Liquid receiver on a reverse cycle VRV system

I'm in the process of building a water-condensed dual head ducted air-con system.

This is all theory in as much as I have all the parts, some of it brazed together and some of it partially installed.

I have a scroll compressor, which feeds a (quite large) brazed plate heat exchanger as a condenser. From there I go into a LLFD, a large liquid receiver and then a pumpdown solenoid before branching into a couple of EEV's. Because the lines are long, the evaps are reasonably big and the BPHX holds so little liquid, I figured I'd need the receiver to be able to pump down at the end of a cycle. II don't want to leave a lot of static pressure in the evaps/lines while cycled off, and I've always liked the idea of pumping down to obviate the requirement for a sump heater (never gets below about 5 degrees C here)

All my EEV's are rated for bi-di, and I've had thoughts about putting a reversing valve in.

My thinking goes along these lines :

Branch the liquid line around the liquid receiver. Put a check valve in line with the receiver so it only gets liquid in a cooling cycle, and put an additional EEV with opposing check valve across the receiver.

The theory being that at the end of the heat cycle, I can flip the reversing valve, close the liquid line solenoid and pump it down.

I've never dismantled a VRV heat pump before, so I'm coming from a perspective of barely knowing enough to be dangerous. I'm almost content to leave it as a cool only system (and I'm basically convinced it will work), but I've got this lovely reversing valve and 1/2" EEV here begging to be used.

The FCU's are pretty standard, so I figure if the whole thing is a failure I can just install a couple of small split compressor/condenser units and be done with it.

Questions, explanations, advice or smacks around the head are most welcome.

Shame nobody answered your post.

Do not pump down the receiver. This is time consuming, complicates control, two receivers are needed anyway and you would never be sure of the amount of oil left in the receiver.

Instead use 4 check valves in a wheatstone bridge fashion installing the receiver in the lines that never change direction and the liquid line in the other two.

Like shown here:
http://hvactutorial.wordpress.com/ai...iping-diagram/
¡Good luck!

i know this is off topic but im stuck. Ive got a question about an ac system im working on now but ive only just joined this site and i cant find where is can start a new thread from where do i do this?

I think first choosing the right forum and then pressing the button "+ Post New Thread" might do it!

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Originally Posted by aramis

Shame nobody answered your post.

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Well, I wouldn't say that. You did.

Thanks for the headsup. That looks quite a bit more complex than I think I need to get on the first pass, so I'll leave the reversing valve out and just try a cool only system for the first iteration. I've just scored a 15KW scroll compressor and an 8M2 plate heat exchanger to use as the condenser, so I'm making progress.

I recovered some EEV's from a scrapped Daikin unit a while ago, so I've just finished building a controller and brazing one of them into a little widget I can use to replace the fixed orifice in a 7KW heat pump I have here. I might see if I can make that all work before I start getting funky with non-return valves in a bridge, and brazing in a reversing valve.

I really appreciate the answer, thanks!

Actually, that Daikin diagram prompts another question.

The head units have thermistors on both ends of the coil. If the outlets of all the coils are into a common manifold, shouldn't the SST be the same on all coils, and therefore the liquid temperature post EXV be the same?

I can understand at the output of the coil, as it'd be needed to control the superheat, but what is the purpose of the sensor at the coil inlet?

Hi Brad.

2 thermistors for more accurate refrigeration control. A VRV system with different size coils at different heights and more/less bends in the pipework will mean the flow to all fan coils wont be the same.

Also the outdoor unit may record SST and SCT but if subcooling to fan coils is different then for a fixed EEV opening more or less rerigerant will be needed for the same amount of cooling. Some have a thermistor mid coil also for this reason.

Remember also that if you had a heat recovery system then the liquid coming out of one fan coil (that coil in heating) would eventually go to a fan coil in cooling. Who knows what the subcooling and head pressure would be due to altering loads. By the time the outdoor unit had measured pressures and indoor units adjusted accordingly refrigerant flow could have shot about all over the place.

Hope that helps.

Cheers,
Andy.

In heating the flow through the coil would be reversed so the outlet thermistor is now the inlet thermistor. Not sure of my facts here but maybe the outlet thermistor is used to maintain refrigerant flow in heating to that paticular fan coil.
More load on the fan coil would mean this thermistor would see a greater fall in temperature meaning more duty required from outdoor unit.

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Originally Posted by Tayters

Also the outdoor unit may record SST and SCT but if subcooling to fan coils is different then for a fixed EEV opening more or less rerigerant will be needed for the same amount of cooling. Some have a thermistor mid coil also for this reason.

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That makes sense.

All the bolt-on EXV controllers I've seen take the temp and pressure at the outlet of the coil (as per convention) to determine superheat. Could you reliably determine superheat by simply measuring both ends of the coil? (with suitable qualifications about limited application to unrestrictive coils with low pressure drops)

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Originally Posted by Tayters

Hope that helps.

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Heaps. Cheers for that Andy.

Good post tayters i understood all of it .

unlike that diagram I only under stand 70% of it .

Whats with the capillary and the solinod on the oil return ?

What do the strainers look like? similar to water ones?

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Originally Posted by Simeonx1

Whats with the capillary and the solinod on the oil return ?

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The oil separator is subject to both hot and cold air depending on the cycle the system is in. With hot air around it you open the solenoid valve so that the oil returns quickly to the compressor. With cold air around it will probably condense refrigerant so you pass it through the cap tube to expand it.

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Originally Posted by Simeonx1

What do the strainers look like? similar to water ones?

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Sorry, never opened one but I suppose it is a common mesh filter to protect the cap tube with finer mesh than water filters, depending on the cap size.

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Originally Posted by BradC

That makes sense.

All the bolt-on EXV controllers I've seen take the temp and pressure at the outlet of the coil (as per convention) to determine superheat. Could you reliably determine superheat by simply measuring both ends of the coil? (with suitable qualifications about limited application to unrestrictive coils with low pressure drops)
Heaps. Cheers for that Andy.

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Yes, in general for azeotropes (or near): you can measure superheat more reliably with 2 temperature sensors than temperature/pressure unless you compensate for the different time constants the transducers have.

Usually system pressure time constant + pressure transducer time constant are shorter than temperature TC, but you also have to consider the time it takes to look up saturation tables for the controller.

For non azeotropes you must calibrate/compensate.

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Originally Posted by BradC

(with suitable qualifications about limited application to unrestrictive coils with low pressure drops)

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All designs should keep pressure drop in the coils low for all types of refrigerants or you have to measure the coil’s pressure differential too, and compensate.

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You must follow the controller’s manufacturer manual as to where, how and number of sensors must be installed, for it's control logic success depends on it.

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Originally Posted by aramis

You must follow the controller’s manufacturer manual as to where, how and number of sensors must be installed, for it's control logic success depends on it.

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Which is all well and good if the controller *has* a manufacturer. :D
http://www.fnarfbargle.com/private/120213-gear/EEV.JPG

It doesn’t look like the ones I get, will ask the rep anyway.

You will have so much fun!

Yeah, it's a bit of a mongrel. The Fujikoki valve came from an old Daikin, the pressure sensor is a Carel ratiometric and the temp sensor is a little digital jobbie from fleabay. I figure that combined with some other random hardware makes a good lab to learn about superheat and evaporator pressure control algorithms.

I found a good spreadsheet floating around George Goble generated from Refprop and used that to extract the P/T tables.

Back to the original topic, the more I look at this, the more I wonder about the receiver.

I have the receiver as I want to be able to pump down the system each time it cycles off and I have two evaporators which contain varying volumes of refrigerant as the load varies during operation.

The condenser is a big PHX. I have configured the HX and reciever to sit on the same base. I had planned on pumping through the receiver, however I think I see an issue.

I'm condensing at below 30C (Calculated condensation temp is about 25C). Most of the time the system is in operation, the ambient air temp is going to be quite a bit above this. I asume the heat absorbed by the receiver from the ambient air is going to cause the heating of the gas/liquid it contains. As this will be at a higher pressure than the planned condensation temperature, I assume this is going to flood the condenser (prevent it draining) until the condenstation temperature gets close to the ambient.

I thought about two options to combat this.
a) Insulate the receiver, and actively subcool the liquid both entering and leaving (a couple of tube-in-tube subcoolers)
b) Bypass the receiver in normal operation.

I'm unsure if (a) will keep the receiver temp below Tc, and short of using three solenoids, I'm unsure of how I might achieve (b) given the varying volume of refrigerant in the two evaporators and the requirement to store it in the receiver during system operation.

All the system diagrams I've examined are air cooled, and therefore the temperature of the receiver is naturally below Tc.

Can anyone enlighten me?

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Originally Posted by BradC

I asume the heat absorbed by the receiver from the ambient air is going to cause the heating of the gas/liquid it contains…

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This assumption is valid only if the energy absorbed by the receiver (from the air) is greater than the energy you need to bring the subcooled liquid from your PHE back to saturation and then super heat it (only then you would produce the pressure differential).

This effect will depend on the refrigerant velocity and the air velocity around the receiver.

Common practice is to insulate not only the receiver but all of the liquid line when ambient temperature is most of the time well above condensing temperature.

Insulating the liquid line is a mistake when the opposite occurs.

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Originally Posted by BradC

a) ..., and actively subcool the liquid both entering and leaving (a couple of tube-in-tube subcoolers)

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You are using EEV with an orifice that should be dimensioned taking into account the average amount of subcooling your system has.

Having too much subcooling affects the stability of the control of the EEV if the minimum it can open already meets the demand of the evaporator due to excessive subcooling.

So if you cannot choose your orifice you should test how the system operates without/with insulation. Maybe preventing air currents around the receiver is enough or maybe a small amount of insulation around it is enough.

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Originally Posted by BradC

I asume the heat absorbed by the receiver from the ambient air is going to cause the heating of the gas/liquid it contains…

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b) Bypass the receiver in normal operation.
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The only reason to bypass the receiver is if it is indeed reducing the system subcooling and you want the subcooled refrigerant from the condenser go straight to the liquid line with neither the pressure losses nor temperature gains of a receiver.

In order to do this effectively you have to use a number of designs like the “Through Type” receiver, widely used with shell and tube condensers, you can read about it in Ashrae’s Fundamentals Chapter 2 under Receivers. In all of these designs you have to respect a certain height between the receiver, condenser and liquid line depending on pressure drops. A correct design is problematic and an empirical design, impossible.

If you simply bypass the receiver then your “normal” operation of the evaporator must be a constant load or you will not benefit from the ability of the receiver to store and deliver excess refrigerant to the evaporator. If this were the case I would save my EEVs for a more demanding application.

I´d stick to the first part of plan a, if more subcooling is then needed ok proceed to subcool more, if everything fails then go to plan b but get data for pressure drops first (these are small values and you need differential pressure gages).

Have fun!

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Originally Posted by aramis

Common practice is to insulate not only the receiver but all of the liquid line when ambient temperature is most of the time well above condensing temperature.

Insulating the liquid line is a mistake when the opposite occurs.

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This will almost always be the case.

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Originally Posted by aramis

You are using EEV with an orifice that should be dimensioned taking into account the average amount of subcooling your system has.

Having too much subcooling affects the stability of the control of the EEV if the minimum it can open already meets the demand of the evaporator due to excessive subcooling.

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I discovered this accidentally last weekend. The EEV I have is probably slightly undersized for the application at standard air cooled subcooling levels, however as I increased the superheat by closing the eev the subcooling also increased and my overall supply air temperature dropped.

I have to say I was absolutely astonished until I thought about what was actually happening.

I've actually revised my control algorithm to maximise the Td across the evaporator coil rather than maintain a steady superheat. As the outdoor unit has quite a large accumulator, I can sustain temporary control glitches that might ordinarily cause a bit of floodback. Overall this control strategy has dropped supply air temps by between 1 and 3C on average, and additionally reduced overall power consumption as it appears to be running with a slightly lower mass flow (and resulting lower condensing temperatures), but higher superheat/subcooling.

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Originally Posted by aramis

Have fun!

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I'm actually having a ball. I really appreciate the feedback. I'm way out of my depth, but you guys keep throwing me enough life rings to stay afloat.

I forgot to mention in my previous post that my Daikin supplier said he does not know your valve either.

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I discovered this accidentally last weekend. The EEV I have is probably slightly undersized for the application at standard air cooled subcooling levels, however as I increased the superheat by closing the eev the subcooling also increased and my overall supply air temperature dropped.

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For a slightly undersized valve you have two strategies to play with:

You mentioned this already. Subcooling affects the SYSTEM performance. You can get more capacity from your system by increasing subcooling. This is a good strategy but don’t exaggerate.

The EEV’s performance is affected by the applied pressure differential, so if you need more from your fixed EEV you need to increase the pressure differential. You can easily do so controlling your PHE flow. Of course you also have the opposite effect if your suction pressure lowers, that the compressor reduces capacity. So you must play with this balance until you get what you want or may need a bigger compressor.

Also don’t forget to check for high discharge temperatures, the dark side of increasing superheat.

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I have to say I was absolutely astonished until I thought about what was actually happening.

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This is called learning! Good for the brain and soul!

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I've actually revised my control algorithm to maximise the Td across the evaporator coil rather than maintain a steady superheat. As the outdoor unit has quite a large accumulator, I can sustain temporary control glitches that might ordinarily cause a bit of floodback.

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What differential are you talking about here?

It is most normal for superheat to vary, this only means that your EEV is working as it should: trying to keep superheat stable! Though there are a number of controllers designed for other purposes like keeping discharge temperature low but I haven’t seen those in ACs. Without manuals, only by trial and error you can discover where to place your sensors.

Accumulators must be big enough to capture most of the liquid refrigerant going to the compressor but small enough not to trap oil. So be careful at low demand that oil gets back to the compressor.

Cheers