Does Accumulator really prevent liquid return to compressor?

[dsltung]

Dear All,

I have a problem with my air source heat pump which has accumulator just before compressor. Bubbles appeared at the oil level glass of compressor, superheat was about 8 degree C, Refrigerant R407C, LP was about 50 psi. I think liquid refrigerant was there.

Does accumulator stops liquid return to compressor?

Can accumulator be used for R407C system?

How to size the volumn of accumulator for a system?

Thanks for all your advise.

Dennis

[nike123]

Dear All,

I have a problem with my air source heat pump which has accumulator just before compressor. Bubbles appeared at the oil level glass of compressor, superheat was about 8 degree C, Refrigerant R407C, LP was about 50 psi. I think liquid refrigerant was there.

How did you measured superheat? Did you used saturated gas temperature for calculation of superheat?


Does accumulator stops liquid return to compressor?

Yes, if properly sized for flow and volume!


Can accumulator be used for R407C system? http://www.refrigeration-engineer.com/forums/showthread.php?p=102637


How to size the volumn of accumulator for a system?

Suction Accumulator Sizing
By Kem Russell, PE
VP Engineering

Sizing of suction accumulators can be challenging due to the varying conditions that exist in a system. There are however a few principles that should be considered when determining the diameter and length of a suction accumulator.
First, what is the refrigerant that will be used? Due to the differences in refrigerant properties, in particular, vapor and liquid density, the design velocity through a vessel will change. As the temperature of the refrigerant decreases, the vapor density reduces and the vapor velocity through the vessel can usually be increased. For example, for ammonia at 20°F a design vapor velocity of 120 to 130 fpm is generally appropriate. If the temperature is -40°F, a vapor velocity of 220 to 240 fpm may be appropriate.
Second, will the pressure (and temperature) in the vessel vary over time? As mentioned above, as the vapor temperature changes the vapor density changes. For example, in a processing facility the refrigerant temperature may start out high (and likely will) then decrease as the system, and product load and flow rate balance out. Under these conditions the design velocity through the suction vessel may need to be lower than what the normal operating conditions would dictate. For example a -40°F system might better be designed for velocity in the range of 180 to 200 fpm to deal with the warmer vapor during startup.
Third, what is the vessel orientation? Will the vessel be horizontal or vertical? The positioning of the inlet and outlet connections of the vessel will also be important. For the vertical vessel usually the entire diameter of the vessel can be used as the separating area. However, for a horizontal vessel any liquid in the vessel will reduce the vapor area available for droplet separation. For horizontal vessels it is important to have the inlet and outlet connections far enough apart that the critical size droplets can fall to the liquid surface before they travel the horizontal distance between the connection points.
Fourth, what is the flow rate through the vessel? This can be determined from the CFM of the connected compressors, or the refrigeration capacity of the compressors. Ultimately, the CFM will be needed in determining the vapor area required for proper separation in the suction accumulator. For determining the vessel area required the equations used is usually Area = Compressor CFM/Design Vapor Velocity.
As you might guess, there are many factors that should be addressed when sizing a suction accumulator. There are some "rules of thumb", but following correct design principles tempered with experience will make the difference between success and trouble.




http://www.hvacrinfo.com/Tecumseh_bulletins/EngineeringRecommendation12.pdf
http://www.henrytech.com/Tips/HT-TT14.pdf

[nike123]

What is compressor model #?
What is total refrigerant charge in that circle?
Check proper function of crankcase heater.

[dsltung]

Yes, I used saturated gas temperature to calculate the superheat. It is a scroll compressor, "Danfoss 10HP". I think the system charge may be a little higher than require. If I reduce the charge, will the suction pressure increase?

Thank again for the information, it must be very useful and I will study it. In the mean time, I still have the following unclear in my mind:

(1) There is a small hole in the accumulator for oil return, will the liquid refrigerant return together with oil to the compressor?

(2) What is the pressure drop with the accumulator, is the accumulator cause low suction pressure (50psi)? Or others?

(3) If accumulator can store liquid refrigerant, can it subsitute liquid receiver?

Thanks for your help.

[nike123]

Yes, I used saturated gas temperature to calculate the superheat. It is a scroll compressor, "Danfoss 10HP". I think the system charge may be a little higher than require. If I reduce the charge, will the suction pressure increase?

Yes, if overcharged!
Check your charge by measuring subcooling when system temperature are near design temperatures. It should be around 7K (at condensation temperature around 40°C)



(1) There is a small hole in the accumulator for oil return, will the liquid refrigerant return together with oil to the compressor?


No, that is orifice and act as expansion device for liquid refrigerant.


(2) What is the pressure drop with the accumulator, is the accumulator cause low suction pressure (50psi)? Or others?

Small if properly sized, and cannot be cause for low pressure.


(3) If accumulator can store liquid refrigerant, can it subsitute liquid receiver?

That is not his primary function. His function is to ensure that no liquid is going to compressor and to evaporate any liquid and not to store excess refrigerant.
If you store refrigerant in it, then you reducing his capacity.

[dsltung]

Thanks for your reply, I have better picture now.

If bubbles appears in the oil level glass, does that mean the accumulator is full of liquid? Will oil itself generate bubbles?

When oil level rise, higher then the oil level when the compressor was brand new, does that mean liquid return to compressor? Is there any other reason cause oil level increase?

[nike123]

[quote=dsltung;137782]Thanks for your reply, I have better picture now.

If bubbles appears in the oil level glass, does that mean the accumulator is full of liquid? Will oil itself generate bubbles?
Bubles in oil glass mean two thing:
Evaporation of liquid refrigerant and/or churning
http://www.refrigeration-engineer.com/forums/showpost.php?p=94982&postcount=9
http://www.refrigeration-engineer.com/forums/showthread.php?t=11690&highlight=churning


When oil level rise, higher then the oil level when the compressor was brand new, does that mean liquid return to compressor?
Oil level should be monitored when compressor works and some time is passed. Check sticker near sight glass.
If then oil level is high, than you have excessive liquid refrigerant in crankcase. If level is high only in standstill, then crankcase heater is faulty.


Is there any other reason cause oil level increase?Only if additional oil is added!

[Gary]

If I reduce the charge, will the suction pressure increase?


No. More than anything else, the suction pressure is determined by the temperature and volume of the air flowing through the evaporator coil.

[dsltung]

Thanks a lot.