Graduation Year

2006

Document Type

Thesis

Degree

M.S.M.E.

Degree Granting Department

Mechanical Engineering

Major Professor

Muhammad Rahman, Ph.D.

Co-Major Professor

Luis Rosario, Ph.D.

Committee Member

Autar Kaw, Ph.D.

Committee Member

Roger Crane, Ph.D.

Keywords

Coefficient, Performance, Single-Stage, Two-Stage, Refrigerant

Abstract

Carbon dioxide is a natural refrigerant that has been considered for certain

refrigeration and air conditioning applications. The coefficient of performance (COP) of

carbon dioxide cycles is low compared to classical vapor compression cycles. The aim of

this portion of the thesis is to present a thermodynamic analysis of carbon dioxide cycles

in order to evaluate the potential performance of a refrigeration cycle using carbon

dioxide. A thermodynamic model for the cycle is proposed which can simulate the

operation of a carbon dioxide refrigeration cycle. This model takes into account the

practical effects of the thermo-physical properties of carbon dioxide as a refrigerant in a

trans-critical cycle. One and two-stage compression processes were considered for

comparison purposes. A sensitivity analysis has been conducted so that cycle

performance can be estimated. The effect of cycle components on system capacity and

cycle performance was investigated.

The second portion of the thesis deals with the concept of reheat air conditioning,

and looks at the performance of different reheat cycles. The thesis looks at reheat

systems that utilize different placements of the reheat coil. The overall performance of

these reheat systems is then calculated. These systems require no additional electric

power to reheat the air after it is cooled and dehumidified in the evaporator. Instead, they

use heat from the condenser heat exchanger to reheat the air during partial load

conditions. Four different reheat configurations are discussed and analyzed to determine

performance levels. Visual Basic programs were written for each of the four cycles to

simulate the different configurations and to evaluate key performance parameters.

Graphs were developed based on these programs, where critical variables were changed

to monitor trends in coefficient of performance. The thermodynamic cycle of each reheat

configuration is developed, with equations presented with figures depicting the cycles.

Refrigerant 134a was used in the programs throughout the reheat section of the thesis.

The reheat coefficient of performance is used as the basis for cycle comparison. The

relative performance of the four cycles is illustrated in the figures and explained in the

Results and Discussion section at the end of chapter 3.

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