Refrigeration ExperimentMSU Chilled Water Plant Tour

WorksheetDue 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 PMNovember 4 (Thursday) 9:30-11:30 AMNovember 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.

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 = 1

T

T - 1

max

H

L

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:

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.

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 QL) 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 1st

Law of Thermodynamics.

Figure 2. Schematic View of Trane Absorption Unit