Refrigeration Cycle

REFRIGERATION CYCLE

There are many types of mechanical refrigeration systems. They come in many varieties, shapes, sizes, and component arrangements depending on end application.

If one tries to understand each system separately, it would be a long and difficult task.

If on other hand one learns the basic refrigeration, the job becomes much easier, since the principles of mechanical refrigeration and essential components of system are the same, no matter how big or small is the system and how it is packaged.

As we have learned there are four basic components and refrigerant fluid circulated through each for purpose of moving heat. The refrigerant always flows in one particular direction when it is in cooling mode cycle.

A refrigerant is a fluid that picks up the heat by evaporating at a low temperature and pressure and rejects heat by condensing at high pressure and high temperature.

PRINCIPLE OF OPERATION

The normal Carnot cycle uses heat energy and delivers mechanical work as energy output whereas in Refrigeration and Air Conditioning it is the reverse Carnot cycle operation. This means work is supplied as energy input and heat is transferred from lower energy level to higher energy level.

The normal strategy of mechanical refrigeration is first to get heat into refrigerant. Then elevate its energy level and temperature level and pump it to a place where heat can be removed from it. The refrigerant makes this heat transfer possible.

Most of heat transfer takes place through latent heat by changing phase from liquid to vapour in evaporator and from gas to liquid in condenser.

In open cycle refrigeration the substance doing cooling is open to atmosphere. The common examples are old fashioned ice box, an earthen pot for water, liquid nitrogen or solid carbon dioxide commonly known as dry ice. Even refrigerant if allowed to escape to atmosphere through a cooling coil and pressure reducing valve, it would give cooling. However it is extremely expensive to allow the gas to go to waste, it is also illegal to vent the gas to atmosphere in most of the countries. We use close cycle refrigeration as we want to make use of gas again and again.

Even in closed cycle refrigeration if we do not use compressor and metering device, a closed cycle refrigeration system will not achieve the objective as heat will then flow in a reverse direction till the temperatures are equalized. The heat will flow downhill from ambient to inside the premises if the ambient temperature is higher than inside.

As water cannot move from lower level to higher level without pump,  and since we have to reject heat uphill, we require compressor and mechanical energy to raise the level of refrigerant.

Normal evaporating temperatures for Air Conditioning application  are between 40 to 45° F(4.4⁰c-7⁰c} , while the room is being maintained at 75°F(23.9⁰c) and the supply air to the room is around 50 to 55°F.(10⁰c-12.8⁰c)

Similarly for 95⁰ F(350c) ambient temperature, the air-cooled condenser operates at around 120⁰ F(48.9⁰c) sat. Temp.              (170 ⁰ F(76.7⁰c) actual gas temp.). This corresponds to 263 psig.(18.8 bar)

Normally the design is based on 15 to 25° F(8.8⁰c-13.8⁰c) rise/fall in temperature across condenser/evaporator.

The metering device and compressor separate system components in high side and low side of the system.

The high side comprises of  part of compressor, discharge line from  compressor to condenser, condenser, liquid line, and inlet of expansion valve.

The low side comprises of outlet of metering device, evaporator, suction line, and compressor inlet.

The function of 4 major components is as under:

1.   An evaporator to absorb heat into the system.

2.   A condenser to reject heat from system.

3.   A compressor to establish temperatures and pressures necessary to force heat to flow upwards.

4.   A liquid metering device to regulate the flow of refrigerant and in the process to establish two pressure levels.

An evaporator when used to cool liquid  which could be water/ brine is called chiller. Normally for Air Conditioning duty liquid chillers are used above 100 Tons capacity invariably. Similarly condensers could be air cooled or water cooled.

The compressor is a mechanical device that boosts pressure from low side to high side of refrigeration system. Heat of compression added is about 22 to 30 % to the heat absorbed in evaporator.

The condenser therefore has to reject  this additional heat. Hence condensers are normally 15 to 20% bigger  in surface area than coolers for identical design & temperature differences.

Component functions- Summary

1.   Evaporator absorbs heat from conditioned space into system. This causes refrigerant to get converted  from low temperature , low pressure, saturated liquid /vapour mixture into  low temp., low pressure superheated gas.

2.   As gas enters compressor it is reduced in volume, then compressed into high temperature, high pressure, Superheated gas.

3.   As gas passes through condenser, heat is rejected so that it changes into high temperature, high pressure, sub-cooled liquid.

4.   The metering device takes this liquid and regulates flow to evaporator and puts it through desired pressure drop. In this process, refrigerant changes into low temperature, low pressure, saturated liquid/ vapour mixture. The cycle repeats.

The pressure enthalpy diagram helps to

1.   Trouble shoot mechanical refrigeration system

2.   See function of each part & how they work together to change pressure and move heat.

3.   Predict pressures and temperatures to be expected at various places in the system.

4.   Gives an important tool for learning other principles about refrigeration systems.