Repsol YPF

Compression systems (or direct expansion cycles)

A compression system can be one of two different types:

a) Heat pump, which will be covered in a separate section.
b) Water-cooling units.

The difference between these systems and current electrical appliances is that the engine that activates the system's refrigerating compressor runs on gas.

In compression systems, 'cold' production is achieved by provoking the evaporation and condensation of a refrigerating fluid, which follows the cycle shown in the diagram below:

Compression cycle with a gas motor

1-2: The refrigerant enters the compressor under certain pressure and temperature conditions. The compressor raises the pressure of the refrigerant through adiabatic compression (without heat exchange), which also raises the temperature of the refrigerant.
2-3: The refrigerant moves into the condenser, which is a heat exchanger situated on the outside, and condenses, going from vapour to liquid form and releasing heat, which passes directly into the outside air or to a water source that will then transfer it to the air.
3-4: The refrigerant passes through an expansion valve, which reduces its pressure and temperature.
4-1: On leaving the expansion valve, the refrigerant enters the evaporator, situated on the inside of the room that needs to be cooled, and evaporates. In order to do this, it takes the necessary heat from the air inside the room in which it is situated, thereby causing a drop in the air temperature in the room.
Following this, the cycle begins again.

Water-cooling plants are based on a classic compression cycle, in which the compressor is operated by a gas engine. They are basically conventional cooling units, but the fact that there is a combustion engine allows the residual heat from this to be recovered using the necessary exchangers, thus achieving temperatures that can reach around 80°C. Consequently, they can be used to meet the installation's heat demands, basically in the form of hot water.

These units are particularly suitable in plants in which there is a combined demand for cooling and hot water, such as ACS, and allow for a considerable energy saving. There are currently models being marketed that provide refrigerating power ranging from 100 to 1200 kW. The cold water return and delivery temperatures are around 12°C and 6.5°C respectively.