Mycom relief valve issue?

[Mojjee]

Hi Again, some of you are kinda familiar with my system from previous postings. I have an ammonia ice rink system with 3 compressors a 7x7, 9x9 and a Mycom. We switch between the Mycom and the other 2 as needed. The problem I am experiencing is that with the 7x7 and 9x9 running I have a normal level in the evap. When I run the Mycom the level starts to increase almost to the high level cutout. It happened today again. After shutting the compressor down I noticed the line that comes from the outlet of the relief valve, which is piped to the suction line going to the compressor was frosted. I am assuming there was liquid in this line possibly leaking by the relief. Does this sound like it makes sense and would it possibly cause my level problem in the evap?

Thanks

[samlee12]

The relief valve is piped from the high side of the compressor and back into the suction side of the compressor. If somehow the pressure built up to the relieving pressure on the high side the relief valve would open and blow discharge gas back into the low side via suction line.If it was leaking by you would be blowing hot gas through line don,t think it would be frosted at that point.If the relief is closed the outlet side of relief is on the low side, frosting up because low side is cold thus condensing water vapor from surrounding air is condensed and frozen. As for the liquid level in evaporator (i think you mean flooded chiller) When the 7x7 and 9x9 are running it is at a certain level and if the Mycom is started the level is higher. What is the suction pressure when 7x7 and 9x9 are running and what is The pressure when Mycom is running, perhaps Mycom has more capacity than both others compressor thus reducing the vapor pressure at top of evaporator thus raising the level. I could be wrong just a thought.

[RANGER1]

Agree with samlee12 comments .
Also what size mycom L and what speed ?
Its a big capacity compressor and when turned on it should load slowly between each step .
If you only have pressure switches with no load delay , maybe you have a manual switch .
If you can manually load it slowly it may reduce surging in chiller .
You may also have liquid carryover so keep an eye on cold sump , foamy oil and cylinder banks ( between head and top of sidecover ) .

[josef]

Hi,
suction -10°C NH3
Mycom L series, RPM 1450
4L-325,3m3/h - 179,3KW
6L-488,0m3/h - 265,4KW
8L-650,6m3/h - 349,0KW

suction -10°F NH3
Frick 7x7
RPM-515 - 36,1KW
RPM-475 - 33,4KW
RPM-440 - 31,1KW
RPM-400 - 28,1KW
RPM-345 - 24,4KW

suction -10°F NH3
Frick 9x9
RPM-475 - 71,2KW
RPM-440 - 65,9KW
RPM-400 - 59,9KW
RPM-370 - 55,4KW
RPM-345 - 51,4KW

I agree with the samlee12, performance difference


Sorry, the difference ° C and ° F, 4 ° I believe is that there is no need to convert more, the difference is minimal.
Mojje, your performance is built 7x7 + 9x9 Frick, Mycom can be used safely only on half power

[Mojjee]

Thanks Sam, So if the relief were leaking it would act like a hot gas bypass, defrosting, not making frost. So the relief is not my problem.
I know the Mycom has much more capacity than the other two but the folks before me used to run it quite a bit. I'm wondering if this problem existed all along or if somethings' different now. Will the Mycom work on reduced power ? Our chiller plant was reconfigured last year to add a new chiller. The electrical room was moved and the MCCs updated. I think I'll go and check past logs and look for pressure differences. Thank you.

[US Iceman]

How much does the suction pressure reduce when the Mycom compressor is started? What you are describing is often due to a suddenly lower pressure in the surge drum. This lower pressure causes the refrigerant to boil faster and causes the apparent liquid level to rise.

This is like a pan of water boiling on the stove. As the boiling increases the bubbles rise and overflows if the heat is not reduced. If you reduced the heat input the boiling action slows down and the bubbles (apparent liquid level) begin to fall.

When you start the Mycom compressor, start it on minimum capacity. Allow it to run at this capacit for a while. If the suction pressure continues to increase allow another stage of capacity to be added. If you do this slowly the boiling action is less violent than if you start the compressor and allow it to reach full capacity shortly after it is started.

After shutting the compressor down I noticed the line that comes from the outlet of the relief valve, which is piped to the suction line going to the compressor was frosted. I am assuming there was liquid in this line possibly leaking by the relief.

Which relief valve? The one on the compressor or someplace else?

[Mojjee]

Hi Iceman,
The relief is on the discharge side of the compressor. Going thru some Mycom literature I found that there should be a capacity control valve that can be either manually operated or controlled by a solenoid. I'm doing more research.

[US Iceman]

Going thru some Mycom literature I found that there should be a capacity control valve that can be either manually operated or controlled by a solenoid.

That would be the capacity control solenoids, which we normally call unloaders.

How is your system being controlled?

Do you physically start and stop the compressors yourself? Or, do you have a pressure switch(s) that start and stop the compressors depending on where the suction pressure is?

[Mojjee]

DSCF0024.jpgThe BAS starts and stops it based on the bottom ice temp. I found a knob that I can't identify from any of the literature I took a picture and will post. The knob is at the 1 o'clock position from the center of the hub.

[Mojjee]

If,by chance that is a manual capacity control valve, would I be able to place the valve in a permanent position where the compressor could operate at less than 100% all the time? Maybe 75% ?

[IceMan_4000]

DSCF0024.jpgThe BAS starts and stops it based on the bottom ice temp. I found a knob that I can't identify from any of the literature I took a picture and will post. The knob is at the 1 o'clock position from the center of the hub.

This valve will control your oil pressure not your load unload capacities

[US Iceman]

The BAS starts and stops it based on the bottom ice temp.

That's what I was afraid of. The control system is probably monitoring ice temperature and when the temperature rises the control system may start and load the compressor too quickly. Refrigeration systems work better when the control system responds slowly.

Most control guys do a good job of controlling things, but they don't know much about refrigeration system operation. The high liquid level you see when the Mycom compressor starts is either caused by the sudden decrease of suction pressure (compressor starting or loading) or the accumulator is a little short in the design requirements needed to keep the liquid level at a low suitable level when this occurs.

[IceMan_4000]

4L at 50% capacity




DATE 05-08-2009
REFRIGERANT AMMONIA
MODEL N4L
BOOSTER N
CAPACITY [kW] 58.2
CAPACITY [TR] 16.5
ABSORBED POWER [kW] 16.2

SPEED [Rpm] 1000
LOAD [%] 50.0
CONDENSING TEMP. [degC] 32.2
EVAPORATIVE TEMP. [degC] -12.2
SUCTION SUPERHEAT [degC] 0.00
LIQUID SUBCOOLING [degC] 5.00
SUCTION TEMP. [degC] -12.2
SUCTION PRES. [MPaA] 0.266
DISCHARGE PRES. [MPaA] 1.24
SUCTION PRES.LOSS [MPaA] 0.000
DISCHARGE PRES.LOSS [MPaA] 0.000

SWEPT VOLUME [m3/h] 112
DISCHARGE TEMP. [degC] 96.8
REFRIG. FLOW RATE(SUC.) [m3/h] 85.3
REFRIG. FLOW RATE(DIS.) [m3/h] 26.2
REFRIG. FLOW RATE(SUC.) [Kg/h] 187.2
REFRIG. FLOW RATE(DIS.) [Kg/h] 187.2
OIL HEAT REJECTION [kW] 0.70
JACKET HEAT REJECTION [kW] 5.10

COP [-] 3.60

--- SUPERHEAT in not counted in refrigeration capacity ---

--- WITH WATERCOOLED OIL COOLER ---


at 100%

DATE 05-08-2009
REFRIGERANT AMMONIA
MODEL N4L
BOOSTER N
CAPACITY [kW] 116.3
CAPACITY [TR] 33.1
ABSORBED POWER [kW] 30.0

SPEED [Rpm] 1000
LOAD [%] 100
CONDENSING TEMP. [degC] 32.2
EVAPORATIVE TEMP. [degC] -12.2
SUCTION SUPERHEAT [degC] 0.00
LIQUID SUBCOOLING [degC] 5.00
SUCTION TEMP. [degC] -12.2
SUCTION PRES. [MPaA] 0.266
DISCHARGE PRES. [MPaA] 1.24
SUCTION PRES.LOSS [MPaA] 0.000
DISCHARGE PRES.LOSS [MPaA] 0.000

SWEPT VOLUME [m3/h] 224
DISCHARGE TEMP. [degC] 96.8
REFRIG. FLOW RATE(SUC.) [m3/h] 171
REFRIG. FLOW RATE(DIS.) [m3/h] 52.5
REFRIG. FLOW RATE(SUC.) [Kg/h] 374.4
REFRIG. FLOW RATE(DIS.) [Kg/h] 374.4
OIL HEAT REJECTION [kW] 0.70
JACKET HEAT REJECTION [kW] 5.10

COP [-] 3.88

--- SUPERHEAT in not counted in refrigeration capacity ---

--- WITH WATERCOOLED OIL COOLER ---


You only have 2 options for capacity with a 4L

50% or 100%

The L compressor usually has solenoids mounted on the body for loading the compressor. You must energize the solenoids to load.

A 4 cylinder will be 50% loaded when off.


In Canada we are no longer allowed to pipe the reliefs this way unless the vessels have been size properly

If you are injecting discharge into the suction side of the system you could be pushing hot gas directly into your chiller causing the lift.

I would suggest changing out this relief and piping it into your relief stack

[josef]

US Iceman Yes, the problem is not float Herl, oil system, oil pressure problem in Mycom ....., ignorance cooling systems, improperly designed Mycom compressor.

[RANGER1]

The mycom relief valve is not a balanced port relief so it should never be piped back into suction or system .
Should go to atmosphere .
It would only relieve at setting of ( usual ) 1950 kpa + suction pressure

[samlee12]

By Mojjee's picture he has a Mycom 6b the unloader has a manual valve to unload bank on left hand side . The manual unloader valve in picture is located on uper left hand side just above galvanized 90 degree elbow. Remove the valve stem cap and turn valve all the way counter clock wise this will manually unload compreessor their is probably another identical valve on the right hand side to unload right side bank , middle bank is loaded full time.

[samlee12]

You can change these manual valves with electrical control solenoid valves and can be control by main control system to energize solenoid to unload compressor and de-energize solenoid to load compressor.

[samlee12]

The knob at 1 oclock tha mojjee was reffering to is the vavle to either increase or decrease oil pressure, depends on turning valve in or out will either decrease oil pressure or increase.

[samlee12]

Sorry for all the different posts i just keep thinking of things to say I agree that relief valves should be piped into the common relief stack.

[samlee12]

Mycom reccomends 27 psi oil pressure.

[US Iceman]

samlee12, you can go back and edit your own posts if you need to. That will prevent the small posts. Thanks.

[samlee12]

Thanks, never noticed the edit block, sorry.

[Mojjee]

Samlee, the machine is a N6WB, and the unloaders are exactly how you describe them. I didn't think the relief should have been piped back into the suction either as all our other reliefs are piped to a deluge tank with water inside. The unloader valves are closed which means its at 100% ? Can I open them and leave them half way open and run at 50% (+/-) ?

[Mojjee]

Samlee, What would be 50%? Close one side and leave the other open ? I am thinking about the future when we install solenoid valves. Or will we have to re-do the piping so that one valve will operate both sides?

[samlee12]

When the compressor is at 100% it is fully loaded all three banks consisting of two cylinders each are pumping. If you turn one stem out and unload one bank you are decreasing capacity by 33% (100%-one bank of 3 you are down to 66% load if you open other valve you are now unloading two of the three banks and are down to 33% load, the last bank has no unloader mechanism.I'm pretty sure you can run unloaded for a long time without damage, perhaps some else can confirm as there are a lot of smart people at this site. you can also change the manual valves to solenoid valve and solenoid valves can be controled by wiring them in series with pressure switches set to cut out and cut in at specific suction pressures. hope this helps.oh and before i forget you don't have to change piping each bank will have its own solenoid.As far as unloader vavles go fully closed is loaded ,fully opened is unloaded there is no half way either fully closed or fully open.

[Mojjee]

Thanks Samlee, I got it now.

[josef]

Samlee, good advice, but only for short periods, cylinder liner is not cool refrigerant and will be destroyed (excessive wear)

[Mojjee]

josef, can you explain a little more ? If its going to be a problem I can run at 66% on one bank then alternate ?

[RANGER1]

Samlee, good advice, but only for short periods, cylinder liner is not cool refrigerant and will be destroyed (excessive wear)

From my experience you can run mycom WA , WB ,K, C on minimum capacity all day with no problem . Why is L type of machine different if it has water cooling on heads/oil cooler ?

[josef]

Hi RANGER1 yes, you are right, all day, but not for months or maybe years? reply Mojjee what time you want to run the compressor at 66%. Cooling is the head - not on the cylinder, friction?

[Mojjee]

Its water cooled. In the heads. I want to run at 66% at night. I notice at 66% I get some flop in the belts,at 100% they smooth right back out. Does the machine not like to run at less than 100% ?

[IceMan_4000]

Its water cooled. In the heads. I want to run at 66% at night. I notice at 66% I get some flop in the belts,at 100% they smooth right back out. Does the machine not like to run at less than 100% ?

How many belts do you have on this compressor.

I have noticed that people tend to over belt the machine.

we supply a large flywheel that must accommodate many situations

At 100% you the machine will use 59.2 BHP

When you unload to 66% you will only use 41 BHP

Call you belt supply there is a calculation depending on the belt and the length on how much HP one belt can take

If you have 75 BHP of belts and only require 41 BHP you will see alot of belt flap.

Reduce the amount of belts to match your maximum hp

[Mojjee]

Cool. I will do the math. Thanks