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View Full Version : Experianced engineers, who is correct?



james 100
23-01-2009, 12:45 AM
Please help settle this.....

Engineer 1 attends a multi deck running warm due to excess ice build up. Unit switched off to defrost.

Two days later engineer 1 & 2 arrive to "deep clean" a less than two year old unit that looked in good condition, good evap air flow, no sludge build up and clear drains.

Engineer 1 states the unit must have been short of refrigerant. This is a twin condensing unit capillary fed with one controller for both units.

Engineer 1 & 2 attach gauges to each unit. Standing pressures as expected, switch on finds both units within 10 PSIg of each other. I appreciate this is no way of checking the gas charge but a good indication.

Engineer 2 after confirming all evap fans are working as should be, unit is cutting out at temperature and gas charge appears to be ok asks what state the condensors were in? Enginer 1 said they were ok but had been blown out with OFN anyway.

Engineer 2 put the cause of the ice build up down to the condensors being blocked, Engineer 1 said that would have no bearing on an iced up evap what so ever and if the condensors were even partially blocked the unit would have cut out on HP and not caused ice build up :confused:. Engineer 2 pointed out the unit had no HP device fitted ;)... No reply.

Engineer 2 has not been in the trade long and although he has prooved to himself that a blocked condensor or failed condensor fan motor can cause ice build up on an evaporator, he would like engineers with experiance to comment on this case.

Thanks in advance. :)

multisync
23-01-2009, 07:09 AM
Please help settle this.....



Engineer 2 has not been in the trade long and although he has prooved to himself that a blocked condensor or failed condensor fan motor can cause ice build up on an evaporator, he would like engineers with experiance to comment on this case.

Thanks in advance. :)

Can engineer 2 prove to us how a blocked condenser/failed cond fan (ie increased head pressure) causes ice build up


Multisync
London

just-chillin
23-01-2009, 07:20 AM
iced up cases, more often than not customer has overloaded case, blocked vents, is the case near the front door of shop/ warehouse door, any air blowing in to case , heating vent blowing into case, shop too warm, deck panels not fitted correctly, the old Craig Nichol cases if the base panels were 1/4 inch out the case would ice up, have you checked defrost timer does it have heaters. I guess the case is integral do the condencer fans blow front of case or to rear , if they blow to the front just watch where the store staff put the stock when loading see the warm air go straight into the case. is coil fully iced up or only 1/2 or at ends of coil. if none of these you need to dig deeper pressures temps. superheats, air flows etc. but start simple check the defrosts.

monkey spanners
23-01-2009, 05:05 PM
I don't usually work on this type of equipment but.... I have twice been called to look at frozen evaporators in fosters style cabinets and the only fault i've found is a duff condenser fan motor.
My theory was that without the condenser fan it would never reach set point and as these relied on off cycle defrost they never got a chance to defrost.

Never got called back so it was either fixed or i was judged so incompitent they called out someone else :D

Jon

750 Valve
24-01-2009, 10:42 AM
as per monkey spanners - the only way a blocked cond could cause an ice up is if the head pressure increased and the compressor did not have the capacity to maintain the suction pressures when under normal operation and did not cut out on stat when the case was designed to.

If this is true then it does beg a few questions though - does the evap coil have too many FPI for its application (assuming case still defrosted as per normal it shoul;dn't have iced up due to continual operation unless coil selection was poor)? should the compressor selection for required heatload be a little greater (spare capacity)? could the condensing unit be selected at a more appropriate ambient temp to allow for a slightly dirty condenser?

although I agree it could happen it should not

james 100
24-01-2009, 11:45 AM
Multisync, please see monkey spanners reply. If the unit is not condensing the metering device is not fed and the unit will never get to set temp and so run continually until the evap is totally iced up.

nike123
24-01-2009, 12:14 PM
If we exclude excessive moisture from air at evaporator, than cause of higher than normal ice build up is high TD.
High TD could be because high evaporator superheat.
High evaporator superheat could be because of lack of refrigerant, restriction in liquid line, restricted metering device, TEV bulb charge or faulty or wrongly adjusted TEV, oil logged evaporator, low condensing pressure, no subcooling.

If you have high condensing pressure, than on TEV opening stroke valve releases more refrigerant and therefore reduces superheat. If superheat is reduced, TD is reduced, and ice formation is reduced.
If, on the other hand, condenser is dirty and there is no subcooling, than TEV is fed with liquid/gas mixture and superheat is higher. That could cause excesive ice formation.

On capillary fed system could be different scenario with superheat. I need to study more this pos (http://www.refrigeration-engineer.com/forums/showthread.php?t=16920&highlight=capillary)t!

I assuming this:
If condenser is dirty so much that there is no subcooling, than capillary tube is fed with liquid/gas mixture. That mean liquid refrigerant starvation at capillary tube and higher than normal superheat and therefore higher than normal ice formation at evaporator.

multisync
24-01-2009, 05:03 PM
OK lets run with this.

What happens to during this fault condition.?
One would assume
The discharge pressure increases
The mass flow rate increases
The suction pressure increases
As it's circular further rises add to the initial rise so it increases .
The pressure rises above zero saturation. (no ice)
The suction gas becomes high superheat and the compressor cuts out on it's O/L.

I scanned his article and couldn't see anything there that would indicate this event happening

I suspect need Chef to expand on this as a possibility ..

I will PM him.

Chef
25-01-2009, 08:36 AM
Hi Multisync - I got your PM.

If, as is highly likely, the system when running at its design condition it will have at the inlet to the
capilliary a quality of around 0.2 to 0.25, ie gas/liquid and so there will be no subcooling at all.

It gets complicated when analysing changes to the system conditions whilst the exit of the condensor is in the two phase region. Normally one would expect liquid to be output from the condensor but once inside the two phase region things change a lot.

If there is now a blocked condensor/fan failure this causes the condensor temperature to rise and the condensors heat rejection falls the postion of h3 on the PH diagram will move upward and to the right, if there is sufficient reduction in performance of the condensor it may even move above the saturation line but it is more likely to stay within the two phase area.

This means only a small increase in mass flow (as the refrigerant is still a gas/liquid mixture with a large
pressure drop) for the increase in head pressure but a large reduction in the cooling effect as h4 also moves to the right.

With less cooling effect the system may never reach its set point as pointed out by monkey spanners.
Scenario 2 is that h3 does move above the saturation line and some subcooling is envoked, this requires more liquid to be held within the condensor and so less is available in the evaporator, an increase in the TD and a reduction of the pressure to maintain equilibrium.

An overall increase of mass flow rate but h4-h1 is still lower so reduced cooling again. The lower evap temp may cause some excess icing but it may still never reach the set point.

The reduction of refrigerant in the evaporator will tend to cause one end to ice before the other, is this the case?

If you can give me the pressures and temps in the circuit I can try and run it in a simulation and get a PH plot out for the change in the condensor condition.

Chef:)

james 100
25-01-2009, 11:55 PM
Chef.

Thanks for the in depth reply, i managed to understand a few sentences!! I will inform all involved and post any feed back.

As for further investigation, the cost of the plant/engineers involved/service provider are not worth your time and effort to that extent although its good to know people like you are out there.

Thanks again to yourself and the others who contributed.