10-08-2013, 01:00 PM
Thermal Analysis of Air Conditioning System
Introduction
The basic concept behind air conditioning is known to have been applied in ancient Egypt, where
reeds were hung in windows and were moistened with trickling water. Air conditioning is the
process of alternating the properties of air primarily temperature and humidity to more suitable
or favorable conditions. More, generally, air conditioning can refer to any form of technological
cooling, heating, ventilation or disinfection that modifies the condition of air. Comparison
between controlled and uncontrolled cycle is made in case of a case study on thermal analysis of
car air conditioning cycle. People more and more time spend in vehicles (cars, trains, planes,
buses or subway). This is the reason why the thermal comfort has more and more paid attention.
In one hand people try to make comfort (in each situation) whatever they are at home, office or
in car. In the other hand the thermal conditions in the cabin of vehicles directly influences on the
driver's and passengers safety. The investigations presented in this paper are the part of larger
project, which assumes complex modeling of thermal state of car interior.
Thermal Comfort In Vehicles
Car compartment is a place where often thermal discomfort is obvious. In summer, the most
important phenomenon of energy transfer is sun radiation. In hot summer day radiation heat flux
can be even several times higher than convection heat flux. It can be easy to notice when one
takes car to shady road after driving in the sun. Coldness and freshness feeling is almost
immediate. Temperature in a vehicle cabin is closely related with the occurrence of traffic
accidents . In hot summer days internal temperature often exceeds +30°C. Zlatoper has created
the ranking list of the factors which affect the traffic accidents in United States and placed the
temperature on the third position. So it is obvious, that the thermal conditions in the cabin of
vehicles directly influences on the driver's and passengers safety. Both too high and too low
ambient temperature influences human physical and mental state. Driver's efficiency researches
indicate that it can be even 35% higher at +20°C than at +35°C.Decrease of efficiency at +5°C
can be the same as that at +35°C.It is impossible to ensure temperature at the same level as
outside compartment using only ventilation, even with the strong flow. The internal temperature
is almost always higher.Radiation heat flux with greenhouse effect in car compartment is higher
that heat flux which canbe refuse with ventilation.
Non Controlled Cycle
In this case as expansion valve is orifice and everything depends on orifice effective throttle area.
If the area is too high the compressor works properly (without fluid droplets) only in some range.
If effective throttle area is too low the refrigerant at evaporator outlet is always superheated but
the temperature at compressor outlet can be too high. That high temperature involves the
problems. First is higher compressor material durability, second is a problem withliquid phase at
condenser outlet, especially with higher ambient air temperature.
Controlled Cycle
In this case there is no problem with too high temperature, because expansion valve always
assures 1K of superheating. So there is no problem with high temperature material durability,
because temperature at compressor outlet is lower too. If the temperature at compressor outlet is
lower it is also easier to obtain liquid phase at condenser outlet. One can say the controlled cycle
is more “flexible”. In this case heat flux in evaporator can be about 1.5 kW higher than in
noncontrolled one, which means that we can reject 1.5 kW of heat flux more from car
compartment. For 0.055 kg charge, compressor inlet (evaporator outlet)temperature is about
40°C at 45°C air temperature.
Conclusion
It can be noticed that the decrease of the refrigerant charge decreases COP both for R134aand for
CO2 as refrigerant.. For R134a case, decrease is higher in controlled cycle because the
temperature in evaporator is higher which causes lower heat flux in evaporator and higher
compressor power. Beneficial is that compressor works always with gas phase. For
noncontrolled R134a cycle efficiency is higher but the hazard of compressor work with liquid
appears. For CO2 case COP values are on the same level for all temperature, except 0.15 kg
charge when it decreases significantly with air temperature. It can be noticed that COP values are
lower than for R134a cycle. In all cases COP is lower when refrigerant charge is below its
nominal value.