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ong
13-07-2005, 03:22 PM
Can anyone explain to me why discharge air on the C.U is extremly hot...beside overcharging,what other reasons could led to this outcome?

frank
13-07-2005, 04:52 PM
What do you class as "extremely hot" ?

Can we have a measurement, say in deg C or deg F?

What ambient temperature is going on the the condenser?

What refrigerant are you using?

Etc, etc.

ong
13-07-2005, 05:34 PM
ambient air-35 degree celcius
discharage air-60-65 degree celcius

frank
13-07-2005, 07:27 PM
discharage air-60-65 degree celcius

Have you taken any pressure readings of the discharge gas entering the condenser or the liquid line temperature leaving the condenser?

What refrigerant are you using?

Temprite
14-07-2005, 11:54 AM
ambient air-35 degree celcius
discharage air-60-65 degree celcius

Sure the condensers not blocked? :)

Deejey
14-07-2005, 12:51 PM
what is the appplication its on...

ong
14-07-2005, 03:17 PM
this is an aircon unit (R22 gas) - no pressure reading taken... this a question given by my boss during a training lesson yesterday... he did not give us the answer...but he told us one of the reason is due to overcharging...

chillyhamster
14-07-2005, 06:10 PM
over charging decreases the effective condenser cooling capacity

Peter_1
14-07-2005, 07:44 PM
Increases the HP pressure, all right but a so dramatical temperature change?? I doubt it.
Surface that can condens will be smaller and condens at higher temperatures but the rest of the condensor will subcool the liquid and will not contribute to a general warming up of the blown air.

Such a warming up: air in the system I should say.

chemi-cool
14-07-2005, 08:43 PM
air in the system I should say

I was going to say this ;)

Chemi :)

gerryboy00
15-07-2005, 04:02 AM
Such a warming up: air in the system I should say.



eh?........

wambat
15-07-2005, 05:49 AM
Other symptoms of an over charged system beside the possibility of air are:.
High discharge temperature.
High condenser subcooling.
High condensing pressures.
Higher condenser splits.
Normal to high evaporator pressures.
Normal superheats.
High compression ratio. Here are some explanations,

With an overcharged system, the high discharge temperature of is caused by the high compression ratio. Liquid backed up in the condenser will flood some of the condensing surface area, causing high head pressures. All of the heat being absorbed in the evaporator and suction line, along with motor heat and high heat of compression from the high compression ratio, has to be rejected into a smaller condenser because of backed-up liquid.

Because of the overcharge of refrigerant in the system, the condenser will have too much liquid backed up at its bottom, causing high subcooling. Where any liquid in the condenser lower than the condensing temperature is considered subcooling. You can measure this at the condenser outlet with a thermometer or thermocouple. Subtract the condensing out temperature from the condensing temperature to get the amount of liquid subcooling. A forced-air condenser should have at least 5/10 degrees of liquid subcooling. However, subcooling amounts do depend on system piping configurations and liquid line static and friction pressure drops. Condenser subcooling is an excellent indicator of the systemís refrigerant charge, but it is not the only one. The lower the refrigerant charge, the lower the subcooling. The higher the charge, the higher the subcooling.

Subcooled liquid backed up in the condenser will cause a reduced condensing surface area and raise condensing pressures. Now that the condensing pressures are raised, there is more of a temperature difference between the ambient and condensing temperature, causing greater heat flow to compensate for the reduced condensing surface area. The system will still reject heat, but at higher condensing pressure and temperature.

Because of the higher condensing pressures ó thus higher condensing temperatures ó there will be a greater temperature difference (split) between the ambient and condensing temperature which is what the instructor is talking about.

If these systems have TXV metering devices, the TXV will still try to maintain its evaporator superheat, and the evaporator pressure will be normal to slightly high, depending on the amount of overcharge. If the overcharge is excessive, the evaporatorís higher pressure would be caused by the decreased mass flow rate through the compressor from the high compression ratios causing low volumetric efficiencies. The evaporator would have a harder time keeping up with the higher heat loads from the warming entering-air temperature. The TXV will have a tendency to overfeed on its opening stroke due to the high head pressures, unless it is a balance port TXV.

The TXV will try to maintain superheat even at an excessive overcharge. As I mentioned above, the TXV may overfeed slightly during its opening strokes, but then should catch up to itself if still within its operating ranges.

benijoseph
16-07-2005, 07:50 PM
[QUOTE=frank]Have you taken any pressure readings of the discharge gas entering the condenser or the liquid line temperature leaving the condenser?

Mr, ONG,
U cant escape from franks question?with out knowing
the pressure ,how U can come to the concluction? that con-er is hot, better belive your BOSS, He might have taken the reading!
regards,
beni joseph, :D

Peter_1
16-07-2005, 08:00 PM
better believe your BOSS, He might have taken the reading

I mostly don't believe anything from anyone (speaking in terms of a technical Refr. explanation)
I have to believe it myself, I remain sceptical, I have to understand it and the explanation given me must be clarified in my mind before I believe it.
If not, doubt will remain in my mind, not saying what was said was also false.
But as long I don't understand it, I will search for the answer (which will be many times the explanation they gave me)

botrous
16-07-2005, 10:21 PM
I stand at Peters side . . .I have to beleive it myself

benijoseph
17-07-2005, 06:44 PM
I mostly don't believe anything from anyone -----((which will be many times the explanation they gave me)

U R right Mr peter , We should not believe other's , but at the same time when a person can't give proper feed back what are asked?, then what, Mr ong I doubt is afraid to ask his BOSS , that is the reason ,why he could not get readings, without geting proper parameter it will only a guesswork.
regards,
beni joseph,

Peter_1
17-07-2005, 09:59 PM
You're right Beni Joseph but his boss should also explain him why he thinks overcharged adn also explain how you can determine this.

Lc_shi
18-07-2005, 04:02 AM
if the discharge air is so high,the condenseing temp and pressure must be higher.I wonder if it still works. Overcharge is affect the condensing ,most time it can be measured by the consuming power or current. other reason may be related to the fan air volume. ask your boss to give a clear explanation.we can learn sth from him :)

Gary
18-07-2005, 12:24 PM
If the condenser delta-T (air out minus air in) is high, there is an airflow problem. Nothing else will cause this.

Peter_1
18-07-2005, 01:47 PM
If the condenser delta-T (air out minus air in) is high, there is an airflow problem. Nothing else will cause this.
I disagree Gary:
Air in the system, 100% sure!
Wrong refrigerant in the system.
Part of the refrigerant is another gas.
Fractionation (with the new gasses)
Rests of N2 in the system.
Mechanical excessive wear inside the compressor.

Saying that it's only an airflow problem is a litlle bit to easy.

piston broke
18-07-2005, 07:43 PM
Non-condensables in the system also make it impossible to fully charge the system with the correct charge of refrigerant.

It appears short of refrigerant (because it is) yet the working pressures are high.

Therefore the compressor overheats and also starts to draw too much current.

It's a pain but start again. Decant, pressure test, vac and re-charge.

P.

Peter_1
18-07-2005, 08:14 PM
Strange all these different approaches of a problem.
Obviously was the original question not that easy.
Any other answers and/or explanations?

piston broke
18-07-2005, 08:24 PM
The original question was too vague.

It's detective work really, the more info you have the easier it is to deduce.

The experience of the persons involved also help and sometimes a stroke of luck is needed.

It's a 'lemon entry' dear Watson.

P.

Gary
19-07-2005, 01:14 PM
A lot of things can cause high head pressure, but only airflow problems can cause high delta-T. I did a lot of testing on this. If I am wrong, I would be very interested in seeing the evidence.

ong
19-07-2005, 02:10 PM
i think i better get to all of u soon what actually my boss want to tell me beside overcharging of gas...is this problem common to most technicians or engineers?

Johnny Rod
20-07-2005, 10:27 AM
If the air flow was correct then you could still get a high delta T if the condenser temperature is higher than usual, as you'd be putting the more heat into the same amount of air. Hence the other guys' suggestions of what leads to a high condenser temp, e.g. non-condensables.

Put us out of our misery, ong.

Gary
20-07-2005, 12:57 PM
Noncondensables will not add to the heat output, nor will overcharge. Heavy load on the evaporator will add to the condenser delta-T, but not to the extent that the delta-T will be abnormally high. Only airflow problems will do that.

Peter_1
20-07-2005, 03:44 PM
I still disagree but it's no use for me arguing any longer on this matter, especially because I encounterd and measured many times the opposite
I'm only thinking now on a r134a compressor which is wrongly filled with R404a.

ong
21-07-2005, 11:38 AM
Hi everyone,

i got the answers from my boss already.. he told me 4 reason that causes the hot discharge air:

1) overcharge..
2) mixture of air in refrigerant
3) cod condensing pipe
4) undersize fan

r u all satify with his answers?

Temprite
21-07-2005, 12:11 PM
Hi Peter
I found out about a wrong gas on a system filled by
somebody else and it was working when the weather was
cooler but now on the peak time summer it failed.when
i changed the location of the compressor found the nameplate and found out that these guys used to fill R409a on R12
williams double door fridge which is O'K BUT found the
compressor is for R134a..........the fridge used to bring the temp down....intresting ain't it..... :D :D

Hello Mr cooling magic
Did this fridge have a 134a comp on an r12 condensing unit?

From what I have discovered is the r12 condensing units dont really have big enough condensers to handle 134a or other r12 drop ins.

If we have dead compressor on r12 condensing unit we always order complete new condensing unit not just comp.

Living near the ocean doesnt help either the condensers dont last as long.

homelover
21-07-2005, 02:39 PM
Blocked condencer,
Non condensibles in system,
Wrong fan motor or blade,

:cool: HOMELOVER

Johnny Rod
21-07-2005, 03:00 PM
You have cod in your condenser?

piston broke
21-07-2005, 07:32 PM
Yes now thats interesting. You could have gotten away with halibut or even a bit of skate but cod just means trouble.

PIST :D

frank
21-07-2005, 07:44 PM
I think your boss has missed out the possibility of the condenser being undersized or mis-matched with the evaporator.

wambat
22-07-2005, 12:45 AM
Noncondensables will not add to the heat output, nor will overcharge. Heavy load on the evaporator will add to the condenser delta-T, but not to the extent that the delta-T will be abnormally high. Only airflow problems will do that.
This is correct because noncondensibles do not follow the PT relationship, however they do tend to raise the COP which will add more heat to the condenser because higher required KW. So the only way to add more heat(Q) to a condenser is a larger heat load on the evaporator or a lower COP assuming a constant RPM. Now with regards to the TD off the condenser, that is controlled by the heat quantity and CFM because T = Q/1.08 x CFM so in this case, either the total heat went up or the CFM went down. Well you might say because the condenser lost efficiency and that is true if the coefficient of heat transfer went down (U) factor, but that doesn't affect the total heat, only that the heat is not disapated with proper efficiency :) The bottom line is what caused the TD to go so high the answer in my estimation is whatever caused the evaporator heat load to increase or whatever caused the
lower COP and that could be all of the following: Air in the condenser,dirty condenser, low air flow,increased evap load

gerryboy00
22-07-2005, 02:51 AM
Gary says it all


Noncondensables will not add to the heat output, nor will overcharge. Only airflow problems will do that.


Only airflow problems will do that


Non-condensable blanket the tubes of the condenser, thus reducing the heat transfer surface area of the condenser


......takes up room in the condensor, lowering the system's ability to remove heat.

Peter_1
22-07-2005, 08:25 AM
Gerryboy,

Ever serviced a unit where someone added the wrong gas in it?

Compressor is a constant massflow pumping unit but the enthalpy of the R404a gas compared to R134a is much higher.
So...?

Peter_1
22-07-2005, 08:26 AM
Gary says it all




Only airflow problems will do that


Non-condensable blanket the tubes of the condenser, thus reducing the heat transfer surface area of the condenser


......takes up room in the condensor, lowering the system's ability to remove heat.
So....rejecting the absorbed heat at a higher pressure and a higher DT.

Johnny Rod
22-07-2005, 08:45 AM
I thought that air in the system raised the condenser pressure, thus also the compression ratio of the compressor, and with it the motor current and so more heat into the system from the compressor. This heat has to come out somewhere, so if it's not the condenser then where? And if it is, surely you're putting more heat into the same air flow rate? My understanding of these things is fairly rudimentary, which is why I ask the question.

Thanks for the answers Ong, I had assumed that all the bits were spec'd right but the system wasn't running correctly, so yes, other things like undersized condenser are also possibilities. I'll get the bigger one out of the wagon...

ong
22-07-2005, 01:58 PM
sorry typo error..it should be clog in the condensing unit piping...

wambat
22-07-2005, 11:37 PM
sorry typo error..it should be clog in the condensing unit piping...
What does a clog look like :confused:

chillin out
22-07-2005, 11:54 PM
What does a clog look like
Thats they shoes they wear in holland.

chemi-cool
23-07-2005, 05:49 AM
Here is a nice clog.


Chemi :)

P.S. How do you get them inside a refrigeratin system? :D

wambat
23-07-2005, 07:14 AM
Thats they shoes they wear in holland.
OK then what is clogging...A pair of shoes? :rolleyes:

chillin out
23-07-2005, 12:44 PM
OK then what is clogging...A pair of shoes?
Now look your just being silly.
Its obviously a kind of dance. :D

Gary
27-07-2005, 04:18 AM
I can see where something which increases the heat load picked up by the evaporator, such as a different (more efficient) refrigerant or an oversized evaporator fan, would increase the heat in the condenser.

I can also see where swapping for a larger compressor and/or smaller condenser would increase delta-T, but these are not normal trouble shooting scenarios.

On almost all systems, if the delta-T is higher than normal for that system, there is an air flow problem. This is an extremely important fundamental trouble shooting principle.

Gary
27-07-2005, 02:02 PM
ambient air-35 degree celcius
discharage air-60-65 degree celcius

65C = 149F
60C = 140F

35C = 95F

65C(149F) - 35C(95F) = 30K(54F) dT
60C(140F) - 35C(95F) = 25K(45F) dT

In the real world, delta-T this high can ONLY be caused by airflow problems. I would challenge anyone to try to make these numbers occur by any means other than airflow. You will have to severely undersize the condenser to make it happen.

Alan B
27-07-2005, 04:41 PM
Gary,
A slight aside - why has your website and the link above been highjacked by merchant america?
AlanB.

Gary
29-07-2005, 06:24 AM
Gary,
A slight aside - why has your website and the link above been highjacked by merchant america?
AlanB.

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