Flood back

[skyboy6142]

Hi,

If an expansion valve is super-heat controller why we use accumulator in suction line in refrigeration systems to prevent flood back ?

Thanks.

[Rob White]

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Belt and braces approach for safety plus if you have multiple evaporators on a system, if they are independently controlled they have defrosts and valves opening and closing which all add to the complexity of the system and need the extra protection for the compressor.

Rob

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[mbc]

Hi
The an expansion valve controls inlet liquid by meassuring super heat and pressure of the evaporator in dry (DX) systems
Accumalator do 2 jobs
1) prevents liquids back to comperssor
2) subcooling the liquid ( 4 pipe accumulator)

[Glenn Moore]

A thermostatic expansion valve or an electronic expansion valve is a pure superheat controller, and if set up correctly an accumulator in a system is not necessary .
Accumulators are really designed to be used typically on heat pump systems where the system flow is reversed during heating and cooling modes ,and the system charge during changeover is unstable. At this point some liquid flood back can occur before the expansion valve has readjusted itself which obviously can cause compressor damage ,broken valves and pistons and con rods. Also reverse cycle defrost systems can have a similar problem during defrost.
As Rob says many engineers fit accumulators as an extra safety device very often to overcome a system problem , maybe pipe work or system design, or poor system commissioning. Poor condenser fan control set up can cause the tev to hunt as fans cycle
Accumulators are often called “slop pots” due to the job they do, ie stop any liquid that has escaped the evaporator.
Fitting an accumulator can cause other issues such as poor oil return ,and also can cause liquid to condense in the accumulator if some sort of heat source is not fitted to boil of any liquid that spills into the accumulator.
If the small bleed hole in the accumulator pick up pipe becomes blocked or choked with system dirt etc this can cause oil to become trapped inside the vessel.
Multiple compressor packs or compressors with unloaders are examples where accumulators maybe helpful .if you have a system where you have several compressors , the suction pressure will rise and fall as the compressors cut in and out, a similar situation can occur when the compressors have unloaders .one of the control points of a thermostatic expansion valve is the suction pressure under the diaphragm “via the equalising connection”. When compressors or unloaders switch in and out ,this affects the pressure under the diaphragm, ie when a compressor or unloader switches in ,the suction pressure drops lower , this drop in pressure under the Tevs diaphragm will cause the tev to open and over inject liquid until the valve readjusts itself ,this can cause liquid slugs back to the compressor. Electronic valves do not have this problem as filters in the electronics smooth out these pressure swings and are quick to control
So accumulators have a purpose in some systems and in other systems they are an expensive luxury and may cause more problems than they solve