16 Feb 2017

Refrigeration 101

To get ready to study for an HVAC associate’s degree, there are a few things you’ll need to be familiar with. The process of refrigeration is one of them. We’ll go through the basics of refrigeration here to get you started.

Heat Transfer

All refrigeration systems operate on one single principle: heat transfer. In any space, there’s heat. Naturally, the less heat there is in the air, the cooler it will feel. Refrigeration simply takes heat from the air in a space and transfers it somewhere outside.

Special refrigerant fluids are used to accomplish this. It’s run through a pipe system that allows it to absorb heat from inside an air conditioned space and carry it outside, where the heat is dispersed by a condenser. The way refrigerant is circulated through the system depends on how it’s designed. Two layouts for refrigeration systems are direct expansion and secondary loop.

Direct Expansion

The direct expansion system, or DX for short, operates solely on the pressure changes in the refrigerant fluid. In order for the system to function, the fluid changes state from a low-pressure liquid to a high-pressure gas. The whole system works something like this:

  • The refrigerant begins absorbing heat in the evaporator.
  • The refrigerant boils and becomes a gas, increasing pressure.
  • The pressure change causes the compressor to draw it down the line.
  • Once the refrigerant passes through the compressor, it is cooled by the condenser.
  • Being cooled to a lower-pressure state, the refrigerant moves on to an expansion valve.
  • The expansion valve relieves pressure and allows the cycle to begin again.

The reliance on state changes from liquid to gas means the system requires a great deal of refrigerant to function. It also means the piping needs to be made of very strong, resilient material.

Secondary Loop

A secondary loop system, or SN system, uses a pump to circulate fluid through a secondary loop (hence the name) that’s connected to a DX system. The refrigerant in the secondary loop absorbs heat from the refrigerated space through a heat exchanger. A pump circulates it to a chiller, which doubles as the evaporator in the DX side of the system. There, the heat is absorbed into the DX system where it follows its usual course.

A secondary loop system can function on half as much refrigerant as a strictly DX system, making it more environmentally friendly. Also, the secondary loop doesn’t have to change states, which puts less stress on the SN side of the system. As such, this method has become much more popular in recent years for HVAC systems.