Saturday, January 7, 2017

Domestic refrigerator suction line accumulator

In refrigeration, suction line accumulator (sometimes called liquid trap) is a small tank that is used to deliver refrigerant gas to the compressor to avoid compressor flood-back, liquid refrigerant compression (in some cases), and the potential of external water vapor condensation on suction tube (if not insulated). Suction line accumulator is commonly placed between the exit of evaporator and the capillary-suction line heat exchanger. Suction line accumulator is a passive protection element (protect compressor) which is not functioning the whole time, but in certain situations it does its job.

Domestic refrigerators..., some have suction line accumulators and some don't. So, why this difference? To answer this question we have to ask the question:

What is the function of suction line accumulator?

As we said before, the accumulator function is to receive saturated mixture refrigerant (liquid+gas) and deliver a saturated vapor refrigerant gas. If the refrigerator is in the full-load state, the refrigerant state at the exit of evaporator (before accumulator) is probably saturated vapor and may be a superheated gas because the heat load is high enough to boil and evaporate the whole refrigerant inside the evaporator.

So, "What will happen in case of small loads?". At small loads the evaporator is flooded with liquid refrigerant and the exit of evaporator contains too much liquid refrigerant. The following are the cases where the liquid refrigerant return is possible:

[1] Overcharged refrigerator
[2] Oversized compressor: using compressor with higher cooling capacity
[3] Excessive accumulation of frost on evaporator which leads to thermal insulation of evaporator and evaporator clogging
[4] Stalled (stopped) evaporator fan
[5] Low ambient temperature
[6] Compressor startup
[7] Very small evaporator size
[8] Low evaporator heat exchange effectiveness

How suction line accumulator works?

Suction line accumulator has a tank and deflector tube. In this configuration the liquid refrigerant settles down in the bottom of tank. Also, the expansion of refrigerant in accumulator tank causes pressure drop which lower evaporation temperature and evaporates some liquid refrigerant as well. The deflector tube also splashes liquid refrigerant on accumulator walls causing more evaporation of liquid refrigerant and this is why most accumulators used in refrigerators are bare (non-insulated). Additionally, in frost-free refrigerators the defrost heater always evaporate the trapped liquid inside accumulator during the defrost period.

Commercial suction line accumulator. Source: Modern refrigeration and air conditioning

Suction line accumulators may be found in different orientations: horizontal, inclined, and vertical. The higher the inclination angle, the higher volume of liquid refrigerant it can trap.

Suction line accumulators are mostly used in refrigerators with single speed compressor with on-off control. Also they are plenty in refrigerators using refrigerant 134a as well as refrigerators having mechanical control.

Modern refrigerators with variable speed compressors or linear compressors can modulate different loads by controlling the refrigerant mass flow in the refrigeration circuit so it is not mandatory to use suction line accumulator. Most refrigerators with R600a, don't have suction line accumulators because the total amount of refrigerant inside the circuit is small compared to R134a.

Some designers don't use suction line accumulator as they rely on the heat of compressor motor which can evaporates the return liquid. Others may use long suction tube path outside the foam insulation to make sure that no liquid refrigerant will return.

How to calculate accumulator size (Accumulator sizing)?:

For a predefined refrigerant charge, the simplest and safest way is to assume that the whole refrigerant charge in refrigeration circuit is trapped inside accumulator in liquid state at evaporator pressure of ASHRAE conditions (evaporator temperature equals -23.3 C), calculate the density of liquid, and then the volume of this liquid (see the image below of an actual example)