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frank
15-02-2005, 05:54 PM
One of our controls specialists that we use has just rang for a bit of advice.

He's got a job with a heat pump on and he's written the software for defrost (as advised by the client - We*the*ite) (not to mention any names) as follows:

1 When the coil temperature reaches 0degC then the reversing valve changes over for a period of 8 minutes or until the coil reaches 60deg C

2 After a defrost the system goes into a 20 minute anti cycle stage and does not run.

3 If the coil drops to 0degC during the anti cycle time then it must defrost again before going back into normal heating mode.


The system was commissioned by Wea*her*te in July but they have just rang our engineer mate and said that the system has failed with a burst coil due to the defrost software being incorrect and saying that he must stand the cost of the repairs! :eek:

Any thoughts??

Apparently, the coil sensor is on the "pipe just before the compressor" :eek: and only the first bottom 1/4 of the coil ices up :rolleyes:

Among other things my first opinion was that if the software was written to the clients instructions and tested to comply then the refrig eng who commissioned the system must have been happy with the software at that time as he's signed off the commissioning for the system. Who do you think is right?

rbartlett
15-02-2005, 06:47 PM
why did he say the coil burst?..if it was because the coil was to get to 60 deg'c before termination (i did get a tad confused by no/1)

then I would agree that expecting a fridge coil to need 60 deg'c to defrost is a tad excessive.

if you/yours had designed the system to need a 60 deg c indoor coil to DEFROST and it failed -at xxx pressure to which corresponds- then you are liable..

if only because you should have ensured the system (evap) was capable of withstanding the pressures relating to 60 deg'c

besides there are not many refrigeration applications where the evaparator doesn't reach 0 deg c (those what do -I would hazzard a guess- don't need a hot gas defrost)

however if i have misunderstood....

cheers

richard

frank
15-02-2005, 08:04 PM
Hi Richard

This job has nothing to do with me. It is a job that the controls engineer that we use has had dealings with. He wrote the defrost software (amongst other software) based on the clients spec. Bear in mind that this is a heat pump and the coil we are talking about is the outdoor coil sat on a high roof. (heating mode).

Now that the system has failed he is being blamed for writing software that has caused the coil to burst.

He rang me asking for advice and questioning if defrost problems could cause a coil to burst. As he is not a fridgie he is looking for technical assistance in responding to the claim that he is to blame.

I've offered advice and he is going to reply tomorrow but I thought this problem was worthy of a few comments from the forum.

The coil getting to 60degC after or during defrost is highly unlikely but bear in mind that he only wrote the software based on instructions given. He wouldn't know if 60C was good or bad.

The basis is that they are trying to blame him for the problem.

Thoughts on defrost cycles or defrost problems I will print out and pass on, together with any other relevant thoughts the members have to offer - much better than him just relying on my observations alone. :)

Argus
15-02-2005, 08:38 PM
....it would help to know what refrigerant is in the system at 60 degrees.
________
Ford Pinto (http://www.ford-wiki.com/wiki/Ford_Pinto)

frank
15-02-2005, 09:02 PM
....it would help to know what refrigerant is in the system at 60 degrees.

hot gas at 60C could be R407C or R22 or ?

At this stage I don't know but 60C is not that high

dill
15-02-2005, 10:13 PM
hi frank, I would be interested to know where the coil burst, if it was crushed, wether it is a split or a large rupture. these different types of burst can lead you to determine the cause. the defrost program is being blamed but the cause could lay elswere.

RogGoetsch
16-02-2005, 02:08 AM
Hmmmm. Nothing about this sounds right. A picture of the "burst" coil would be in order.

I have serviced hundreds of iced-up coils over the years. Never have I seen ice accumulating on the outside of a coil cause a rupture. What I have seen several times are holes punctured in the coil by a screwdriver some one has used to try to remove the ice.

Water freezing inside a pipe will expand and rupture the pipe. Water freezing on the outside of a pipe or coil will not. Water needs to be trapped in a confined space before freezing in order to produce stresses sufficient to cause a rupture. Moisture freezing as it condenses fills every available space but does not crush the pipes it freezes on or between.

A mechanical joint, such as a flare, can be damaged by water continually freezing, melting and re-freezing if it is seeps into the joint, but this would be a problem with the design or fabrication of the joint. For this to happen somehow with a coil would mean a design or fabrication problem with the coil.

Of course there is also the problem of the defrost initiation and termination temps not making much sense, and the 20 minute anti-cycle seems unnecessary. Why an anti-cycle at all if the compressor has not shut down? Maybe because the temps are so unrealistic that the unit can't operate correctly?

Supposedly these are the customer specs yet the programmer is blamed?

As I said, none of this makes any sense. What's the real story?

Rog

Jase
16-02-2005, 07:45 AM
Where exactly are the sensors positioned on the unit. As you have mentioned that the coil sensor is 'on the pipe before the compressor', I would assume that this is the temperature phiall for the defrost thermostat timer. Is this pipe insulated well?
They must have wired there control circuitry through this 'sensor' thinking that this would control the unit @ whichever parameters they have set. If so, then the unit would short-cycle as the 'sensor' would be reading 0 degrees, as set by the controls.
The best bet is to re-configure controls and let the unit control it's own defrost period.

Jase

frank
16-02-2005, 08:50 AM
I will be speaking to the controls guy tonight to get some feed back and further info.

One of the things that I,ve requested is details of the failed coil including photo's and copies of the commissioning report.

frank
16-02-2005, 08:53 AM
As I said, none of this makes any sense. What's the real story?

Hi Rog

My sentiments exactly. Either the tech looking after the kit doesn't have much of a clue about his job or they are just trying to pass the buck (and costs) on to the controls guy.

I'll post further info as I get it

frank
18-02-2005, 05:25 PM
Well - an update

The unit is one of 4 rooftop Wea*heri*e models serving a Tesco shop floor in Birmingham (UK). All 4 units have the same software controlling defrost but only this one has failed.

Our controls guy met the "engineer" from We**herit* on site on Thursday to view the problem. It appears that the coil has split at the bottom of the coil just after the pipe entry but there is no ice heave damage to the fins in the surrounding area :confused: so it is difficult to say that the damage is caused by the defrost - or lack of it! :confused:

The sensor for the coil temperature is located on the coil header which would be the common suction in heating mode.

Apparently, when the adjacent system was put into defrost (by the engineer blocking the evaporator to lower its temperature ) the defrost lasted 30 seconds before the coil reached 60Deg C and defrost terminated. He asked for the defrost termination temperature to be increased to 70degC to allow a longer defrost time to clear the bottom of the coil :D

The engineer was adamant that he did not want any frost on the coil and that the defrost must start when the coil dropped to 0degC even when our guy suggested that it would be better with a timed defrost. Seems like this guy lacks some basic knowledge.

When the defrost termination temperature was raised to 70degC the unit had a defrost time of about 50secs and the Weath*r*te engineer was quite pleased that the system was now operating correctly and to his satisfaction.

What baffles me is that he is now happy that the defrost settings and operation are to his liking on 3 out of 4 systems but he is still adamant that it was the defrost that caused the coil to fail on the 4th unit :eek:

I think this guy is just trying to pass the buck and should just own up to not commissioning the units correctly in the first place. As the ice build up on the coil was not restriced I fail to see how a short timed defrost could cause a coil to split/crack/burst or whatever you want to call it

chemi-cool
18-02-2005, 08:24 PM
Hi Frank,

After reading all the posts again, it looks to me as there was a fault with the copper tube and not with the defrost setup.

Ice is built on the outside, so it has nothing to do with it.

How exactly did it burst, along the pipe or in one spot?

Chemi :)

frank
20-02-2005, 06:13 PM
Thanks Chemi - that was my deduction as well and I've printed out all the posts to support our controls guy in his fight against what appears to be an attempt to make him pay for some one else's problems.

Why aren't people more honest? :confused:

chemi-cool
20-02-2005, 06:28 PM
Hi Frank,

Honesty means that some times people have to come with: "Sorry, I have made a mistake"

Chemi :)