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Renato RR
29-03-2006, 11:46 AM
Two Bitzer screw compressor on R22 conected with heat recovery and then to air heat exchanger (Condenser).
I calculate thet i need pipe NO 50 from two comressors and heat recover.Calculate for 250kW each circle,Tcond=55 C,Tevap=0C,liquid subcoling = 10C,superheat at suction=12C

2 oil traps

20m horizontal

5m vertical distance

4 elbow

In this case I woul have 11m/s in discharge line and 0,8m/s in liquid line.So I thinking about reduction in discharge line to get 17m/s.

This is all for full load but what when compressor start to unload I dont know steps.(25%,50% or 75% etc.)

So you see I am afraid that due the unload I would hawe problem with velocity (oil return) to compressor.
Althaugh Bitzer screw have oil separator inside.



What is your sudgestion??

NoNickName
29-03-2006, 12:16 PM
You can't have a full canister and a drunken wife at the same time. :)

Renato RR
29-03-2006, 12:40 PM
Yes but I dont wont to have past out wife.

Renato

NoNickName
29-03-2006, 01:03 PM
Set the microprocessor to a very short maximum time for unloading, in order to make sure it won't work for long at reduced capacity. This will negatively affect temperature stability, but will positively affect reliability.
Please note that 25% on bitzers is normally only used for starting and not for thermoregulation, otherwise you will most probably end up with a liquid injection request.

Renato RR
29-03-2006, 02:01 PM
I have liquid injection on compressor.

Renato:(

Andy
29-03-2006, 10:16 PM
Two Bitzer screw compressor on R22 conected with heat recovery and then to air heat exchanger (Condenser).
I calculate thet i need pipe NO 50 from two comressors and heat recover.Calculate for 250kW each circle,Tcond=55 C,Tevap=0C,liquid subcoling = 10C,superheat at suction=12C

2 oil traps

20m horizontal

5m vertical distance

4 elbow

In this case I woul have 11m/s in discharge line and 0,8m/s in liquid line.So I thinking about reduction in discharge line to get 17m/s.

This is all for full load but what when compressor start to unload I dont know steps.(25%,50% or 75% etc.)

So you see I am afraid that due the unload I would hawe problem with velocity (oil return) to compressor.
Althaugh Bitzer screw have oil separator inside.



What is your sudgestion??

You velocity seems alittle low:) What sort of loss have you in K and what about your suction line, would that not be the critical one for oil return. 25% capacity on a bitzer, never heard of that, the ones I use are 50% minimum. I never use the capacity reduction except for unloaded start, all they are is just a hot gas by-pass port, not a slide unloader, the compressorjust overheats when unloaded.

Kind Regards. Andy:)

NoNickName
29-03-2006, 10:42 PM
Yes, bitzers have 4 steps, that is 100% plus 2 unloaders (75 and 50) plus the 25 which should only be used for starting.
I actually use just the 75%, no solenoid on the 50% and one solenoid for the intermittent oil injection.

Lc_shi
30-03-2006, 02:42 AM
Two Bitzer screw compressor on R22 conected with heat recovery and then to air heat exchanger (Condenser).
I calculate thet i need pipe NO 50 from two comressors and heat recover.Calculate for 250kW each circle,Tcond=55 C,Tevap=0C,liquid subcoling = 10C,superheat at suction=12C

2 oil traps

20m horizontal

5m vertical distance

4 elbow

In this case I woul have 11m/s in discharge line and 0,8m/s in liquid line.So I thinking about reduction in discharge line to get 17m/s.

This is all for full load but what when compressor start to unload I dont know steps.(25%,50% or 75% etc.)

So you see I am afraid that due the unload I would hawe problem with velocity (oil return) to compressor.
Althaugh Bitzer screw have oil separator inside.



What is your sudgestion??
Discharge line 17m/s should be no problem. But how about suction line? 25% is only for start up(most suppliers will tell it:). I think may you need to install liquid injection and keep watch oil level indicator in start operation and confirm oil system is OK

rgds
LC

Renato RR
30-03-2006, 07:20 AM
Yes you are right about 25%.This compressor have oil separator inside housing so that will give me some time until unload stops.

Anywone have idea how much will oil separator slow the reduction of oil on suction line.
Some sors claims 4-5 working hours.

Should I instold ICS Danfoss pilot ventil to make bypass in case that heat recovery takes all heat from sistem?

Renato

NoNickName
30-03-2006, 07:26 AM
bitzer has a very good demister on discharge, so I wouldn't worry, unless refrigerant owns a low isoentropic exponent (low discharge temperature, like 404).
Try to keep the oil as warm as possible, and connect the oil switch level sensor, just to be sure.