Increase condenser size to increase subcooling?

[Drew]

If we increase the size of an air cooled condenser do we increase the subcooling or do we just lower the SCT or both?

[nike123]

By increasing condenser size you lower condenser TD.
BY lowering your condenser TD, SCT drops.

Subcooling depend on amount of liquid refrigerant in condenser. If you overcharge it, it will rise. If you undercharge it, it will fall.

If outdoor temperature is too low and you don't have fan speed control, subcooling could rise.

[desA]

^ A question:

How would the parameters outlined above change between a tube-in-tube, shell-&-tube, tube-&-fin & plate-type condenser?

[nike123]

^ A question:

How would the parameters outlined above change between a tube-in-tube, shell-&-tube, tube-&-fin & plate-type condenser?

If I correctly understand your question, I suppose same, if we assume that with "bigger condenser" we mean bigger mass flow of secondary media as well as bigger active surface.

[desA]

What I was wondering, if the physical 'shape' of each type made a difference.

For the tube-in-tube, for instance, it seems to lend itself to allowing a longer contact length for the sub-cooling - simply because the fluid can't get away - & probably better sub-cooling than a vertical-fall plate.

For the vertical plate, the liquid hold-up region would probably not back up the plates as much, I'd expect - so allowing less sub-cooling.

I've seen this quoted on plate CBE HX's where their design sub-cooling is around 3.5'C. These would probably require a dedicated counterflow sub-cooler to get the sub-cooling down a little further e.g. for a heat-pump.

For the tube-&-fin I'd expect a similar response to the tube-in-tube.

Not quite sure what to expect from the shell-&-tube - perhaps similar to the tube-in-tube - or should that be plate HX? Guess it depends on which side the refrigerant fluid sits (which side is it - the tube-side?)

[Gary]

Nike is correct. Subcooling (measured at the receiver outlet or condenser outlet if there is no receiver) is about refrigerant charge, regardless of type of condenser.

Receiver/condenser outlet subcooling is a measure of surplus refrigerant.

Subcooling added along the way to the evaporator is a whole different story.

[Drew]

Thanks again guys.

Under low ambient conditions , without head pressre control, a system would show a low head pressure and possible high subcooling. The low head pressure would be from the more effeicient condenser and the sub cooling would be more from the refrigerant backing up a bit in the condenser due to being slightly 'overcharged' for that condition(due to increased flow rate of refrigerant)??

[Drew]

The reason i ask is that i have a package unit that on cold mornings during its reverse cycle defrost, with outdoor fan off, trips on its HP manual switch before the end of defrost still with ice on its coil? The defrost logic is that it will either run for max 10min or terminate by a probe that is positioned towards the end of the outdoor coil at 10 C. The gas charge is good and both independant stages are doing it. The probe is very subcooled still with a 80 c SCT, which trips out the switch. I have installed HP auto switches to the outdoor fans to cycle them on towards the end of defrost when the pressures really gets to high stop the compressors tripping.
The suppliers say that this cannot happen.

[Gary]

The reason i ask is that i have a package unit that on cold mornings during its reverse cycle defrost, with outdoor fan off, trips on its HP switch before the end of defrost still with ice on its coil? The defrost logic is that it will either run for max 10min or terminate by a probe that is positioned towards the end of the outdoor coil at 10 C. The gas charge is good and both independant stages are doing it. The probe is very subcooled still with a 80 c HP, which trips out. I have installed HP auto switches to stop it tripping.
The suppliers say that this cannot happen.

How do you know the gas charge is good?

[Gary]

The reason i ask is that i have a package unit that on cold mornings during its reverse cycle defrost, with outdoor fan off, trips on its HP switch before the end of defrost still with ice on its coil? The defrost logic is that it will either run for max 10min or terminate by a probe that is positioned towards the end of the outdoor coil at 10 C. The gas charge is good and both independant stages are doing it. The probe is very subcooled still with a 80 c HP, which trips out. I have installed HP auto switches to stop it tripping.
The suppliers say that this cannot happen.

This doesn't make sense. How can the coil possibly have ice on it with the HP at 80C? I'm wondering if you have air in the system... or possibly the wrong refrigerant in the system?

[desA]

Removed... by author

[michaelm]

By increasing the surface area of the condenser more energy will rejected from the refrigerant. Cooling below the boiling point is subcooling. Bigger surface should increase subcooling, of course, surface of the condenser was sufficient before the increase to condense the refrigerant to liquid.

[Drew]

The gas charge is good because I reclaimed, evacuated and recharged with the correct amount of 407C. Under normal running conditions the operating pressures are good in either heating or cooling.
The outdoor coils are fed with a TEXV leading to a 4 way distributor which feeds the 4 circuits.
The defrost termination sensor is on the lowest circuit at the bottom of the coil. In the early morning when the coils ice up the spread of ice is even through out.

Could the following be happening?:
When the system calls for defrost (-3C probe temp with 30min minium intervals) the ice on the top 3/4 of the coil melts off beautifully. All this ice cold water runs over the lower section of coil cooling the refrigerant down by the probe to such a degree that it takes to long to reach the 10C defrost termination point. In the mean time the head pressure climbs and cuts off at the preset hp.

Ive thought that maybe one of the distributors are blocked , but it is doing the same thing on both independant stages. I have also heard of the same brand name unit doing the same thing.

Thats why i asked about subcooling. Could the refrigerant be subcooled to such a degree.?

I have learnt today that if you want to increase subcooling you need to maintain your SCT at the same pressure and cool your liquid further.This increases your subcooling.
You cannot just decrease your SCT by increasing the condenser size. This will lower the SCT but not increase the subcooling much.
So for good subcooling you need an oversized condenser with fan speedcontrol which will maintain SCT. ? This makes sense on a pressure/ enthalpy graph.

I realise that im going on about two issues, but i feel that if i can understand how subcooling can vary on a sytem then i can understand this fault.

[Peter_1]

Nike is correct. Subcooling (measured at the receiver outlet or condenser outlet if there is no receiver) is about refrigerant charge, regardless of type of condenser.

Receiver/condenser outlet subcooling is a measure of surplus refrigerant.

Subcooling added along the way to the evaporator is a whole different story.

Very important remark in your sentence:"without a receiver"
With a receiver, you only can have subcooling with a build-in subcooler in your condensor (which most AC's have these days) which are the last tubes flowing upwards.

[Drew]

By increasing the surface area of the condenser more energy will rejected from the refrigerant. Cooling below the boiling point is subcooling. Bigger surface should increase subcooling, of course, surface of the condenser was sufficient before the increase to condense the refrigerant to liquid.

Increasing the surface area of a condenser will only increase your subcooling if you maintain your HP at a higher pressure.

Would this be true:

A system running at SCT 45C with a condenser fan running flat out without capacity control on a standard subcooled condenser is fitted with a bigger condenser. This would lower the SCT to say 40C, but the subcooling would be about the same? If now we fitted speed control to the fan of the bigger fitted condenser and forced the SCT back up to 45C would this then create an increased subcooling???

The idea would be to increase condenser size but maintain the higher SCT by fan speed control??

[Gary]

Cooling the liquid is a good thing. Raising the SCT is a bad thing. But this has its limits. There must be enough high side pressure to properly feed the TXV.

Just enough. Not too much. In refrigeration, balance is everything.

[Gary]

The gas charge is good because I reclaimed, evacuated and recharged with the correct amount of 407C. Under normal running conditions the operating pressures are good in either heating or cooling.
The outdoor coils are fed with a TEXV leading to a 4 way distributor which feeds the 4 circuits.
The defrost termination sensor is on the lowest circuit at the bottom of the coil. In the early morning when the coils ice up the spread of ice is even through out.

Could the following be happening?:
When the system calls for defrost (-3C probe temp with 30min minium intervals) the ice on the top 3/4 of the coil melts off beautifully. All this ice cold water runs over the lower section of coil cooling the refrigerant down by the probe to such a degree that it takes to long to reach the 10C defrost termination point. In the mean time the head pressure climbs and cuts off at the preset hp.

What is the low side doing during defrost? Is the indoor fan running? Are there electric heaters running indoors during defrost? If so, are the heaters upstream or downstream from the coil?

[Gary]

Let's see if I can clarify the subcooling thing:

The compressor must pump against the high side pressure. Lowering the pressure makes the compressor's job easier, increasing it's efficiency. On the other hand, we need sufficient high side pressure to push the refrigerant through the TXV or the coil will be starved.

At the receiver/condenser outlet we want a solid stream of liquid, but we do not want liquid to back up into the condenser, because this limits it's ability to reject heat thus raising high side pressure.

We check the subcooling at the receiver outlet (or condenser outlet if there is no receiver) in order to determine the quality of the liquid at that point. Vapor bubbles disappear between 10-15F/5.5-8.5K subcooling. Less than this means there is not solid liquid at the outlet. More than this means liquid is starting to back up into the condenser.

After the liquid has left the outlet, any further cooling (without raising SCT) is a plus as the colder liquid will result in less flashing at the TXV outlet, thereby increasing the efficiency of the coil.

Excess receiver outlet subcooling = Bad

Liquid cooling between receiver outlet and TXV inlet = Good

[Peter_1]

We have a plant running where the liquid leaving the receiver is excessive subcooled with a PHE, one side R404a and the other side a glycol net running under a freezer as a soil frost buildup prevention.

Liquid lines isolated all the way.

The SC we achieve is in the region of 20°C to 25°C (65°F/77°F) which increases system performance a lot.
This subcooling doesn't cost us almost nothing, only a pump running which we needed anyhow.

[Gary]

We have a plant running where the liquid leaving the receiver is excessive subcooled with a PHE, one side R404a and the other side a glycol net running under a freezer as a soil frost buildup prevention.

Liquid lines isolated all the way.

The SC we achieve is in the region of 20°C to 25°C (65°F/77°F) which increases system performance a lot.
This subcooling doesn't cost us almost nothing, only a pump running which we needed anyhow.

This is a good example of liquid cooling without backing liquid up into the condenser and raising SCT.

I'm thinking life would be a little easier if the industry referred to this as "liquid cooling" instead of "subcooling", but who am I to tell the industry what to call anything?

When we chill our favorite beverages we are technically subcooling them, but we don't call it subcooling because that would unnecessarily confuse everyone.

[US Iceman]

I think the thing that has gotten lost is what subcooling actually means and how it is achieved.

Subcooling is a good when you do not have to pay for it. Larger condensers help to lower discharge pressure; that's good also.

One thing everyone needs to remember is that subcooling in a condenser should not be done by backing up liquid in the condenser coil. This can increase the discharge pressure if there is too much liquid refrigerant inside it. However, if you use a subcooling coil in the condenser that is different. The condenser surface is used for condensing at an appropriate discharge pressure while the subcooling coil provides the ability to lower the liquid temperature to within a few degrees of the ambient air (or water).

[nike123]

Nice discusion! Let's rate this thread.

[Drew]

So .... the way i understand it:

Refigeratiion effect is increased by increasing condenser size (which lowers SCT) and increases refrigerant flow and compressor capacity.

Refrigeration effect is also increased by subcooling the liquid without letting the SCT drop.

So if we have a good sized condenser (low SCT) with good sub cooling we have an effecient system with regard to high side?

The idea becomes clear if you plot the cycles on a pressure / enthalpy graph. If you have a large condenser with a system condensing at 40C with a standard 4 - 7 C subcooling it will have the same refrigeration effect as the system having a 45C SCT with a bigger sub cooling.

Therefore from a design point of view choose the lowest SCT possible( depending on the ambient) , match a TEXV to the correct pressure drop , try and obtain a free sort of subcooling ( not at the expense of super heating the suction line further). Use fan speed control to maintain desigh SCT.

[US Iceman]

Therefore from a design point of view choose the lowest SCT possible( depending on the ambient) , match a TEXV to the correct pressure drop , try and obtain a free sort of subcooling ( not at the expense of super heating the suction line further). Use fan speed control to maintain design SCT.

I would say you have a good idea of what to do now.

[Drew]

Even with the hp auto switches working to prevent the high head pressure during the reverse cycle defrost the system has tripped on HP manual during the low ambient condition this morning.

I have seen this system tripping on high head pressure (465psig) even though i have installed hp auto switches which cycle on the outdoor fan at 325psig. Why would the hp climb beyond this point.?Normally the system wouldnt need hp auto's? But since ive found the system tripping on HP manual ive been forced to fit the hp autos to bring on the outdoor fans to prevent the high head pressures. It seemed to have worked on most cooler mornings except for this morning when it was a little colder than normal when it tripped again. How could the hp climb eventhough i am driving on the fans which should be lowering the hp.
It only happens on low ambient conditions between 6-7am after starting at 6am.
I will be going to site at 6am tommorrow to catch it red handed.

The unit is a APAC unit. 2 stage. The defrost logic is that if the oudoor coil falls below 3C and is out of the minimum time period the stage 1 will reverse cycle with outdoor fan 1 off. The stage 2 will carry on running in heating with the indoor fan on to offset the stage 1 cooling effect. The indoor coil is interlaced so the stage on defrost's suction will be raised by the system still in heating??This in term will raise the head. I thought by fitting the hp autos to drive the outdoor fans on this would correct the fault?

Could it have something to do with the colder ambients causing the refrigerant to back up in the outdoor coil and accumulator then when the system reverse cycles (defrost) their is some sort of hydrolic action happeneing forcing the hp up? So it might not be the normal raising of head pressure caused by the lack of outdoor fan , but by liquid refrigerant sitting in the coil. When the system reverse cycles superheated vapour from the indoor coil is pumped onto this liquid in the outdoor coil. this liquid refrigerant then boils off creating the high head pressure.??

Almost like an overcharged system with high subcooling and high head pressure.?
Due to the liquid sitting in the outdoor coil under low ambient conditions when it reverse cycles and turns into a condenser their is already liquid backed up and this takes a while to balance out again but in due course the hp manual trips?

[Gary]

I'm thinking there must be some sort of blockage between the point at which the HP is measured and the outdoor coil. What devices are between these two points?

[Drew]

Their is a bi directional drier, sight glass and bi directional texv on the out going side of the coil.Only the 4 way valve on the ingoing though.
Could it be refrigerant sitttin in the coil due to the low ambients causing a sort of overcharge for a period?

[Gary]

That would seem highly improbable.

More likely a (temporary?) restriction across the 4 way valve when it switches to defrost mode.

[desA]

More likely a (temporary?) restriction across the 4 way valve when it switches to defrost mode.

I was shown a 'sticky' 4-way valve a month, or so back, by a manufacturer.

A single manufacturing dimension, marginally over-tolerance, caused the valve to stick under certain situations.

You may want to test the valve's operation.

[Drew]

Thanks Guys,
I went to site this morning at 6am.
Unit was running well with even frost of ice, which is expected at 0c ambient. The defrost was working. The defrost happened every 20min , last about 4 min terminated by the probe (10c). During the last 30sec of the defrost the outdoor fan cycled on at 60c , diff 8c. R407c. The fan cycled on because of the hp autos i had fitted. So ,all looks fine at the moment.

Tommorrow i'll go again at 6am and monitor.

Dont know why i had that weird hp trip yesterday?
I know that the hp autos are helping the hp towards the end of defrost
So could i also have a sticking valve causing the same original issue? Two faults causing HP cut outs? That will teach me not to tell the client that it was fixed.

Im getting lots of second faults lateley. Same with a Daikin unit im working on. VRV with L8 and E5 errors (compressor lock and high amps, I think) Replaced compressor and on starting up a U2 fault appeared (outdoor board) If the unit is clever enough to tell that the compressor is screwed why couldnt it advise me about the PCB?? But i'll leave that for another thread!

[Drew]

That would seem highly improbable.

"Highly improbable things seem to happen to me.

[Gary]

Where are you hooking up your high side gauge? Is there more than one access fitting on the high side?

[Drew]

I only have access at the compressor service port.
All the other fittings are brased in.

[Gary]

You might want to check the temps on all 4 lines at the 4 way valve. In particular, if the slide sticks, hot discharge gas can cross over to the suction side. This would be evidenced by an increase is suction line temp from one side of the valve to the other.

[Gary]

While you are monitoring the system, it would be a good idea to gather all of the data with the system in normal operation to see if we are missing something basic.

This would include in and out air temps for both coils, high and low side pressures/temps, and line temps at the exit of both coils.

[DeB]

Drew, is this a split or package unit? Was it charged on heating or cooling cycle? The problem is possibly due to high liquid level in the outdoor coil. The reverse cycle defrost is causing the high velocity hot gas to condense in the outdoor coil before the cold liquid level has dropped sufficiently in the condenser thereby reducing condensing area. I would try delaying the compressor start at defrost or recharging system on the heating cycle with a minimal refrigerant charge.

[Magoo]

Hi Drew, sounds like you have a Bradway heat pump.

magoo

[Drew]

Could it have something to do with the colder ambients causing the refrigerant to back up in the outdoor coil and accumulator then when the system reverse cycles (defrost) their is some sort of hydrolic action happeneing forcing the hp up?

Almost like an overcharged system with high subcooling and high head pressure.?
Due to the liquid sitting in the outdoor coil under low ambient conditions when it reverse cycles and turns into a condenser their is already liquid backed up and this takes a while to balance out again but in due course the hp manual trips?

Hey DeB,

Would you then agree with the above?

It is a rooftop package unit.

So... the refrigerant that sat in the outdoor coil during the normal heating cycle that was more than normal due to the lower ambients. With the low ambients causing the outdoor coil to be inefficient,and not vapourising the refrigerant the liquid sits in the coil. The TEXV even in its most closed position will still be to wide open. When we go into defrost the outdoor coils surface area is reduced due to the liquid. Therefore we have a 'small condenser' . Head pressure must go up and possibly trip.
A clue that this could be right is that the hp auto's that i fitted to lower the hp towards the end of the defrost have been adjusted with a 300psig differential. I thought that the fans would cycle on for quite a while at this setting, but they only ran for a few seconds and the hp dropped hugely. I thought it could be due to the low ambient, but it could also be from the minimul vapour fitting in the outdoor coil.
Therefore the fans dont need to run long to the lower hp. The other clue is the high sub cooling. Liquid backing up??

The unit was still running well this morning with the hp autos kicking in towards the end of defrost which kept the hp low enough to prevent the saftey trip.

During normal running conditions (heat or cool) the pressures are perfect. The air flow across both coils is good. The refrigerant has been weighed in.
The fault doesnt happen often enough for me to check the temps on the 4 way valve. But after all this theory it could also be sticking 4 way valves . But as both independant stages are tripping periodicallly it seems unlikely.?

It is an APAC unit and the manufacturers arent sure of the fault , but ive heard of simular complaints from people using these units at low ambients.

[Gary]

During normal running conditions (heat or cool) the pressures are perfect. The air flow across both coils is good. The refrigerant has been weighed in.

I would much prefer to see the numbers and draw my own conclusions. Possibly there is something in there that will reveal a hidden problem.