How does my fridge cool the food?

Refrigeration gas system…..

What happens to cool the food when the fridge compressor is switched on?

The gas system is composed of a compressor, a network of pipe, a filter dryer and an evaporator, all connected together in a continuous sealed loop. Within this loop a vacuum is created and a measured amount of refrigerant gas is then inserted, free of any moisture.

Starting at the compressor:
Low pressure gas is pressurised by an electrical pump called a compressor, the action of compressing the gas increases the heat in the gas as it flows into the High Pressure pipework side of the system.

• Much like the heat generated by a bicycle tyre pump when blowing up a deflated tyre.

This hot gas is piped via the pipework tubing around the cabinet door openings, buried just inside the insulation of the cabinet. These pipes can be called “dew pipes” or “hot pipes” and their purpose is to prevent Fridge and Freezer door seals from freezing to the cabinet by keeping the contact area very slightly warm.

• This effect is not usually noticeable during normal working conditions

The hot gas is then routed to a condenser unit, which can be a series of pipe work coils (which are blown with cooling air from a condenser cooling fan motor) or a flat panel of pipe work, spaced out by cooling fins or vanes. However the condenser is formulated it has a specific purpose:
The gas is cooled by the fins and vanes releasing heat into the room (the condenser is warmer than the ambient room temperature) or the air flow from the condenser fan motor across the condenser pipes, this cooling starts the liquifying process of what was refrigerant gas into liquid refrigerant.

• Heat flows from a warm body to a cooler body easily. The theory goes that two identical cups of water, 1 @ 50 degrees and 1 @ 70 degrees in the same conditions and ambient temperature will both cool to 10 degrees at the same time. The hotter liquid looses heat more quickly.

Across the condenser tubing there will be a significant temperature drop, referred to as “the glide”, and at the end of the condenser pipework is a filter dryer unit.
The filter dryer unit has the purpose of absorbing oil and impurities from the liquid refrigerant passing through it. This debris has come from the compressor, as the refrigerant gas can absorb oil under certain conditions during use.

• If the filter dryer becomes ineffective at removing oil from the liquid, a system blockage or a reduction in pressure will occur.

The “liquid refrigerant” is now delivered to the evaporator unit by a small delivery pipe with an internal diameter of approx 1mm. This pipe is called either the measuring pipe or the delivery tube although it is commonly called the capillary tube.
It’s important to note that, the compressor sizing, the condenser and the length and diameters of pipework are all calculated to deliver liquid refrigerant to the evaporator expansion device at the correct volume and pressure.

• The evaporator expansion chamber is a venturi device

The effect of delivering a known amount of pressurised liquid from a small tube into a large expansion device immediately lowers the pressure of the liquid.
 This allows some of the liquid to vaporise which dramatically reduces the temperature; sometimes referred to as “boil off”, this evaporation process cools the remaining liquid. Expect R134a @ -34 degrees and R600a @ -28 degrees approx.

• This is now the low pressure side of the gas system and the compressor is effectively sucking on this return piping to bring the liquid back to the compressor

As the liquid passes through the evaporator the liquid absorbs heat from the inside of the fridge cabinet and as the liquid warms up it vaporises back into a gas.
Within the fridge cabinet, ice droplets can form on the very cold surfaces of the liner or evaporator plate, the water vapour coming from the air within the cabinet. Each door opening will increase the water vapour within the cabinet, as cool air spills from the lower cabinet and warm air is sucked in the upper cabinet to replace the air volume.

And ending at the compressor
At all times when the compressor is running, high pressure is being pushed out of the HP port, but also the low pressure side is “sucking” on the return tube, bringing back the refrigerant gas so that the cycle is constant.
To aid the warming process of the vapour/liquid refrigerant that is leaving the evaporator on its return journey to the compressor, the 1mm delivery tube is routed inside the return tubing. Therefore the heat from the delivery tubing is warming the return liquid/gas ensuring that only gas is returned to the compressor, as it is not possible to compress a liquid

A few definitions

• Absorption:
  A reaction process in which heat is taken (absorbed) up by a liquid or solid.
 
• Fridge Compressor:
  This is a pump which compresses refrigerant gas, and consequently heats the gas.
 

• Condensation:
  The process of changing from gas or vapour to a liquid.
 
• Evaporation:
  A change from a solid or liquid to gas or vapour. It occurs when some molecules of a liquid have enough energy to escape into the gas phase and this has an overall cooling effect on the liquid.
 
• Refrigerant:
  A chemical substance used as a fluid in a refrigeration system. There are many different types of fluid used, depending on the system design. Most commonly used are hydrofluorocarbons R134a (HFCs) and hydrocarbons R600a (HCs) which is HFC free

• Refrigeration:
  This is the transfer of heat from a substance to be cooled to somewhere else. Heat flows naturally from a warm substance to a colder one.