PDA

View Full Version : Benji







benji
23-01-2008, 01:19 PM
Hi guys know im new but just one question got a problem with a 404a system. I keep getting high head pressure on the system ive checked the condenser, the suction and discharge valve plates of the compressor. Its a copland and it runs a blast freezer. Im starting to think that the tev could be a problem but it could also be air in the system, i also rodded out the shell and tube condensor but no luck, any answers? anybody.:mad:

Josip
23-01-2008, 01:44 PM
Hi, benji :)

Welcome to RE forums...


Hi guys know im new but just one question got a problem with a 404a system. I keep getting high head pressure on the system ive checked the condenser, the suction and discharge valve plates of the compressor. Its a copland and it runs a blast freezer. Im starting to think that the tev could be a problem but it could also be air in the system, i also rodded out the shell and tube condensor but no luck, any answers? anybody.:mad:

Please check this...

http://www.refrigeration-engineer.com/forums/showthread.php?t=10456

.....if you need something more try to search forums...

Best regards, Josip :)

grump
23-01-2008, 03:43 PM
Hi Benji,Shell and tube condenser Water cooled?? Check if there is a strainer in the condensing water pipework
make sure you have a full flow of condensing water at the correct temperature .Is the system cooled by a central plant,?/are you on the end of the line/To your knowledge,has the system ever worked to specification?
I hope i have interpreted your difficulties correctly if not,please get back to me/us,Kind Regards and welcome.Grump :D:D:D

Grizzly
23-01-2008, 07:02 PM
I know it sounds stupid but how are you measuring this High head pressure. If it is via a pressure transducer it could be faulty. Have you actually observed this fault. Or are you returning to the plant to find a HP FAULT. iF IT'S A HP SWITCH (mechanical) have you proved that.. Only you don't say if you have eliminated these issues?
Cheers Grizzly

nike123
23-01-2008, 08:34 PM
Hi guys know im new but just one question got a problem with a 404a system. I keep getting high head pressure on the system ive checked the condenser, the suction and discharge valve plates of the compressor. Its a copland and it runs a blast freezer. Im starting to think that the tev could be a problem but it could also be air in the system, i also rodded out the shell and tube condensor but no luck, any answers? anybody.:mad:

Did you checked entering and exiting temperatures and pressure of water in condenser, and proper functioning and adjustment of water regulating valve?
Shall and tube condenser ability to transfer heat to cooling medium (water) directly depend at proper water flow and functioning of water regulating valve.Thus, you need to check this first.

benji
23-01-2008, 08:45 PM
I know it sounds stupid but how are you measuring this High head pressure. If it is via a pressure transducer it could be faulty. Have you actually observed this fault. Or are you returning to the plant to find a HP FAULT. iF IT'S A HP SWITCH (mechanical) have you proved that.. Only you don't say if you have eliminated these issues?
Cheers Grizzly



hey griz.,liquid line hot due to high head pressure,feel i need to check exp.valve.....thanx all the same for info thus far. cheers

nike123
23-01-2008, 09:01 PM
hey griz.,liquid line hot due to high head pressure,feel i need to check exp.valve.....thanx all the same for info thus far. cheers

Oh my Good, you touched liquid line who appears to be hot, and diagnosed possible exp. valve fault.:eek:
That is "quick-ie"!:D:D

The Viking
23-01-2008, 09:13 PM
1ST, Before you try and adjust anything...

Attach your gauges!

Check super-heat and sub-cooling, then you will have a better idea of what happens in your system.
And only then will you know what adjustments should be made.


Good luck,

nike123
23-01-2008, 09:40 PM
I found this on HVACProTech forum posted by great guy Wayne Shirley. Maybe this could be of help to you and others!
Carefuly read red marked text.

he illustration below is a fairly typical cross section of a thermostatic expansion valve.
http://farm3.static.flickr.com/2166/2214315757_22c5a28cde_o_d.jpg

..The valves open or close, as a result of the pressures P1, P2 and P3. P1 is the sensing bulb pressure, P2 the evaporator (suction) pressure and P3 the spring pressure. The sensing bulb pressure varies with sensing bulb temperature, which is, more or less, the suction line temperature. If the suction line warms, the sensing bulb warms, the refrigerant inside the bulb warms and expands, so the pressure increases...vice versa for when the suction line cools.

The spring and suction pressures have a closing effect on the valve. The bulb pressure has an opening effect on the valve. When a system is at steady state operation, the refrigerant flow is constant, so P1=P2+P3. When the heat load on the evaporator coil changes, the rate of refrigerant evaporation or boiling, inside the coil will change. This affects both the evaporator (suction) pressure and suction line (sensing bulb) temperature, so P1 isnít equal to P2+P3. At that point the valve will open or close a little, trying to get the pressure formula back to an equality...

The whole design point of a TXV is to maintain a constant superheat value. Letís say the valve is adjusted or pre-set (for non-adjustable TXVís) for 10 degrees. When the system is off, the suction line temperature is, more or less, equal to the evaporator temperature...so the saturated sensing bulb and evaporator pressures are equal. So P1 and P2 cancel each other out leaving only the spring pressure doing any work, which closes the valve.

When the system starts, the suction pressure begins to drop, faster than the sensing bulb is cooling...P1 is greater than P2 so the valve opens a little. As more liquid enters the evaporator coil, more vapor is produced, raising the suction pressure. At the same time, the suction line begins to cool, causing the sensing bulb to cool, and bulb pressure to decrease. P2 becomes greater than P1, and the valve closes a little. As the evaporator ďloadsĒ up and the system reaches steady state operation, the suction line / sensing bulb temperature reaches a value producing a bulb pressure with a saturated temperature 10 degrees higher than the saturated temperature of the evaporator coil...so you end up with 10 degrees superheat.

If the heat load increases on the coil, the liquid evaporates faster and earlier in the coil tubing, allowing more time for the vapor to warm, or increasing the superheat. The suction line / sensing bulb warms, raising the bulb pressure. P1 becomes greater than P2...the valve opens a little. The additional refrigerant will eventually cause the suction line to cool down, creating the same 10 degree relationship between suction pressure and bulb pressure. If the heat load decreases on the evaporator coil, the reverse action takes place. The suction line / sensing bulb will cool a little, causing P1 to be less than P2. The suction pressure will close the valve a little, reducing the amount of liquid entering the coil until the relationship between P1 and P2 reaches the 10 degree point again...

The spring pressure is necessary only to create the desired amount of superheat... For most residential heat pumps, the spring pressure is non-adjustable, so during system operation, only P1 and P2 are of concern to us.

Itís interesting to note the explanation for TXV operation makes no mention of system head pressure. There is some minimum head pressure required in order for a TXV to operate, but above that value, the valve will function independent of head pressure variations. Sudden changes in head pressure can result in temporary variations of the valve operation, but it will eventually compensate, and return to design operation.

OK...all thatís crystal clear, right? Maybe not ... I spent years looking at the sketch and seemingly simple formula, all the while trying to figure out why I couldnít eliminate or diagnose a faulty TXV. So, in that respect (troubleshooting), letís take a different approach.

Forget about the P1ís, P2ís and P3ís and the adjuster screw on adjustable TXV models. All you want to know on a service call is whether or not the valve is faulty. If it is, youíre going to replace it...if not, youíre going to look elsewhere for a problem. In that situation, you only need to remember what the TXV does, and decide if itís doing it...to do that, you need to measure the system vital signs: low pressure, high pressure, superheat and subcooling. With those four values, you can diagnose most any problem with any system, TXV or otherwise.

Letís look at a typical R22 A/C or heat pump system operating in the cool cycle, with a TXV feeding the indoor coil. We would expect to see vital signs:

Low pressure...70 +/-
high pressure...225 +/-
Superheat...15 +/-
Subcooling...15 +/-

First of all, if you measure the superheat and get 15 degrees, regardless of the other numbers, the TXV most likely isnít faulty, because itís doing what it was designed to do...

If the suction pressure is 40 psi (low) and the superheat is normal, the problem is low airflow across the indoor coil.

If the suction pressure is 40 psi and the superheat is 25 (high), the problem is either low charge, faulty TXV or a liquid line restriction (filter or strainer or something)...now you need more information...measure head pressure and subcooing...if the subcooling is low, say less than 10 degrees, the problem is low charge. The head pressure will be low or lower, at least not high (unless the condenser coil is also dirty).

If the subcooling is normal to higher than normal, the problem is the TXV, or a liquid line restriction. Head pressure wonít necessarily be noticeably higher than normal (unless someone has come along before you and overcharged the system, trying to get the suction pressure up).

Now itís a matter of deciding between the TXV or a restriction...restrictions occur at potential restriction locations: filters and strainers...3/8 tubing doesnít normally restrict (if it does, you really have a problem!) . And liquid flowing through a restriction will undergo some pressure drop creating some flash gas, which will cause a temperature drop...so a restricted filter will have a noticeable temperature drop through or across it. Itís the same with a strainer located ďin frontĒ of the TXV ...it can plug. Then you would see a temperature drop before the valve. But if the liquid line appears constant in temperature up to and into the valve, then the problem is the valve...

Heat pumps in the heat cycle, with TXVís feeding the outdoor coil are approached in the same manner. Suction pressures will vary substantially, following the outdoor temperature. But the valve will maintain a constant superheat. The problem you encounter most often is accessing the suction line to get a temperature measurement, to calculate superheat.

Of course, when itís not possible to measure all the vital signs, you can always just add some refrigerant. If the suction pressure comes up, the problem was low charge. If, after adding a pound or two or three, the suction pressure remains unchanged, you have to suspect a faulty valve or liquid line restriction.

Iíve personally never seen a TXV fail in a way that produces low superheat, which is not to say it canít happen. In this case, it is possible the sensing bulb has lost contact with the suction line (though not likely...the straps holding the bulb in place usually outlast any other component in the system). If the bulb were to be sensing the air temperature around it, the valve would ďthinkĒ the suction line had warmed up, and force the valve to open, resulting in overfeeding of the coil and low superheat.

Now anything is possible, but barring really screwy situations, the information offered should get you through the majority of field situations with TXV systems...if anyone sees something I overlooked or misinterpreted, please post your comments or corrections...

Grizzly
23-01-2008, 10:08 PM
I found this on HVACProTech forum posted by great guy Wayne Shirley. Maybe this could be of help to you and others!
Carefuly read red marked text.
Nike
What a good link, I feel it was needed in this case.
Thanks Grizzly

grump
23-01-2008, 10:36 PM
Benji,It is a good idea to check water flows air flows,etc
Prior to messing with the TXV in the meantime please keep us updated on your progress. Grump:D:D:D

benji
24-01-2008, 08:19 AM
hi guys i found the problem actually, we have four compressors ok and each shell and tube condenser is attached to one water supply header and one outlet header, the pump on the cooling tower was replaced by someone else and the pump is over rated for the application? so what i did was i throttled the water returning to the cooling tower from the shell and tube condenser keeping the water in the longer in the condenser coz what was happening is with the pump pressure at 3 bar the water went in and out not giving the condenser water time to remove the heat.

grump
24-01-2008, 01:22 PM
Hi Benji.Pleased to hear you got it sorted,some of us have been in HVAC for many years,many engineers get fixated on the TXV (me included) in reality it is a modulating valve controlled by pressure /temperature the link by, nike 123, was very helpful:D:D:D. Grump

grump
24-01-2008, 02:25 PM
Benli you could fit head pressure control valves.Have you heard of Penne/Penn controls?You wont have to rely on setting the water flow by hand these valves will take care of the problem you are having with the condensing water pump, ie open the gate valves fully the penne valve will maintain head pressure. Grump :D:D:D

benji
25-01-2008, 07:42 AM
Hi are you talking about the pressure switch?:confused:

benji
25-01-2008, 07:44 AM
aaah it has to be tev on the high side?

benji
25-01-2008, 07:45 AM
Benli you could fit head pressure control valves.Have you heard of Penne/Penn controls?You wont have to rely on setting the water flow by hand these valves will take care of the problem you are having with the condensing water pump, ie open the gate valves fully the penne valve will maintain head pressure. Grump :D:D:D

could you send me a foto of one

nike123
25-01-2008, 08:56 AM
Benji

Grump talks about water regulating valve whose place is on water pipe feeding shell and tube condenser. This valve, thru capillary tube connected at high side, sense pressure in high side and adjust water flow keeping that pressure at constant preadjusted level. You need for each condenser one of these.

http://farm3.static.flickr.com/2083/2217720825_52fb9566fd_o_d.jpg

This is one of them:
http://tinyurl.com/3xp2e7
http://tinyurl.com/2j3qo9

benji
26-01-2008, 08:24 AM
Thanks mate, i had a tough day at work today had to fit a gearbox on a spiral freezer, job done start up electrical faults on scada spent the day there but managed to get it up and running, ****ing plc messing around otherwise ok.

Polestar
27-01-2008, 12:40 AM
I would stick to what you know,you mention air in the system is a unit one you visit regularly is it likely could another enineer have caused air in the system ? If you suspect it its simple (weigh in or sensible fill level)invert so you only draw liquid allow to settle for short period reintroduce liquid real careful into suction but hold back a kilo or so repeat a couple of times using same cylinder to recieve check pressures to see if improved if it has then reclaim and do the full monty if not I would pump down and check the TEV filter is not choked, crack the line see if she is loaded up to the TEV if is loaded and your suction is low its the valve orifice or filter thats shot. Had one once where the coil was oil ladened gave me some odd pressures and symptons while you have the TEV down is it possible to purge some ofn in to it just to check she is not loaded with oil past the manifold ?

benji
29-01-2008, 07:57 AM
Hi grump, how u doin , im ok just one thing one of our plants has a fmc pack screw compressor package with thermo siphon receiver what happens is that the oil is not returning i checked the oil return valve its ok i also know how the thermosiphon receiver works but just can seem to figure out why the oil is not returning, the package does pulldown a freezer room, system is R22......

grump
29-01-2008, 11:14 AM
Hi Benji.Can you check the oil and suction pressure, are you sure oil is not returning,don,t rely on the compressor site glass,i would be more concerned about the differential (oil above suction) The low temperature guys could be of more help as my time was spent in HVAC Why don,t you start a new thread with this problem.
In the meantime i would be interested to hear back from you with your oil and suction pressures also did you open the links by Nike123? (The Penn Valve by Johnson Controls) Keep coming back Grump :D:D:D

benji
29-01-2008, 11:46 AM
first of all i dont know how to start a new thread and i dont know how to delete all the rest but never the less the problem is the oil i know for a fact is laying some where in the system cos what these guys do is they put in oil and after a day or two they remove oil which means that when the system goes on defrost the oil returns?....

benji
29-01-2008, 11:56 AM
Hi nike123 benji here i have a problem wondering if you could help we have a thermosiphon receiver and fmc screw compressor package what happens is that when the system runs the oil does not return so what the gys do is they fill oil because the compressor trips on oil pressure and after a day or two they drain oil which means that when the system goes on defrost the oil returns any idea?....

Gary
29-01-2008, 03:12 PM
What kind of oil are you using?

nike123
29-01-2008, 04:37 PM
Hi nike123 benji here i have a problem wondering if you could help we have a thermosiphon receiver and fmc screw compressor package what happens is that when the system runs the oil does not return so what the gys do is they fill oil because the compressor trips on oil pressure and after a day or two they drain oil which means that when the system goes on defrost the oil returns any idea?....
I don't know, I only once worked width screw compressors, when I, and my mentor for refrigeration (these days, I was electrician with affection to refrigeration) commissioned quick-chill tunnel for one of biggest meat industry in my country at that time. That "tunnel" has worked (and still works) on Copeland screw compressor with R22 as refrigerant and Evaporation temperature of -42įC. My assignment is to make Elliwell controller EWPC 900 parameters for that application to work for us. I know, that my mentor tell me once, that Copeland engeneers did not believed that this setup will work satisfactory. But when we show them oil cooler they said that that solution is something new.
I told you this, because i think, that oil return is very serious thing, and problem you are experiencing, need attention of reputable engineer at that field.

benji
30-01-2008, 07:32 AM
well what i found today is that the oil sep. could be faulty they say the system is running ro years and the seps filters was a problem in the begining so i might just open the sep and check the filters.

benji
30-01-2008, 07:35 AM
well its not difficult you know its just some brain work and know how it works, so i checked the oil sep and got a feeling the oil sep is not doing its job....

Gary
30-01-2008, 09:07 AM
well what i found today is that the oil sep. could be faulty they say the system is running ro years and the seps filters was a problem in the begining so i might just open the sep and check the filters.

Sounds like it might be one of the big Temprite filter/separators. When the filter plugs, it blows out the bottom O-ring gasket under the filter cartridge and you can see a lot of action (bubbling, swirling and such) in the bottom sight glass.

grump
30-01-2008, 02:25 PM
Hi Benj i dont know the difference between A POST AND A THREAD EITHER IN THE MEANTIME LISTEN TO GARY HE WILL SORT YOU OUT . gRUMP:D:D:D