Fundamentals of the automotive cabin climate control system

Nitin Gupta, Manish Jindgar and Ravinder Dasila, Freescale Semiconductor -December 21, 2012

Heating Ventilation and Air Conditioning (HVAC) is the technology for indoor and automotive ambient comfort. HVAC facilitates in managing the pleasant climate inside the cabin by controlling the degree of hotness/coolness.

There were times when having an air conditioner in a car was considered one of the big features, but today air conditioners have become standard equipment even in entry-level automobiles. The desire for even more comfort and luxury has led to the development of the climate control system inside an automobile. The primary purpose of automatic climate control is to manage the temperature of a given area for the comfort of onboard passengers.

HVAC was first introduced into automobiles in the early 1960s, and is available in most of the high-end vehicles today. It is a complex system consisting of mechanical/electronic switches or knobs in the frontend. The backend of the system comprises one or more blower motors, actuators (for fresh air circulation control, air-flow control and temperature control), and refrigeration unit coupled with many ducts through which air is transferred to the cabin.

The basic principle behind the operation of HVAC unit is conduction and convection. Heat is transferred from a low-temperature region to a high-temperature region in the vehicle, due to the pressure difference. This process of heat transfer is called Refrigeration. Figure 1 shows the cycle diagram of the complete refrigeration process.

Figure 1: Refrigeration cycle diagram.

An air conditioning system comprises five major components:

  1. Evaporator
  2. Compressor
  3. Condenser
  4. Receiver/Drier
  5. Expansion device

The five major components are divided into two pressure regions: the high-pressure side is the condenser and receiver/drier unit, and the low-pressure side is the air conditioning evaporator. The dividing point between high and low pressure cuts through the compressor and the expansion valve.

The following section discusses in detail each piece of the HVAC system (see Figure 2).

Figure 2: An air conditioning system comprises five major components.

An evaporator is a heat exchange device in the refrigeration cycle. The liquid refrigerant, coming out of expansion value and entering into the evaporator, is at lower temperature and lower pressure.

On passing through the evaporator coils refrigerant absorbs the heat from the air that is blown through the coils and gets converted to low temperature, low pressure vapor. The liquid refrigerant is made to flow from the bottom to the top of the evaporator coils to ensure that liquid refrigerant boils before it leaves the evaporator coils.

The tasks performed by the evaporator can be summarized as follows:

  1. Absorption of heat
  2. Boiling of all the refrigerant to vapors

The air blown by the blower in turn gets cooler, on transferring the heat, and is passed into the cabin through the vents.

Since air conditioning evaporator provides the cooling by absorbing the heat from the surrounding medium, it could serve dual purpose when placed very close to dashboard of vehicle. It absorbs the heat from the air that is passed through it and also absorbs heat from inside the vehicle to maintain the required temperature.

The air conditioning compressor is known as the heart of the central air conditioning units. The compressor absorbs vapor refrigerant from the suction line and compresses the vapors to high superheat vapor. The temperature of the vapor is normally two and a half times higher than the temperature of the outside air.

Since heat always flows from hot to cold, the refrigerant must be much hotter than the outside air to be able to move heat out of the system. As the refrigerant flows across the compressor, it also removes heat of compression, motor winding heat, mechanical friction, and other heat absorbed in the suction line. Another key task of the air conditioner compressor is to generate the flow of refrigerant in the system.

The tasks performed by compressor can be summarized as follows:

  1. Superheat
  2. Remove latent heat or (condense)
  3. Remove more sensable heat or (subcooled)
  4. Generate the flow of refrigerant

Next: Condenser

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