27-06-2012, 02:41 PM
Thermodynamics Design Project: Automotive Air-Conditioning System
Thermodynamics Design .doc (Size: 1.54 MB / Downloads: 82)
Introduction:
The Montreal Protocol has ceased production on R-12 (Freon). As a result, new automotive air conditioning systems have been switched to R-134a. R-134a is a high pressure refrigerant with zero ozone depleting potential but with a global warming potential that is not acceptable. As a result, new compressor designs along with substitute refrigerants are being investigated. The purpose of this project is to evaluate the feasibility of a new compressor design; a constrained rotary vain design for use in an automotive air-conditioning system. The new constrained rotary vane compressor will be placed in the vapor compression refrigeration cycle for the purposes of this project. The proposed compressor has a compression ratio of 3.6, but the ratio can be adjusted up to 6.0 if justification is given. The compressor utilizes four vanes as shown in Figure 1. This allows four inlet volumes to be compressed each rotation. The compressor body, vanes, and rotor can be produced for various compressor widths depending on the density of the refrigerant chosen by the designer.
System Analysis:
Choice of Refrigerant:
The choices of possible refrigerants for the system include R-134a, R-124, or R-152a. The refrigerant that is being replaced is R-12, therefore, a saturation curve that is relatively close to the saturation curve of R-12 is desirable.
Evaporator Calculations:
The air flow rate in the evaporator can be calculated based on the temperature difference of the refrigerant R-134a and the temperature difference of the air. In the worst case scenario, the ambient air temperature will enter the heat exchanger at around 100ºF and will need to drop to 65ºF to provide the passengers with a comfortable environment. The temperatures of the refrigerant are the temperatures of state 4 and state 1. The temperatures are therefore:
Non Engineering Explanation
The refrigeration system that was designed is intended to provide cool air to a vehicle using Refrigerant 134a in order to maintain a comfortable environment. To accomplish the refrigeration process, four separate components are needed. The first component is the compressor. The compressor takes the low pressure, low temperature refrigerant and compresses it to a high pressure, and therefore high temperature state using power added by the engine. In the case of this design, the low pressure and temperature is 60 psi and 49.89ºF and the high pressure and temperature is 375 psi and 203.31ºF respectively. It does this so the temperature of the refrigerant is high enough to facilitate heat transfer from the refrigerant to the surrounding environment that takes place in the next component, the condenser. The condenser is designed to exchange heat from the hot refrigerant to the outside environment.