MECH5255 2018 Lecture 13

MECH5255
Air conditioning and refrigeration

Week 13, lecture#13
Semester 2, 2018
M.J. Dunn MECH5255 2018

Admin
Exam:
-Check the exam time does not clash for you
-Three questions answer all questions on topics such as:
-Refrigeration cycles and topologies
-Psychrometrics
-Fans and ducts
-Heat loads
-Systems
-Comfort
Admin
Assignment 3:
-Due this Friday
Major project will take Kaichung and myself a long time

to mark...
Admin
Assignment 2 comments and feedback.
Revision
Worked solutions will be posted online to the following revision
problems in these slides. Only answers to the recommended
practice problems will be provided.
Revision: Refrigeration systems worked problem
A two-stage compression refrigeration system with an adiabatic liquid-vapour
separation unit (as per diagram below) uses R-134a as the working fluid. The
system operates at an evaporator temperature of -32.0°C, the condenser at
1400 kPa, and the separator at 8.90°C. The refrigerant is circulated through
the condenser at a rate of 2.00 kg/s. Determine the rate of cooling and the
power requirement for this system.
Some necessary assumptions:

The refrigerant is a saturated liquid at the
inlet of each expansion (throttling) valve
and a saturated vapour at the inlet of each
compressor. Assume each compressor is
isentropic.
Revision: Refrigeration systems practice problems
Cengel and Boles 7th edition:
11.57, 11.58, 11.59, 11.61, 11-114, 11.121
Revision: Psychrometrics worked problem
A laboratory has a volume of 700 m3 and must have one complete air change per
minute when being used. Outside air at 1 atm, 30°C dry bulb, and 60% relative
humidity is ducted into the laboratory air conditioning equipment and conditioned
to 20°C dry bulb and 50% relative humidity, the required state for air supplied to
the laboratory.
i) Plot the process on a psychrometric chart for an idealised cooling process
and sketch a diagram of the system hardware.
ii) Using both the psychrometric chart AND the exact method using tables and
psychrometric formulas in a table show the enthalpy, specific volume, dry
bulb temperature, wet bulb temperature (psychrometric chart only) dew point,
humidity ratio and relative humidity at the outside air condition, at the exit of
the cooling coil and the room condition.
iii) What outside atmospheric air mass flow rate is required for the air change, in
kg/h?
iv) Determine the mass flow rate of water condensed from the atmospheric air,
in kg/min.
v) The cooling fluid in the air conditioning system is chilled water which has a
15°C temperature rise over the cooling coil in the heat exchange process.
Determine the chilled water mass flow rate for the cooling coil, in kg/min.
Revision: Psychrometrics practice problems
Cengel and Boles 7th edition:
14.122, 14.128, 14.131, 14.135
Revision: Fans and ductwork
A fan with the manufacturers data below in Table 1 developed at 800 rpm, is
connected to a ductwork system that is known to produce a 500 Pa pressure drop
at a flowrate of 4 m3/s. When connected to the ductwork system it is known that
the fan operates at 800 rpm.
i) Determine the flow rate and the developed pressure the fan delivers when
connected to the ductwork system.
Table 1, fan curve developed at 800rpm
Q (m3/s) 0 1 2 3 4 5 6 7 8
Δp (Pa) 800 780 750 710 660 600 500 320 50
Q (m3/s) 0 1 2 3 4 5 6 7 8
Δp (Pa) 800 780 750 710 660 600 500 320 50
System 0 31.25 125 281.25 500 781.25
II) Now consider the case when the fan is connected to the ductwork system the
fan operates at 1200rpm.
Revision: Additional areas
i) Review all of Simon’s lecture notes: Loads, zoning, duct design, system
selection and scheduling.
-Strategies to select rooms and zones
-Impact of loads, sensible and latent, fresh air requirements, solar vs. internal.
-Examples of high and low latent loads
-Scheduling vs. utlisation factors
-Strategies for design and layout of ductwork
-pros and cons of different systems (e.g. chilled water, direct expansion,
unitary/package units, CV vs. VAV)
Revision: Additional areas
i) Review the lecture notes from the IEQ lab tour

• pros an cons of UFAC and chilled beams
• Factors that impact air quality
• Factors that impact human comfort, Fangers model of comfort
• PMV and PPD
• CLO and MET
Some parting thoughts
Be willing to learn, but have skills to offer that are of value
HVAC IS important
Psychrometrics vs. Psychometrics
How to proceed once in a position of influence

Unit of study survey
Your feedback matters!

-previous feedback has led to changes (improvements) such as field tours
https://student-surveys.sydney.edu.au/students/
18
Be willing to learn, but have skills to offer that are of value
HVAC IS important
Psychrometrics vs. Psychometrics
How to proceed once in a position of influence

Thank you!

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MECH5255

Air conditioning and refrigeration

M.J. Dunn MECH5255 2018

Major project will take Kaichung and myself a long time

Some necessary assumptions:

Table 1, fan curve developed at 800rpm

i) Review the lecture notes from the IEQ lab tour

Be willing to learn, but have skills to offer that are of value

Psychrometrics vs. Psychometrics

How to proceed once in a position of influence

Your feedback matters!

Be willing to learn, but have skills to offer that are of value

Psychrometrics vs. Psychometrics

How to proceed once in a position of influence

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