UNSW Sydney
MMAN2700 THERMODYNAMICS
2022 T1
Lab 2: Compressor performance
1. Safety
All staff and students must observe all safety requirements in the laboratory. You must come to the
laboratory dressed for work, NO LOOSE OR BAGGY CLOTHING, NO SANDALS OR BARE FEET.
Before beginning any experiment, inspect all equipment you will use for potential hazards. While using
laboratory equipment, keep alert for any developing hazard, e.g. unusual noise, vibration, unusual data
trends etc.
2. Introduction
Compressed air is a widely used source of power and work in industry. It is safe to use in wet
surroundings and areas with fire risks, where electrical power would be a hazard. Typical uses are:
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Power tools.
Assembly line equipment in factories.
Starting large engines.
Working valves in chemical plants.
An air compressor delivers air at pressure usually to a receiver that stores compressed air. This receiver
is then the source of compressed air for use by other processes. Engineers and technicians need to know
how a compressor works and how it performs. They can then pick or design the correct compressor or
combination of compressors for any job.
The polytropic index for a compressor is a value found by fixing the speed while varying the delivery
pressure. This produces varying temperature and pressure ratios across the compressor and a resulting
chart of the logarithmic values. The gradient of the chart helps to calculate the polytropic index. This
value is characteristic of the machine and is useful in calculating theoretical performance of the
machine.
Since the reciprocating compressor have relatively slow rotational speeds, the thermodynamic processes
of compression and expansion do not follow adiabatic (n =1.4) or isothermal processes (n =1). The
index of the polytropic expansion and compression processes (n) is between these theoretical limits.
Preparation prior to the laboratory period is essential. The preliminary work must be submitted to
Moodle before your allocated lab session (please check carefully your deadline on Moodle).
2.1.
The compressor
Figure 1 below shows the experimental arrangement. A mass flow rate of air ṁ a drawn into a
reciprocating piston compressor at an atmospheric pressure 𝑃𝑃0 and a temperature T1.
The air is compressed to a pressure P1 and a temperature T3. The pressure P1 is caused by a resistance
to the flow. This resistance is created by a flow control valve, which restricts the flow passing from the
reservoir (whose pressure is P3) to the atmosphere.
During the experiment, the demonstrator will vary P1 incrementally from approximately 2 bar to 10 bar
in 2 bar increments. You need to write down the measured values of P1, T1 and T3. The speed of the
compressor is kept at 750 rpm during the experiment.
UNSW Sydney
MMAN2700 THERMODYNAMICS
2022 T1
Figure 1: The compressor lab arrangement. This diagram was taken from the TecQuipment GT103 two-stage compressor
test set user guide.
3. Preliminary work
1. Draw up the following table in your logbook.
Table 1: Experimental data
P1 (gauge) [bar]
T1 [°C]
T3 [°C]
Local atmospheric pressure 𝑝𝑝0 (bar):
2. Write down an expression of 𝑇𝑇3 from the polytropic relationship from knowledge of 𝑃𝑃0, 𝑇𝑇1 and
𝑃𝑃1whilst assuming that the polytropic index n =k.
3. Write an expression for the polytropic index n in terms of 𝑇𝑇1, 𝑇𝑇3, P1 and P0.
4. Experiment
4.1.
Step 1:
Step 2:
Step 3:
Procedure
Prior to commencing the experiment, ensure power is supplied to the reciprocating
compressor and that the rocker switch is set to direct the air flow out towards the receiver
rather than the second stage of the compressor.
Press the green button to turn on the reciprocating compressor, turn the knob to set the
compressor speed to 750 rpm, and monitor the temperatures and pressures within the
system.
Adjust the flow control valve to incrementally vary the pressure from 2 bar to 10 bar in 2
bar increments (turning clockwise closes the valve and thus increases pressure). Write down
the measured values of 𝑃𝑃1, 𝑇𝑇1 and 𝑇𝑇3 at each increment. Record the atmospheric pressure.
UNSW Sydney
4.2.
MMAN2700 THERMODYNAMICS
2022 T1
Analysis
For the complete system,
(a) Calculate 𝑇𝑇3,theo from the polytropic relationship, if the compressor operated in an adiabatic and
reversible (isentropic) manner (assuming the polytropic index n =k.). Give the relative error
between 𝑇𝑇3,theo and T3,exp.
(b) This compressor’s operation however is not isentropic. Therefore, from the measured values of
𝑇𝑇1, 𝑇𝑇3 and 𝑃𝑃1, calculate the polytropic index n for each compression ratio using the equation in
the preliminary work. Give the relative error with n=k.
𝑝𝑝
𝑇𝑇
(c) Plot log ( 3�𝑇𝑇 ) against log ( 1�𝑝𝑝0 ), use the gradient to find the “general” polytropic index of
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the compressor 𝑛𝑛𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 . Compare this value with previous values (question b).
(d) Plot n (take values from question b) against the compression ratio 𝑟𝑟 =P1/ 𝑃𝑃0.
(e) Discuss your findings. What would be the benefits of a 2-stage compressor?
5. Submission requirements
Preliminary work answers are to be made via the Lab 2 prelab submission portal available on Moodle.
All students are to submit their prelab work before the lab (check your deadline on Moodle). For faceto-face students, please bring a copy of your preliminary work to the lab to record your data into.
Submission is to be made via the Lab 2 submission portal made available on Moodle. Your
submission must be typed and be in a single document in word or pdf format. Screenshots or images
of handwritten answers embedded into a document will not be accepted. Your submission is due at
5pm on the day after you attended the lab (again carefully check your deadline).
Late submission will receive 5% penalty per day (including weekends and public holidays) capped at
5 days. Your submission of your analysis results should include the following:
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Cover page including your name, zID, mode of learning (face-to-face or online), course name,
subject code, timeslot of your allocated lab session and assessment task (Lab 2)
Answers to the questions in Section 4.2, analysing your data from the experiments
If you attended the lab face-to-face, please ensure you also include the timeslot of your allocated lab
session.
Format your submission with 12pt Times New Roman font and 1.5 line spacing.
6. Marking
Prelab work (5%) – Lab report (10%)