cold start problem

[isobutane]

I met an AC system that using microchannel heat exchanger as condensor and tube-fin coil as evaporator, the compressor using rotary compressor, capillary for metering device. the system is about 7kW refrigeration capacity and using r22.
every time when the system is hold stopped for about 4 hours, the start of the compressor will cause the evaporating pressure goes down to below 0.25 MPa for about 8~10 minutes.:confused: We add a sight glass before the capillary, find everytime the compressor starts, there are a lot of gas goes into the capillary. If stops the compressor now and start it again, the pressure will go normal above 0.4MPa less than 30 seconds.:mad:
Why would this happen? My first thought is that the charge is low(1200g),about 30% less than that of both fin-tube coil for evaporator and condensor.
b.t.w, the temperature is about 27 c.

[isobutane]

Add more infor: The system is an integrated AC. The condensor and evaporator is about the same height, and the compressor is the lowest position.
I think maybe the gas causes the problem and change the position of the capillary below the outlet of the condensor, that did helps!:). But still short time of low suction pressure(reduce to about 3 minutes)

Does somebody met samilar Phenomenon?

[isobutane]

The pipe has certain traps that keep the refrigerant inside the evparator and piping system.
for evaporator every inlet is about 200mm lower than outlet. and the inlet of the condensor is also 300mm higher than condensor.

So can we say this cold start problem is caused by refrigerant migration? While the condensor and compressor is sucked in same temperature.

When compressor stops, where is the most possible position that refrigerant likes to stay?(The compressor is lowest and pipe traps were used to keep the refrigerant out of compressor. evaporator is the "hottest" component(with fan keeps blow through it) .)

[NoNickName]

pipe trap (or P pipe) is my vote too.

[Brian_UK]

What is the sub-cooling condition?

Without facts and figures we can only guess at what is happening.

[Gary]

Refrigerant will migrate to the coolest part of the system, regardless of relative elevation or traps.

You haven't mentioned the high side pressures and temperatures. If the high side pressure is low (cold condenser), the refrigerant flow will be reduced until the condenser warms up.

[isobutane]

When equalized the high side pressure is about 1.8MPa, and inlet temperature is also 27~28 c. The sub-cool is very high, bigger than 10 at start stage(Pressure is about 1.0MPa). but after the pressure is normal, the sub-cool goes down to about 4 c.

I tried cankase heater yesterday, nothing improved.

Other things I find: at normal pressure, the pressure difference between the inlet and outlet of condensor is very small, I can't even find any difference through the common gauge(minimal interval is 100kPa). Does this has something with the cold start? we find early other system the similar question, in that case, the condensor has 3 circuits, when we reduce the circuits to 2, the problem were solved.What's the requirement of the pressure difference between the coil? I think this relate to the velocity of the refrigerant inside the tube.

Other question: If several parts are in same temperature, then gravity is the most powerful factor? When the trap is design higher than the inlet and outlet of the coil, then can this refrigerant be kept inside the coil?

[amitsaxena]

Dear This phenomenon is called as flooded start as you eva and cond is at elevation hence during the off cycle the ref will stay below the comp oil and when the comp tries to take start again then it sometime fail to suck as oil % is more than the required.
Hence fails during starting.

Thanks
Amit

[isobutane]

I dont think it's flooded start, I tried crankcase heater to keep the compressor warm. Still nothing improved.:mad:

I think this is a design issue. there must be something wrong with the coil and compressor or capillary. The refrigerant pressure drop is too low to be seen, does this influence the start?

[isobutane]

sorry, I mean condenser side refrigerant pressure drop

[Gary]

I'm still trying to translate Mpa to psi.

What superheat are you getting?

[Toosh]

I'm still trying to translate Mpa to psi.

What superheat are you getting?

Hi gary this will do your conversion http://www.chapelsteel.com/mpa-psi.html


Norm

[nike123]

I'm still trying to translate Mpa to psi.

What superheat are you getting?

If you don't have problem with converting Bar to psi than Mpa is 10 times more Bars.

2,5 Mpa = 25 Bar = 362,6 Psia
2,5 Mpag = 25 Barg = 377,3 Psig

Easy for us who uses Bars.;)

[isobutane]

:DDear all

I may find whats the problem, every time the comp. stops, the liquid refrigerant stays parts in evap, parts in cond, when the compressor starts, the cond's liquid were push to evap, but to some reason, the evap's liquid doen't boiled so quickly, and comp can only suck little gas and the flow volumne is very little, its somewhat like the charge of gas in sucking side, the cond pressure the discharge pressure were very low because of the low flow volumne.

If I keeps much of liquid in cond, may be the problem will be solved. like pump down did.

Is that reasonable? and if it is, what's the reason to keep much of liquid inside the evap?

[nike123]

.....................

[Gary]

I dont think it's flooded start, I tried crankcase heater to keep the compressor warm. Still nothing improved.:mad:


On a rotary compressor, the crankcase is on the high side. It does not need and should not have a crankcase heater.

Still wondering about the superheat.

[icecube51]

is this unit not having a liquid vessel?? or an oil separator??

Ice

[Gary]

A rotary compressor comes with an accumulator mounted on the side of the compressor.

[al]

if you added a sight glass and there is no receiver fitted did you pump down? if not this was a perfect time to recover and weigh in the correct charge and go from there.

al

[Gary]

When equalized the high side pressure is about 1.8MPa, and inlet temperature is also 27~28 c.

That translates to 261psi, which puts the saturation temp at about 120F/49C. This would be extremely high, given 82F/28C inlet temp, even if the system were running. If the system is off and pressures equalized, it is pretty much impossible. All of this adds up to air in the system and/or incorrect measurements.

[isobutane]

If the system is off and pressures equalized, it is pretty much impossible. All of this adds up to air in the system.

I make a mistake here, when the comp is running, the high side pressure is about 260psig and inlet air temp is about 27 c .It's very common pressure in air cooled R22 system.

I used crankcase heater to heat the accumulator to see if some liquid is stay there.

and sight glass is temperoray used in the system and I calculated the cond's volumne can hold 2 times of total refrigerant charge. So without an acumulator, I pump all the refrigerant to cond, is there any possible question?

[desA]

Is it possible the OP can post a few pictures of the system? This will assist in overcoming language translation difficulties & put other board members on the same wavelength.

A sketch showing your recorded temp/pressure measurements would also be useful.

[nike123]

Is it possible the OP can post a few pictures of the system? This will assist in overcoming language translation difficulties & put other board members on the same wavelength.

A sketch showing your recorded temp/pressure measurements would also be useful.


Yep, that would be much nicer.

[isobutane]

Yep, that would be much nicer.


I think my english is ok, but now it seems far away from it.:o

See the attachment for the picture of the unit. from the upper side of the unit, there was an evap fan(not shown in the picture).

at blue point, I add two valves for pump down.

[Gary]

at blue point, I add two valves for pump down.

Remove that upper blue valve. A valve in the discharge line can destroy your compressor.

[Gary]

See the attachment for the picture of the unit.


Where the suction line leaves the evaporator, I would install a U shaped trap, then go up to the top of the evap to an upside down trap, and then down to the compressor. This would trap liquid in the coil on the off cycle instead of allowing it to drain (by gravity) into the accumulator.

[NoNickName]

Remove that upper blue valve. A valve in the discharge line can destroy your compressor.

The internal bypass valve is compulsory.

[icecube51]

oil on the compressor head was what i had in mind to Gary,thanks for the comfirmation. therefore the hint of a liquid vessel or oil separator before.

Ice