Refrigeration Experiment
MSU Chilled Water Plant Tour
Worksheet
Due Friday, November 12 by 5:00 pm
(Turn into your TA or their mailbox)
Name:
Signature of TA to Verify Attendance:
1. What devices in an absorption refrigeration system replace the compressor of a
conventional vapor/compression refrigeration system?
2. Describe the four (4) fluid streams within a Trane absorption unit.
3. How does a cooling tower operate?
4. What is the rated capacity of the chilled water plant in tons of refrigeration,
Btu/hr, and kW?
5. Given the chilled water flow rate (from the pump) and the entering and exiting
temperatures of the chilled water stream for one of the absorption units,
calculate the actual cooling load (in tons of refrigeration) the unit is providing.
6. Determine the Carnot cycle COP for the absorption cycle used at the MSU
chilled water plant.
7. Using the Carnot cycle COP and assuming a cooling load of 1250 tons,
determine the required mass flow rate of steam.
MSU Chilled Water Plant Tour
Lecture
This experiment involves a tour of the main chilled water plant on campus and the
completion of worksheet. The main chilled water plant is located on service road;
just eat of the Service Rd-Bogue Rd intersection, as shown on the map overhead.
The tours are scheduled for your regular lab times:
November 2 (Tuesday) 3-5 PM
November 4 (Thursday) 9:30-11:30 AM
November 4 (Thursday) 1-3 PM
You should meet at the plant and wear comfortable clothes and shoes. The tour
will be given by Mike Crouch who is the lead refrigeration technician on campus.
The worksheet consists of some questions about the plant operation and some
simple calculations involving the plant.
The purpose of a refrigeration system is the extraction of heat from a cold space,
so as to maintain its cold temperature. Since the cold space is cold relative to its
surrounding, this heat extraction normally involves driving heat in the direction
opposite to its natural inclination. That is, a refrigeration system must "pump"
heat from a region of low temperature to a region of high temperature. So as not
to violate the second law of thermodynamics this pumping of heat requires a
certain input of energy in the form of work or heat. A simple schematic of a
refrigeration system acting thusly is shown below.
High Temperature Heat Reservoir
at TH
QH
Refrigerator
QL
Low Temperature Heat Reservoir
at TL
Wnet
A refrigeration system generally works by using the fact that a phase change
process is a very effective way of transferring heat.
Hence, the interaction
between the low temperature reservoir and the refrigerator is normally achieved
with a evaporation phase change, whereas the interaction between the high
temperature reservoir and the refrigerator involves a condensation phase change.
Further, by adjusting the pressure the phase change process can be forced to occur
at whatever temperature is appropriate and in whatever direction (evaporation or
condensation) desired for the heat transfer. That is, in order to remove heat from a
low temperature region, a fluid (the refrigerant) can be forced to boil at a low
temperature by lowering the pressure so that energy can be absorbed from the cold
space. Similarly, by boosting the pressure of the refrigerant when it is in contact
with the warm environment it can be forced to condense and release the energy it
absorbed from the cold space. The energy input is what controls the pressures.
A measure of the operation of a refrigeration system is its COP. In general the
COP is defined as
COP =
cooling effect
required energy input
In thermodynamics we learned that the most efficient refrigerator is a Carnot
refrigerator that has
COP max
=
1
TH
- 1
TL
In a conventional vapor/compression refrigeration system, a compressor is used to
control the refrigerant pressure and the required energy input is the work needed to
run the compressor. Here on the MSU campus (as you found out during your tour
of the Simon Power Plant), we have a cogeneration power facility that puts out
both electric power and steam at 90 psig. The absorption refrigeration system is
designed to utilize this steam as the required energy input and significantly reduce
the electricity consumed for air conditioning on campus. Several buildings on
campus are air conditioned with a chilled water system, where the chilled water is
produced at a central plant on campus operating on the absorption refrigeration
cycle. A schematic of this air conditioning system is shown below:
Hot
Air
Warmed
Water
Air/Water
Heat Exchanger
Campus
Building
Cold
Air
Chilled
Water
Condensate
Chilled
Water
Plant
Steam
Power
Plant
Steam
In the absorption refrigeration cycle, the pressure is controlled by two devices
called the generator and absorber instead of a compressor. The refrigerant is a
Lithium/Bromide-Water solution.
A unique phase equilibrium exists for this
solution that connects the Li/Br concentration with the saturation temperature and
pressure.
By appropriately controlling the concentration the pressures of the
condenser and evaporator can be controlled much in the same way a compressor
controls them in a conventional vapor/compression refrigeration system. In order
to maintain the appropriate concentration, energy must be added to the generator
(via the steam, while the absorber must be maintained at a prescribed temperature
and thus requires cooling. Water from the plants cooling tower provides both this
cooling and the high temperature heat reservoir in the condenser. The system is
shown in the schematic below.
CONDENSER
WATER OUT
TO TOWER
STEAM IN
STEAM OUT
CONDENSER
GENERATOR
HEAT
EXCHANGER
LOW PRESSURE
WATER VAPOR
CHILLED
WATER IN
ABSORBER
CONDENSER
WATER IN
FROM TOWER
CHILLED
WATER OUT
EVAPORATOR
GENERATOR ABSORBER
PUMP
PUMP
REFRIGERANT
PUMP
The cooling towers at the chilled water plant provide the heat rejection mechanism
for the absorption system. Energy extracted from the chilled water is eventually
deposited into the cooling tower water. This water then flows through the cooling
tower where some of it evaporates into the air. This evaporation provides the
mechanism to remove the energy from the cooling tower water.
As we found with the power plant tour, the language of academia is very different
form the language of industry.
From our thermodynamics we would have
considered the load of a refrigeration facility (the Q L) to be in kW. In industry the
cooling load is given in tons of refrigeration. A ton of refrigeration is equivalent
to the energy required to freeze one ton of water into ice in an hour. We can relate
this unit to our more conventional units as follows
1 ton of refrigeration = 12,000 Btu/hr = 3.517 kW
Also industry is much more concerned with the capacity of a system and that they
are running at rated capacity than the COP of the system. Industry assumes that
the system manufacturers have maximized the COP in their design and as users
simply want the system to run at the capacity specified by the manufacturer.
Review the worksheet, especially the calculations reminding the students of the 1 st
Law of Thermodynamics.
Figure 2. Schematic View of Trane Absorption Unit