Charging of a Refrigeration Plant

What is refrigeration?

Refrigeration is defined as a process of removing heat from a space called the refrigerated space and pumping it to the ambient. It also includes the process of maintaining and reducing the temperature of the refrigerated space below the general temperature of the ambient.

Carnot Refrigerator
Carnot Refrigerator

Theoretically we can say that a refrigerator is a heat pump which pumps the heat from the low temperature to the high temperature by taking some work input.

What is marine refrigeration?

Refrigeration on board is required for one main reason that is to prevent the cargo from any damage on to prevent the perishable goods so that they are transported in good and healthy conditions.

Refrigeration prevents the growth microorganisms, fermentation, oxidation and drying out of cargoes.

Moreover, the refrigeration systems on board are required to preserve the food items that are stored in the provision rooms for the crew on board.

Marine Refrigeration Plant
Marine Refrigeration Plant (Source-Oceanic technical solutions)

Components of a Marine Refrigeration plant

Compressor

A reciprocating two stage or single stage compressor which is used to compress the refrigerant to high temperature and pressure.

Condenser

A heat exchanger which provides the heat exchange between the refrigerant and the ambient.

Evaporator coils

A heat exchanger which provides the heat exchange between the refrigerant and the refrigerated space.

Thermostatic expansion valve

A valve which provides the isenthalpic expansion of the refrigerant passing through it.

Refrigerator components (Charging of a Refrigeration Plant)
Refrigerator components

What is a Refrigerant?

A refrigerant is a working fluid used in the refrigeration cycles and heat pumps. In most cases they undergo a repeated phase transition from a liquid to a gas and back again and transfers the heat from the refrigerated space to the ambient maintaining lower temperature than the ambient space in the refrigerated space.

What is charging of a refrigerant?

As we have seen earlier refrigerant plays an important role in absorbing the heat from the refrigerated space and throwing it out to the ambient.
Therefore, all the refrigeration systems rely on the amount of refrigerant gas in their systems, to work efficiently.

This is where the refrigerant charging comes into the picture, charging of the refrigerant refers to the replenishment of these gases when system repairs or leaks have caused depletion the refrigerant levels.

The charging of the refrigerant can be carried out by using bulk refrigerant containers or charging kits which generally contain enough gas for only one charge.

How do you charge a refrigeration system?

Step 1: Evacuating the refrigeration system before charging

Evacuating is done with the help of an evacuator pump. The evacuator pump removes all the moisture and the contaminants present in the refrigeration system.

Step 2: Low side refrigerant charging

In this only the refrigerant is allowed to enter the system, it is also referred to as vapour charging. The canister supplying refrigerant is connected to the gauge service port and opened, and the appliance is kept in the running state. The canister containing the refrigerant is kept upright so that only the vapour from of the refrigerant leaves the canister.

This type of charging can be seen in the Split AC systems.

Step 3: High side refrigerant charging

When charging a refrigeration system from the high side, the system being charged is turned OFF. The canister containing the refrigerant gas is placed upside down so that only the pure liquid form of the refrigerant leaves the canister.

Refrigerant charging procedure (Charging of a Refrigeration Plant)
Refrigerant charging procedure

How is refrigerant charge calculated?

The refrigerant charge of the system includes refrigerants present in the compressors, condensers, evaporators and connecting pipes. Therefore, mass of the refrigerant in all these components has to be taken into account to calculate the total refrigerant charge.

The refrigerant charge to be calculated in condenser and evaporator includes two phases, that is, single-phase and two-phase regions.

Therefore,

Here,

(mtotal) – Total charge amount

(mcompressor)- Charge in Compressor

(mthp)-Charge in tubes of high-pressure

(mtlp)-Charge in tubes of low-pressure

(mcon)- Charge in condenser

(mevp)- Charge in evaporator

Also,

suc)- suction density

dis) – discharge density

(Vcompressor)- volume of the compressor

g)- density of gas refrigerant

 (ρl) -density of liquid refrigerant

(mA/D)- refrigerant charge of A/D

ρcong -density of gas section in condenser

ρconl – density of liquid section in condenser

ρevag – density of gas section in evaporator

Lg -length of gas section

Ll – length of liquid section

 ε is void fraction, which is defined as:

Aand Al are the areas occupied by the gas section and liq. Section.

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