment to the substance.

Because at point 4 the cooling material is inside the bell, every heat transferred to it accelerates
the evaporation process, and the substance gradually turns from a mixture of liquid and a small
percentage of gas to gas only. The evaporation process (as the condensation process) occurs with
no change in the substance temperature and pressure.

At point 1, the cooling material finishes its path in the bell only in a gas state at low temperature
and pressure (saturated vapor).

In process 4-1, the substance absorbs energy from the environment and its Enthalpy rises. The
value of this addition is calculated according to the Enthalpies difference between points 4 and 1.

Evaporator h = h1 – h4

The cooling material exits the evaporator at point 1 and enters the compressor.

The above process is a cyclic process.

An important conclusion derives from the closed cooling cycle description in the Molliere
diagram is that the amount of heat expelled to the condenser's environment equals the heat
received in the evaporator + the heat amount needed for the compressor's operation.

P
Evaporator + Compressor h = Condenser h

3
4
h

Heat Amount Received

(h1 – h4) + (h2 – h1) = (h2 – h3)

in the Evaporator

Heat Amount Expelled

from the Condenser
1

2
Compressor's Operation
Energy Needed for the
h

Figure 1-15

To sum it up, it can be said that the cooling system is a unit, which transfers heat energy from the
cooling area to the external environment through the cooling material and investing electric
energy for operating the compressor.

Cooling production:
The Enthalpies difference between the evaporator's input and output points indicates the device
cooling production.

h1 – h 4

The Molliere diagram is calculated for a mass unit, thus the specific change in the Enthalpy must
be multiplied by the cooling material mass, which passes to the time unit in the evaporator.

The cooling production is measured in:

 The British system in BTU/H units.

 The technical system in Kcal/H units.
 The international system in J/H unit.

The Coefficient of Performance (COP). This is the ratio between the cooling produced to the
energy consumed by the compressor.
Evaporatorh h1 – h4
Compressorh h2 – h1
COP = =
The COP is the ratio between the Enthalpies, thus it does not have units.

1.2.3.3 Practical cooling cycle

The practical cooling system cannot achieve the demands placed by the theoretical cooling cycle.
Various constraints, which cannot be ignored, influence the cycle's nature.

a) Superheating:

The theoretical cycle determines that point 1 indicates the end of the cooling material
evaporation, and the end of its path in the evaporator. Sometimes, the evaporation process
ends before the cooling material finishes its path in the evaporator. Each additional heat
transfer from the cooling center to the cooling material (after it finishes its evaporation
process) is expressed in the cooling material temperature rise, and turning it to a
superheated vapor gradually moving away from the bell.

The cooling material leaving the evaporator is hotter than in the moment it finished the
evaporation process. This process is called Superheating.

Sometime there is a concern that the cooling material will not finish its evaporation process
in the evaporator and a certain percentage of liquid will enter the compressor. This
situation occurs when the heat loads operating on the cooling center are low, the heat
transfer to the cooling material is low, and the cooling material does not get enough heat in
order to finish boiling. In this situation, point 1 will move to the left, into the bell.

The compressor is designed to work with a cooling material only in a gas state, and liquid
entrance (even in small percentages) causes damage to the compressor's valves and
jeopardizes its intact operation.

One of the methods to avoid the above problem is to increase the evaporator, thus the
cooling material keeps flowing to the battery, absorbs additional heat from the cooling
space, and finishes its evaporation process. In normal working conditions, point 1 moves to
the right to the superheated vapor region.

b) Sub-cooling:

The theoretical cycle determines that point 3 indicates the end of the cooling material
condensation, and the end of its path in the condenser. Sometimes, the condensation
process ends before the cooling material finishes its path in the condenser. Each additional
heat transfer from the cooling material to the environment (after it finishes its condensation
process) is expressed in the cooling material temperature drop, and turning it to liquid
gradually moving away from the bell.

The cooling material leaving the condenser is colder than in the moment it finished the
condensation process. This process is called Sub-cooling.
 Sometime there is a concern that the cooling material will not finish its condensation
process in the condenser and a certain percentage of gas will enter the metering device.
This situation occurs when the external environment temperature rises and received less
heat. The cooling material is not able to transfer enough heat for it to finish its
condensation process.

An effective condensation process is a process that enables any amount of substance to

condensate, and by doing so, transfers the maximum possible heat to the environment. It is
not advisable that the cooling system will work with a condenser where a certain
percentage of the cooling material will not be able to condensate. This phenomenon
reduces the system's production and causes point 3 to move to the right, into the bell.

One of the methods to avoid the above problem is to increase the condenser, thus the
cooling material keeps flowing to the battery, transfer additional heat to the external
environment, and finishes its condensation process. In normal working conditions, point 3
moves to the right to the liquid region.

The superheating and sub-cooling processes increase the heat passage in the evaporator and
condenser batteries thus contributing to the system's production increase.

A common method for achieving sub-cooling and superheating at the same time is using a heat
exchanger in the evaporator and condenser outputs. The pipe coming out of the condenser can be
wrapped around the pipe coming out of the evaporator, thus heat transfer from the cooling
material at the condenser's output to the cooling material at the evaporator's output occurs.

The cooling material at the evaporator's output warms up and the cooling material at the
condenser's output cools down.

In this case, the Enthalpy change in the 1'-1 superheating process equals the Enthalpy change in
the 3'-3 sub-cooling process.

Additional assumption is that this heat exchanger is isolated from the environment in absolute
isolation.

Procedure:
Step 1: Check that the TPS-3952 panel is properly installed on the refrigeration and air-
conditioning general system TPS-3950 according to the instructions described in the
book's preface.

Step 2: Check that the TPS-3950 MONITOR and PROGRAM switches are at OFF position.

A ground leakage relay, a semi-automatic switch, and a main power switch are
installed in a main power box located on the rear panel.
Step 3: Connect the TPS-3950 power supply cable to the Mains.

Step 4: Check that the high voltage ground leakage relay and the semi-automatic switch are
ON.

Step 5: Set the Auto/Manual switch (located on the bottom left of the simulator) to the
Manual position.

Step 6: Turn ON the main POWER switch located on the main power box on the rear panel.

Step 7: Turn ON the monitor power switch.

Step 8: The FAULT display should display the number 00. If not, use the keys above the
FAULT display to display the number 00 (no fault condition) on the FAULT 7-
segment display and press the ENTER key beneath this display.

Step 9: The STATE display should display the number 00 (no operation program).

Step 10: On the LCD display you should find the following table:

V1 V2 V3 V4 V5 V6 V7 V8 CM OF
- - - - - - - - - -

The '-' sign means OFF condition.

Check that the two taps on the flexible pipes (yellow and red) are open, and that the
pipes are tightened to the module.

TEV mode:

Step 11: Changing the STATE number does not start the operating program (even after
pressing the ENTER key).

Using the pushbuttons under each digit, press the number 11 on the STATE (each
pushbutton changes the digit above it), and press ENTER.

The STATE number after pressing the ENTER key only displays the required
operating program and state.

Step 12: Lower the PROGRAM switch and raise it.

The message 'Program 11' should appear on the LCD.

 The TEV mode states are 11-16.

Note:

You can move from one TEV state to another without lowering and raising the
PROGRAM switch. If you lower and raise the PROGRAM switch, the system
will undergo a dely for safety operation.

The TEV programs are:

State 11 - TEV operation with oC display.

State 12 - TEV operation with oF display.
State 13 - TEV operation with graphic display.
State 14 - TEV operation with oC display and thermal load.
State 15 - TEV operation with oF display and thermal load.
State 16 - TEV operation with graphic display and thermal load.

Step 13: On the LCD display you should find the following table:

V1 V2 V3 V4 V5 V6 V7 V8 CM OF
ON ON - - ON ON - - ON ON
 Step 14: The refrigerant path is marked on the following circuit.
V6

TL
TPS-3951 TS5
EF
TS6
Eva. Evapora
Fan tor

Air
Direction

Quick Quick
Fastener Fastener
TEV1
V5

RV TPS-3950
OF
TS3 TS4
V4
Out Condens
Fan er Capillary
Tube
PS1 PS2
Air Receiver CV2
Direction
TS1 TS2

Sight
Compressor Glass
V1 V2

CV1
Filter
FD Drier

V3

Step 15: The LCD also displays the system pressures and temperature as follows:

LP HP T1 T2 T3 T4 T5 T6 T7 T8

LP - Low Pressure (the suction pressure measured by PS1)

HP - High Pressure (the compression pressure measured by PS2)
T1 - The compressor inlet temperature (measured by TS1)
T2 - The compressor outlet temperature (measured by TS2)
T3 - The condenser outlet air temperature (measured by TS3)
T4 - The condenser inlet air temperature (measured by TS4)
T5 - The evaporator outlet air temperature (measured by TS5)
T6 - The evaporator inlet air temperature (the cooling chamber temperature
measured by TS6)
 T7 - Not relevant to this panel
T8 - Not relevant to this panel
Identify the sensors in the drawing and in the system.
Step 16: Another table that appears on the LCD display is the control parameters:

S1 D1 S2 D2 SP PD E1 L1 E2 L2
20oC 5oC - - - - LO -

S1 - Setup temperature room 1

D1 - Temperature Difference room 1
S2 - Setup temperature room 2 (not relevant)
D2 - Temperature Difference room 2 (not relevant)
SP - Setup Low Pressure
PD - Low Pressure Difference

The setup temperature is the required temperature. When the cooling chamber
temperature goes below this temperature, the refrigeration system should stop
cooling and this is done by stopping the compressor.

The compressor turns ON when the cooling chamber temperature is above S1 + D1.
D1 is determined in order to avoid the system to oscillate. By the way, each time the
compressor is turned OFF, a compressor's operation delay occurs.

There is a linear relationship between the temperature and pressure. This is why we
can control the cooling chamber temperature according to the system low pressure or
the system high pressure. This subject will be described later.

The TEV mode is controlled by temperature and this is why a dash appears in the
pressure squares.

Identify the system's default values of S1 and D1.

Step 17: Immediately after operating the refrigeration, the suction pressure should be high and
going down while the system is cooled according to the following graph.

LP

The Stabilization Point

t
Step 18: Change the STATE no. to 12 and press ENTER.

This state does not change the system's operation; it only changes the display from oC
to oF.

The stabilization point, which is the operation point, is the point where the pressures
in the system are the right ones for cooling and are appropriate for the system's
devices, the refrigerant, the fan speed and the environment.

Observe this sight glass and check that there are no bubbles.

Step 20: When the LP is stable at the stabilized point, record the temperature and pressure
values of the stabilization point.

The cooling chamber temperature should continue to go down.

Step 21: Draw a graph LP against T1, which describes the relationship between the suction
temperature and the suction pressure.

Step 22: The stabilization point values enable us to calculate the COP (Coefficient Of
Performance) of the cooling system.

Use the Molliere diagram in Appendix A to find the value of h1, h2 and h4 in order
to calculate the COP of the system as described in the following steps.

The cooling cycle is described in the following diagram

P
Saturated
Vapor Line
Pressure PSIA

Condensation
3 2
C Line
T2
E Line T1
4 Evaporation 1

h
h4 h1 h2
Enthalpy, BTU per
LB.
1. Draw the evaporation line (E line) according to the LP (the suction pressure
measured by PS1) on the diagram.
 2. Find a point on E line that meets a temperature line according to T1 (the
suction temperature). This is point 1.

3. Find the enthalpy value of point 1 on the enthalpy line h. This is h1.

4. Draw the condensation line (C line) according to HP (the compression pressure

measured by PS2) on the diagram.

5. Go up from point 1 using an entropy line on the Molliere diagram until it meets
the C line. This is point 2.

6. Find the enthalpy value of point 2 on the enthalpy line h. This is h2.

7. On the left side of the bell, find the point with temperature equal to the
compression temperature T2.

8. Draw vertical line up and down from this point until it meets C line (creating
point 3) and E line (creating point 4).

9. Find the enthalpy value of point 4 on the enthalpy line h. This is h4.

10. Calculate the COP according to the following formula:

h 1−h 4
COP=
h 2−h 1
11. From point 3 and point 1 identify the type of cooling (sub-cooling,
superheating etc.).
TABLE OBSERVATIONS:

Observe the temperature and pressure values on the display and record them
every one minute in the following table.

Min. LP HP T1 T2 T3 T4 T5 T6
1
5
10
15
20
25
30
35
1.2.7 Pressure-enthalpy diagram for HFC-134a (SI units)