capacity scenario

[chillyblue]

Just a scenario to get everyone's thought's
this is not true, just hypathetical

A coldroom correctly designed, with a correctly balanced evap, comp, tev, condenser and refrigerant charge. operates correctly.

What happens when we have high ambient temperature.

diagnostics become more difficult because.

My theory
Due to the increased ambient temperature the room temp will increase as more heat transfer will be coming through the walls, doors, ceiling etc.
As the room temperature increases the load on the evaporator will also increase, increasing the evaporator exit superheat, this will cause the tev to open, resulting in more liquid refrigerant flow through the total system.
The ambient temperature will also result in higher condensing temperatures/ pressure and reduce the mass flow rate through the compressor. As the condensing temperature is higher and less efficient this reduces the amount of liquid produced, a indication of lack of refrigerant is seen.

Just assuming you arrive on-site and find the above.

I think at first you would think
you make sure the condenser is clean, you have very low subcooling.
next is it
1. lack of gas
2. incorrectly adjusted tev
3. then the system trips on hp
4. the room temp rises more.

How do you resolve this and diagnose a fault although there is no fault other than high ambient temps.

Normally if the system is running fine under normal conditions, and the fault is high ambient conditions, removing some refrigerant will increase the condensing area, but loosing subcooling, keeping the compressor running but hindering the evaporator (by reducing the refrigeration load) but this is better than the system not running at all!!.

If you are lucky enough to have a condenser/compressor big enough to cope with the extra load, you fill it with gas and you open the tev to re-gain the superheat, great all is well until the load drops off, then you have to much refrigerant in the system and possibly a dangerously low superheat, which could cause compressor damage.

How many of you guys spend hours trying to get a system which your client believes has a fault, but is just not designed for the loadings being asked of it.

Just another scenario, for a cappillary system
under high load conditions, as no adjustments can be made, and because the evaporator is absorbing more heat than the cappillary can feed, a symptom of lack of gas would be observed. With this you reclaim the gas, and recharge with the correct amount (obviously leak testing etc) but the system still provides the same symptoms,
How do you diagnose this.

On a cappilary system (fixed flow rate) and a correct refrigerant charge. I guess because the liquid refrigerant being feed into the evaporator is boiling off quicker, the first few circuits of the evaporator would be fine, but the suction superheat would be high, no fault just higher than normal operating conditions.

As in Engalnd we are expecting some very high ambient temperatures this week, i thought i would ask you guys what you do when you arrive on site and find systems tripped on hp and compressors pulling exsesively high amps and complaints of high room temps.

Thanks CB

[lowcool]

water over the condenser works to bring bring down head pressure and stabilise the refrigerant allowing refrigerant to become efficient again thus heat load is regained quicker reducing the suction pressure,to exhibit a lack of refrigerant by sightglass,condenser would be grossly undersized as always it comes down to design of system,hp control etc,adjustment to close tx valve will assist also in keeping the unit running and hopefully regains that bit when ambient drops off overnight or crank the hp control and hope compressor handles it until a new condenser with capacity control is fitted.with capillary you maybe led to believe that compressor is slightly inefficient as suction pressure would be higher than that of a system running in designed conditions,high discharge temps are similar to that of inefficient comp.condensing operation is paramount if systems are out the design parameters be it over or under condensed
only hypothetical ? it exists

[Yuri B.]

what you do when you arrive on site and find systems tripped on hp and compressors pulling exsesively high amps and complaints of high room temps.
If you are sure AC is faultless but heavily undersized, put the client to one of the three choises:
1. AC continues to pull down air to his/her conventional 21C (while now there is 30C outside); let him/her be prepared to a breakdown at any moment.
2. Content with higher air for a while - let's say 25C (for as long as there is 30C outside); tell the client the AC by that may serve him/her for longer.
3. Change the existing AC for a more powerful, or install an additional AC. Try to be convincing and assure the client it is not extortion.

I believe, while clients have a choice, we do not, apart from explaining them the situation. No miracles, impossible to sqweeze out from a machine more work than it is designed for.

[Gary]

Just a scenario to get everyone's thought's
this is not true, just hypathetical

A coldroom correctly designed, with a correctly balanced evap, comp, tev, condenser and refrigerant charge. operates correctly.

What happens when we have high ambient temperature.

diagnostics become more difficult because.

My theory
Due to the increased ambient temperature the room temp will increase as more heat transfer will be coming through the walls, doors, ceiling etc.
As the room temperature increases the load on the evaporator will also increase, increasing the evaporator exit superheat, this will cause the tev to open, resulting in more liquid refrigerant flow through the total system.
The ambient temperature will also result in higher condensing temperatures/ pressure and reduce the mass flow rate through the compressor.

The mass flow rate increases, not decreases.


As the condensing temperature is higher and less efficient this reduces the amount of liquid produced, a indication of lack of refrigerant is seen.

What would you consider to be an indication of lack of refrigerant?


Just assuming you arrive on-site and find the above.

I think at first you would think
you make sure the condenser is clean, you have very low subcooling.

What do you consider very low subcooling and why is that a problem?


next is it
1. lack of gas
2. incorrectly adjusted tev
3. then the system trips on hp
4. the room temp rises more.

5. None of the above.


How do you resolve this and diagnose a fault although there is no fault other than high ambient temps.

High ambient temps is not a fault, and should not be a problem for a properly designed system.


Normally if the system is running fine under normal conditions, and the fault is high ambient conditions, removing some refrigerant will increase the condensing area, but loosing subcooling, keeping the compressor running but hindering the evaporator (by reducing the refrigeration load) but this is better than the system not running at all!!.

Why would the system not run at all?


If you are lucky enough to have a condenser/compressor big enough to cope with the extra load, you fill it with gas and you open the tev to re-gain the superheat...

No... you don't. When the load is heavy, the superheat is supposed to be high. Leave it alone.


, great all is well until the load drops off, then you have to much refrigerant in the system and possibly a dangerously low superheat, which could cause compressor damage.

That's because you screwed up the TXV when you should have left it alone.


How many of you guys spend hours trying to get a system which your client believes has a fault, but is just not designed for the loadings being asked of it.

If the system is functioning properly but isn't big enough to do the job, I tell the client that the system is functioning properly but isn't big enough to do the job.

[Gary]

Just another scenario, for a cappillary system
under high load conditions, as no adjustments can be made, and because the evaporator is absorbing more heat than the cappillary can feed, a symptom of lack of gas would be observed. With this you reclaim the gas, and recharge with the correct amount (obviously leak testing etc) but the system still provides the same symptoms,
How do you diagnose this.

Heavy load = high superheat. This is not an indication of lack of refrigerant. This is the way it is supposed to work.


On a cappilary system (fixed flow rate) and a correct refrigerant charge. I guess because the liquid refrigerant being feed into the evaporator is boiling off quicker, the first few circuits of the evaporator would be fine, but the suction superheat would be high, no fault just higher than normal operating conditions.

See above.


As in England we are expecting some very high ambient temperatures this week, i thought i would ask you guys what you do when you arrive on site and find systems tripped on hp and compressors pulling exsesively high amps and complaints of high room temps.

I clean the condenser and remove all of the excess refrigerant that the guy before me dumped into the system. Then I start looking for the problem that he mistook for SOG.

[nike123]

If the system is functioning properly but isn't big enough to do the job, I tell the client that the system is functioning properly but isn't big enough to do the job.

It is so simple when you say it.;)

[chillyblue]

sorry i don't know how to use the quotes, how do you do that???

1. The mass flow rate increases, not decreases.??

I believe the mass flow rate decreases because more pressure is needed to over come the discharge valves differential so the discharge space is decreased, also the piston must drop further for the inlet valve to open, this means the intake is less and the outlet is less=decrease in mass flow rate.

2.
What would you consider to be an indication of lack of refrigerant?.

as previously described (low 1-2deg or no subcooling at all), this will not deliver a solid liquid feed to the tev and may contain flash gas inadicate liquid feed to the evaporator.

3. i will tell my client that it's incorrectly designed/sized, but his response will be that it normally works fine and not be overly keen to install thousands of pounds worth of equipment for one or two days a year.

4.High ambient temps is not a fault, and should not be a problem for a properly designed system.

So if a system is design for a ambient of 32deg C as normal in the UK and we experience temps of 36deg C that would not cause a problem??? yes you could design it for 36 deg but at what point do you stop.
also systems deteriate so even under clean and normal conditions, ambients of 27deg can still cause problems as they are no longer 100% efficient.

5. Why would the system not run at all?

If it's tripped on HP with a manual reset.

thanks Gary but i think you may have missed the point.

The point is what can you do to assist a system that normally works fine but is struglling due to higher than designed ambient conditions.

Also the desciption i gave were all faults that could be mistaken I.e high suction superheat would tempt most engineers to open the tev to reduce it, then the increased flow would idicate a lack of gas the scenario snowballs..

I have used water, this works great but can damage the condenser and the water authorities don't really like it.

What i'm really asking is what tips can be used to stop a system tripping on hp without risking the compressor.

Thanks

CB

[Brian_UK]

I may be wrong here but...

If the system is tripping on the HP switch because the ambient temperature has risen a few degrees I would suspect that the system is either overcharged or is poorly designed.

The condenser should be able to handle the increase unless it has been sized so tightly that there is no leeway.

If the only way of keeping the plant going is to add a temporary water spray then go for it.

The water authority is getting paid for the water used so should really be happy not aggrieved (but,yes we do know better).

Assisting by playing with the TEV is not going to solve the problem. The system is working but under a constant pull down load. Would you normally adjust the TEV whilst the system is pulling down or would you normally wait until design space temperature has been reached before deciding if something is wrong?

[Gary]

I believe the mass flow rate decreases because more pressure is needed to over come the discharge valves differential so the discharge space is decreased, also the piston must drop further for the inlet valve to open, this means the intake is less and the outlet is less=decrease in mass flow rate.

You said yourself that the TXV opens, which increases the flow. In a loop system, an increase in flow at one point is an increase in flow at all points.

Flow through the compressor has more to do with suction pressure than discharge pressure. With higher suction pressure the suction gas is more dense, therefore the flow increases.


2. as previously described (low 1-2deg or no subcooling at all), this will not deliver a solid liquid feed to the tev and may contain flash gas inadicate liquid feed to the evaporator.

Any system that has subcooling that low is undercharged. This has nothing to do with hot weather. And lower SC gives you lower discharge pressure, not higher. High subcooling (overcharge) is what drives up the pressure.


3. i will tell my client that it's incorrectly designed/sized, but his response will be that it normally works fine and not be overly keen to install thousands of pounds worth of equipment for one or two days a year.

Yep... that's the response I get, too... except it's dollars.


4.High ambient temps is not a fault, and should not be a problem for a properly designed system.

So if a system is design for a ambient of 32deg C as normal in the UK and we experience temps of 36deg C that would not cause a problem??? yes you could design it for 36 deg but at what point do you stop.
also systems deteriate so even under clean and normal conditions, ambients of 27deg can still cause problems as they are no longer 100% efficient.

At 36C ambient with a heavy load the SCT should be no more than 56C, and probably less. At what point are your HP switches set?



thanks Gary but i think you may have missed the point.

The point is what can you do to assist a system that normally works fine but is struglling due to higher than designed ambient conditions.

Also the desciption i gave were all faults that could be mistaken I.e high suction superheat would tempt most engineers to open the tev to reduce it, then the increased flow would idicate a lack of gas the scenario snowballs..

I have used water, this works great but can damage the condenser and the water authorities don't really like it.

What i'm really asking is what tips can be used to stop a system tripping on hp without risking the compressor.

Clean the condenser and remove the overcharge. This almost always does the trick.

[Gary]

Assisting by playing with the TEV is not going to solve the problem. The system is working but under a constant pull down load. Would you normally adjust the TEV whilst the system is pulling down or would you normally wait until design space temperature has been reached before deciding if something is wrong?

Opening up the TXV would actually increase the discharge pressure.

[lowcool]

just leave it how it is if the owner does not want to spend the bucks on plant maybe he would like to spend it on the liabilities he will create for himself,one fella in here actually came up with a good point surround the condenser wth evap cooler pads with water over them or do what we did in the bush and stream cooled air straight in

[chillyblue]

thanks for the response guys

Went to this job yesterday with a similiar circumstances as my scenario, what do you think??

this is for a walk-in coldroom with a semi-hermetic compressor mounted on a reciever (remote condenser) located approx 8mtrs above the comp reciever set, evap 3mts above comp reciever set and approx 20mtrs away.
Room temp should be 0 to 2 deg C
Fitted gauges toke all the following readings, and believe it may be low on gas low subcooling (leak tested and no leaks found)but was not confident enough to add refrigerant due to the condensing temp readings taken.

Refrigerant R22
hp switch set to cutout compressor at 350psi

Condensing pressure 270psi temp 50deg C
Air on to cond 30 deg C
air off cond 40 deg C
liquid line temp 49 deg C
liquid subcooling 1 deg C

Suction pressure 51psi temp -2deg C
air on to evap 9 deg C
air off evap 7 deg C
suction line temp exiting evap 10 deg C.
evap superheat 12 deg C.

Thanks CB

[Gary]

thanks for the response guys

Went to this job yesterday with a similiar circumstances as my scenario, what do you think??

this is for a walk-in coldroom with a semi-hermetic compressor mounted on a reciever (remote condenser) located approx 8mtrs above the comp reciever set, evap 3mts above comp reciever set and approx 20mtrs away.
Room temp should be 0 to 2 deg C
Fitted gauges toke all the following readings, and believe it may be low on gas low subcooling (leak tested and no leaks found)but was not confident enough to add refrigerant due to the condensing temp readings taken.

Refrigerant R22
hp switch set to cutout compressor at 350psi

Condensing pressure 270psi temp 50deg C
Air on to cond 30 deg C
air off cond 40 deg C
liquid line temp 49 deg C
liquid subcooling 1 deg C

Suction pressure 51psi temp -2deg C
air on to evap 9 deg C
air off evap 7 deg C
suction line temp exiting evap 10 deg C.
evap superheat 12 deg C.

Thanks CB

Add refrigerant until the subcooling is 8-8.5C and then take a new set of temp readings so we can see the comparison.

[chillyblue]

Add refrigerant until the subcooling is 8-8.5C and then take a new set of temp readings so we can see the comparison.

Cheers gary

But i'm concerned the discharge pressure will raise up to the hp cut-out setting?? is that not the case??:eek:

CB

[Gary]

Cheers gary

But i'm concerned the discharge pressure will raise up to the hp cut-out setting?? is that not the case??:eek:

CB

No... the liquid line temp will drop a lot and the discharge pressure will rise a little.

[chillyblue]

Thanks, i'll give it a go, not sure when i'm going back though.

CB

[Brian_UK]

I know that we are a little warm in the UK at the moment but 30°C ON to the condenser seems somewhat high.

Is there any other equipment nearby adding heat or air recirculating around the condenser coil?

[chillyblue]

I know that we are a little warm in the UK at the moment but 30°C ON to the condenser seems somewhat high.

Is there any other equipment nearby adding heat or air recirculating around the condenser coil?

No i don't believe so, it is on the roof and it's a horizontal condenser. I took temp readings no where near the condenser and they were 30 all around. The condenser sits about 1 mtr from the roof surface.

Thanks
CB

[lowcool]

dont suppose this is a new instal on a new bright shiny roof.

[chillyblue]

dont suppose this is a new instal on a new bright shiny roof.

No it's been there for years.

That was my next question.
What would be the syptoms of a undersized condenser, or a condenser that has deterioated and now is lacking or has lost capacity.

CB