Liquid receiver

[Renato RR]

What i need to consider to size liquid receiver.

Renato

[Lc_shi]

How much liquid will you need at lowest capacity of system operation ? Full capacity minus lowest capacity liquid is needed to go into liquid receivermy understanding,only for ref

rgds
LC

[guapo]

Hi,

I think the size of liquid reciever can charge all of refrigerant in the system.

Regards,
Guapo

[yhruhteb]

Hi,

I think the size of liquid reciever can charge all of refrigerant in the system.

Regards,
Guapo

DEPENDS ON THE CAPACITY OF THE UNIT

[mcgru-]

DEPENDS ON THE CAPACITY OF THE UNIT

As usual, it is not neccessary to count seconds when the receiver will be emptied due to high temporary loads.
As usual, the differences in mass flows along the refrigerating contour are no so large to take care about it. So, larger load - larger suction - larger discharge into condenser - more condensate - larger income into receiver.
For example.
Duration of refrigerant's voyage is about 30-60 sec for usual systems - 15m discharge liquid line, 0.5m/s speed. Load 300 g/s (45kW of R22) gives the value for liquid volume in receiver of 18kg. Density is about 1.2kg/dm3, so the volume of receiver may be near 18/1.2*1.25 = 19 liters.
Having 45kW basic evaporator load at Tev=+0C, Tcond=+45C, R22, we may be interested in condensing capacity (starting conditions) of 75kW. the Guentner's GVH-067B/2-L(W) may satisfy us. It's inner tube's volume is 33 liters. Half of them - 16 liters. 10m of 22mm tube from condenser to receiver and farther 15m to evaporator has the volume of 6-7 liters. Sum of them, multiplicated on coef 1.1-1.25, is 24-29 liters. It is larger than 19 liters, so the dependance on the capacity will be not so sensitive...

[NoNickName]

Or, in case you would like to recover all or most of the liquid into the receiver, calculate it for full charge.

[Renato RR]

It is Copeland logic to recover all refrigerant in receive.Well can go wrong with this.

Thanks all,
Renato

[NoNickName]

What logic? Can you please elaborate? What can go wrong?

[Renato RR]

Nothing can go Wrong.In Copeland V volume write the same.But 300l receiver it is big mother ****er.And it isnt cheep.
The trouble is that I can provide such big receiver in that short time so I need to make paralel conection of 2 receiver each of 90l.That is total 180l.

I think thet will be enough.The manufacturer of chiller advice me 50l but I am not agree with his calculation.

Renato

[NoNickName]

300 liters receiver? What a huge PV!
Is that designed under PED certification?
What compressor are you planning to use?

[Renato RR]

Bitzer CSH8571-110Y R134A 400/3/50 X 2.

Renato

[US Iceman]

If the system charge is 300 liters, the receiver needs to be about 125% larger so that you have some vapor space on top of the liquid.

One question; how did you get from 180 liters to 300 liters for the receiver volume? That is quite an increase!

[Renato RR]

1)I have 2 compressors in engine room.
2)Discharge pipe first go to water condenser (sanitary water)
3)Then pipe go 65m in lenght to the air condenser outside the building.(rise 4m)
4)Pipes are from water condenser to air condenser "discharge"65 mm and back "liquid" 42 mm.
5)density of R 134a is for liquid 1160kg/m3 and for wapor 300kg/m3.
6)In situation when water condenser live refrigerant in liquid the 65mm would be full of 1160kg/m3,and when vapor leaves 300kg/m3.
7)Ask for more?

Renato

[Renato RR]

It is water chiller.

Renato

[US Iceman]

OK Guys, let's start over.

What i need to consider to size liquid receiver?

First you need to know the total volume of refrigerant in the system if you want to do a pump down for 100% of the system charge.

You need to know the condenser charge: this depends on the head pressure control method. A flooded condenser coil will hold a lot of liquid during the winter.

Next you need to calculate the volume of the refrigerant piping, evaporators and accessories (driers, etc).

Use the liquid and vapor density at each component's operating condition to find the mass.

Total all of the mass calculations and you have the total refrigerant charge.

The receiver is normally sized to provide a volume equal to approximately 125% of the total system charge.

If you have some components that can trap liquid (maybe the water-cooled condenser), the amount of refrigerant calculated maybe too small.

[Renato RR]

Thanks Ice.

Renato

[Andy]

Ok all

What internal volume is the evaporator and will it need to be pumped out to service.
I work all out in litres, say the coil has an internal volume of 300litres and it is DX, which would mean a liquid content of 200 litres, then calculate the liquid line volume, won't be much in a chiller add it together and you have the net litres required. The gross volume will be
300 divided by 6 and multiplied by 7 350 litres gross.

Hope this helps.

Generally chiller receivers are sized at 50% of the above, due to the fact that they usually don't pumpdown, if they do the first figure is the one to go for.

Kind Regards. Andy

[TopMechanic]

Yeah, you gotta figger that 80% of the receiver can hold 100% of the refrigerant in the system.

[NoNickName]

That compressor would require a total charge (with a DX evaporator and condensing coil) of approx 80 to 110Kg.
I would suggest a liquid receiver around the internal volume of the coil, for a flooded condensing coil control.
Otherwise, for a fan speed controlled condensation, I would suggest a liquid reicever of the volume of the flooded circuits (if used for subcooling), let's say 15-30% of the volume of condensing coil.
If EEV is required or used, reduce the volume further by 30%.

[NoNickName]

Well, thanks for pointing you were designing a split chiller. This is news. If you think there's anything else you would want to write in order to help us to help you, please feel free. Otherwise we could only be guessing.

You can use the 65m pipe run as liquid receiver and reclaim it from the bottom in case of recovery need. Much better than filling the plant with unnecessary volumes.

[Renato RR]

We buy chiller from Italian suplier and we only mount it.But from the start i have bad filling with this.

Soory if i give insufisent information.But I want to heard from you basic design prenciples and then start forvard.Also if I write the novell some of you will not read.

Renato

[NoNickName]

We buy chiller from Italian suplier and we only mount it.But from the start i have bad filling with this.

Soory if i give insufisent information.But I want to heard from you basic design prenciples and then start forvard.Also if I write the novell some of you will not read.

Renato

Oh, then I'll stop helping , until you buy the chillers from us.

[Renato RR]

There is time for ewriting.Now is time for helping.

Renato

[Peter_1]

Oh, then I'll stop helping , until you buy the chillers from us.

NoNickName, he wants a chiller which gives no problems

[NoNickName]

NoNickName, he wants a chiller which gives no problems

Well, charging 300Kg + of refrigerant is not the way to go for "no problems".

[NoNickName]

Flooding the condenser in winter results in high subcooling, which in turn results in low evaporating temperatures and subsequent low compressor performance. Much better Andy's solution: a chopper or frequency converter on fans.

[US Iceman]

...high subcooling, which in turn results in low evaporating temperatures and subsequent low compressor performance

I'm not sure how you equate high subcooling values with loss of compressor performance. The lower pressure ratio due to the decreased discharge pressure and the higher net refrigerating effect provide a substantial boost in compressor performance.

I can see the evaporating temperature dropping a bit, since there is not as much flash gas. But then again, if the compressors have capacity control, this would not be an issue. The compressor would unload and the evaporating pressure would be constant.

I'm not promoting flooded condensers and discharge pressures, but if you are operating an air cooled refrigeration system in the winter with TXV's you have to do something besides VFD's.

No argument about the need to reduce the system charge.

[NoNickName]

No, it is not a matter of flash gas... suppose the liquid refrigerant is 40°C in summer, and it is 20°C in winter.
What would you think the temperature after evaporation would be in the two cases?

It would be lower in the second case, that would results in lower evaporation pressure and lower suction pressure, and lower compressor performance.

I personally do not like to flooded condensers, though at standstill liquid migrates from receiver back to condenser. Air cooled chillers here are operated with just VFD, and we have installations in Russia and Finland.

[Andy]

Doctors differ patients die

As far as I can see you are both correct

The problem is actually TXV's not the head pressure, this do not operate correctly at low pressure drop.

Flooded condensers are not much used in the circles I travel in, perhaps it's just not cold enough in Ireland in the winter, it just about freezes.

Large amounts of subcooling increase system cycle efficiency, but cause liquid distribution problems.

We use EEV's (Danfoss) and float the heat normally.

where we are employing heat recovery we use the original condenser for subcooling, controlling the heat recovery condenser at 45 deg an dthe subcooled one at 30 deg c, increases the standard comercial type system C.O.P from 2.74 in the summer to 3.38 and only slightly reducing the C.O.P from 3.48 compared to floating head in the winter.

Kind Regards. Andy

[US Iceman]

suppose the liquid refrigerant is 40°C in summer, and it is 20°C in winter.
What would you think the temperature after evaporation would be in the two cases?

OK, let's take a look at this.

If the mass flow at the evaporating temperature using40°C liquid refrigerant is 1 kg/min (just an example number), the 1 kg coming out of the evaporator (at the evaporating pressure) is comprised of the flash gas from 40°C to the evaporating pressure. The flash gas will be some percentage of the vapor volume leaving the evaporator. Let's say 15%.

The remainder of the mass flow (say 0.85 kg ; what's left after the flash gas) is the refrigerant that provides the refrigerating effect in the evaporator. This portion of the mass flow also provides some portion of the total vapor volume leaving the evaporator.

The total of the two above constitute the entire mass flow out of the evaporator. (0.85 kg + 0.15 kg/minute). The volume flow would then be 1 kg/minute X the vapor specific volume at the evaporating pressure.

When the liquid temperature is decreased, the amount of flash gas formed decreases. (let's say we are now down to 0.1 kg) We are assuming the evaporating pressure remains constant. (more on this later)

Since we now have 0.1 kg of flash gas per minute, the mass balance available is 0.9 kg/minute available for cooling.

The refrigeration effect is increased due to the increased enthalpy difference so we have more capacity available from the remaining 0.9 kg.

With this we now have 0.9 kg/minute (higher mass flow available that is useful) and the greater enthalpy difference. Therefore the compressor has greater capacity.

Since the volume of flash gas formed is reduced, and the compressor is a constant volume device (assuming it has no capacity control), you would have a decrease in suction pressure.

So I agree with you the suction pressure will decrease if the liquid temperature is lowered, if the compressor does not have capacity control.

If the compressor has capacity control, it should be controlling the suction pressure at a constant pressure.

The lower volume flow coming from the evaporator will balance with the decreased compressor swept volume that is active at approximately the same pressure.

The problem is actually TXV's not the head pressure, this do not operate correctly at low pressure drop.

I agree Andy. The TXV's are the reason for head pressure control in the first place.

Let me back up a minute and state the older systems I have worked on were before the balanced port TXV's were available for refrigeration. With the older TXV's you had to have good head pressure control to feed liquid on very cold days (say below 0°C). Down to this point fan cycling or fan speed control worked just fine.

However, when the air temperature got down to about -28°C, it was a different problem. In this case with the old valves you needed the flooded condenser controls to keep the system running.

With the new balanced port valves I suspect fan speed control may work just fine.

[NoNickName]

So I agree with you the suction pressure will decrease if the liquid temperature is lowered, if the compressor does not have capacity control.

Good, we came at an agreement.
Now, suppose the compressor has capacity control. In this case the COP will be increased by the lower mass flow, but it will be decreased by the disproportion of the motor size to the actual mass flow.
This will also negatively affect the power factor of the plant because

Factors affecting the power factor of an induction motor are size, speed and load. The larger the
motor and faster its speed, the higher the full-load
power factor. The power factor of a motor varies
according to its load. The higher the percentage of
the rated load, the higher the power factor.

[US Iceman]

So you agree the flash gas does have something to do with it then???

Since we are in agreement?

In this case the COP will be increased by the lower mass flow

Only if you consider the use of all of the heat transfer surfaces and the re-balance that occurs. Since less mass flow is circulating at part load, the suction pressure can be increased slightly and the condensing temperature reduced a little.

The refrigeration system itself can become more efficient, but when you consider the motor load and power factor with this, the amount of energy used per kg circulated can increase.

This final effect of this is also determined by the compressor type and if a VFD is used to provide speed regulation. (plus the drive losses too)

[Peter_1]

No, it is not a matter of flash gas... suppose the liquid refrigerant is 40°C in summer, and it is 20°C in winter.
What would you think the temperature after evaporation would be in the two cases?

It would be lower in the second case, that would results in lower evaporation pressure and lower suction pressure, and lower compressor performance.

Haven't read thoroughly the whole thread:
Liquid 40°C in summer and 20°C in winter will give a lower evaporationg temperature in winter conditions at the same condensing pressure??
It will increase the net effect on the evaporator a lot because teh enthaply of the liquid has increased seriously.
The COP of the compressor will increase with +/-25 to 30% and you're talking about a lower performance?

The power factor of a motor varies
according to its load. The higher the percentage of
the rated load, the higher the power factor

This is true but it may not be exagerated to much.
What is then the advantage of the digital scroll?
And form the unloader valves on larger machines and so many small semi-hermetic machines?

The net effect you gain by increasing the suction pressure by unloading is by far much higher then the negative effects of increase of the power factor.

We measured this some 15 years ago on a open 8CC compressor of DWM/Copeland and there was as far as I can remember almost no measurable difference.

And you pay - at least in Belgium - only the power you consume for smalelr applications, not the apparent power.
Line current will be a little bit higher but that's all. So if power factor rises, you could say, who cares!?

I agree on the conclusions, and on the fact that higher subcooling does not equates in better overall performance

I'm almost sure this is a wrong statement. Will try to simulate this the next days.

[NoNickName]

I agree on the conclusions, and on the fact that higher subcooling does not equates in better overall performance

[NoNickName]

Guys, I'm not talking about a theoretical refrigeration circuit. I'm talking about reality. Suppose you've got a 100kW evaporator (you choose the design temperature, I don't care, it's just for example).
Now, the liquid in the TEV will decrease its temperature, with the net result of a lower evaporating temperature. Yes, you've got a higher performance for that amount of liquid, and the evaporator will benefit, but it will give still 100kW at the design temperature... or more but at a lower Te.
Now, suppose you've got more kW to evaporate as a net result of a higher subcooling. The evaporator will react by lowering its average Te. The TEV is a slave of the evaporator and will react accordingly to keep the set SH.
You see there's no gain whatsoever from a high subcooling IN A REAL CASE.
If by chance the evaporator was selected with a safety margin, than you'll probably have a benefit, but who really installs evaporators that generous?
Who cares what the enthalpy gain is, when one cannot take advantage of it. Worst of all: too much enthalpy gain will cause a relative undersizing of the evaporator.

On another basis: why few to no manufacturers are using subcooling circuits in condensing coils?

It sounds strange to me that in Belgium you pay for active power. I think the utility company let you pay for kWh+kvarh and not just kWh, like in most parts of the world. And kvarh are more expensive than kWh.

EDIT: oh, BTW, I wanted to buy digital scrolls from Copeland. They told me they are not on sell.

EDIT2: as you see, I didn't mention flash gas, because I never ever want to see flash gas in any working conditions. And, my experience teach me that the smaller liquid receiver, the better, eventually no liquid receiver is best, when combined with a good fan speed regulator.

[US Iceman]

On another basis: why few to no manufacturers are using subcooling circuits in condensing coils?

They cost extra money and no one will buy then!

[US Iceman]

Worst of all: too much enthalpy gain will cause a relative undersizing of the evaporator.

This might be true if the liquid temperature is lower than the evaporator temperature. Then the liquid entering the evaporator must first be warmed up before it will boil.

You loose evaporator capacity because the evaporator must warm up the liquid, which takes the place of the phase change surface area and delays the superheating of the vapor.

There is such a thing as too much subcooling. One I mentioned above. The other is if the TXV capacity is not adjusted for the increase in capacity due to the lower enthalpy.

[NoNickName]

This might be true if the liquid temperature is lower than the evaporator temperature. Then the liquid entering the evaporator must first be warmed up before it will boil.

I'm not expert of flooded evaporators, on the other hand liquid cannot enter the evaporator at all in a DX operation.

There is such a thing as too much subcooling. One I mentioned above. The other is if the TXV capacity is not adjusted for the increase in capacity due to the lower enthalpy.

How can TXV capacity be adjusted on the fly between summer and winter?

[US Iceman]

...on the other hand liquid cannot enter the evaporator at all in a DX operation

Sure it does. There is about 85% of the entering liquid which is atomized into droplets after the TXV which enters the evaporator coil. This is where the evaporator capacity comes from in the first place. The balance (+/-15%) is the flash gas created during the expansion process, if the liquid temperature is warmer than the evaporating temperature.

If the liquid temperature entering the evaporator is lower than the evaporating temperature, the liquid will not boil.

How can TXV capacity be adjusted on the fly between summer and winter?

You don't. And I'm not suggesting you adjust the TXV's from summer to winter.

The valves have to be selected for the amount of subcooling that may be found. A selection that meets both operating criteria.

In a DX system designed for subcooling (which is a good thing to prevent or reduce the flash gas in the liquid lines) the valve capacity must be corrected for the amount amount of subcooling that will be seen.

The subcooling increases the valve capacity.

[Peter_1]

This might be true if the liquid temperature is lower than the evaporator temperature. Then the liquid entering the evaporator must first be warmed up before it will boil.

You loose evaporator capacity because the evaporator must warm up the liquid, which takes the place of the phase change surface area and delays the superheating of the vapor.

There is such a thing as too much subcooling. One I mentioned above. The other is if the TXV capacity is not adjusted for the increase in capacity due to the lower enthalpy.

Mike,

What about pump systems where the coil/evaporator is fed with pure liquid. Liquid arives at -10°C for a cooling room, so it is injected or fed in the coil at a temperature +/-10 K lower then the room temperature.
It must be warmed up first before it starts to boil.

What is the difference?

The liquid has also a better thermal contact with the copper tubes then a gas/liquid mixture.
The more surface you can cover with low temperature liquid, the larger the capacity will be.

And with a TEV, the portion of the liquid that is transformed to flashgas while decreasing the pressure is +/- 30%. The energy to transform this change is taken from the liquid itself.
The more we feed liquid at evaporating temperature, the less flashgas will form behind the TEV, the more nergy remains in the liquid for a given mass flow, the more surface there will be covered with liquid.

So, I still remain sceptic with the statement that subcooling not allways increases evaporator capacity.

[Peter_1]

It sounds strange to me that in Belgium you pay for active power. I think the utility company let you pay for kWh+kvarh and not just kWh, like in most parts of the world. And kvarh are more expensive than kWh.

Only the big consumers have a kvarh meter.
I don't have one in my shop, let's say that you don't have one till a fuse of 63 A , sometimes 80 A on a 3phase 380 V+N net.
We normally have a kWh meter (which is active power), not a kVArh meter, the reative power.
But, in the transformer cabin is a central kvarh meter which counts this lost power on the main supply. If someone on this transformer is losing too much power via reactive power, then they will try to localize it.

I never saw a reactive power meter - even in Italy - in a butcher, bakery, a small garage..

The big machines are installed in factories but they have there own transformers, at least in Belgium. In these transformer cabins, bot reactive and active power is measured and you see cos-phi capacitors.
But they pay almost 1/2 of the normal tariff/kWh, so it's worth then to measure the kVAr and they counter measurements.

[NoNickName]

I never saw a reactive power meter - even in Italy - in a butcher, bakery, a small garage..

This is off topic, but nowadays also houses in Italy have got a kvarhmeter, though the utility does not let you pay for kvarhs until a certain power installed, and it can well be as much as 63 or 80A, which is BTW the current drawn by a small water chiller (small by our standards).

[Peter_1]

Small to your standards???
That's a chiller of +/- 130 kW and fits perfectly in Montair's higher capacity products.

[US Iceman]

You need to warm up the liquid before it begins to boil, before we reach the beginning of the phase change......OK, I'm with you......or in other words, heat has to be absorbed from the space through the tubes and added to the subcooled liquid to reach the boiling point.

That is right.

There's no necessity that we reach the boiling point.

That is true if you have designed the evaporators for sensible heat transfer, like a chilled water coil.

The capacity of the refrigerant evaporator is based on a phase change. If the refrigerant does not boil, the evaporator capacity will be less than it was designed for. It still does some cooling since the cold liquid will absorb heat. However, if the refrigerant does not boil, it will not provide the design cooling capacity.

Suppose a liquid overfeed system - theoretical coil - and we subcool the liquid that much that it enters a very small coil at a high speed where there can't be added enough heat to even reach the boiling point, this coil will cool and it will work with at it's highest efficiency.

What I am trying to describe is not a theoretical system operation, but a practical one. I agree with you on the theory. It is possible to do what you describe with refrigerants. In this example, the evaporator is working like a chilled water coil. No phase change. However, for a liquid overfeed system this is not the most efficient.

The evaporators are designed to have cold liquid at the evaporating temperature entering the coils. As soon as the refrigerant begins to boil the highest heat transfer occurs at this time.

On a liquid overfeed evaporator the liquid refrigerant is pumped into the coil with additional liquid to provide a completely wet coil surface. Just as you described earlier.

Not all of the liquid will evaporate. Some returns back to the pump receiver. The extra liquid and the phase change result in the highest efficiency of the evaporator coil.

If you compare a DX evaporator of a specific capacity, to the same physical coil using liquid overfeed you will see the coil using liquid overfeed will have about 10-15% more capacity.

The percentage increase is due to the coil surface being wet with boiling refrigerant, since it is not being used to superheat the refrigerant vapor.

If the liquid entering the evaporator is subcooled, it does provide some useful cooling since the liquid is absorbing heat from the space.

However, since the evaporator capacity is designed for a phase change, it may not reach it's full capacity if the evaporator has to warm liquid. If the liquid does not evaporate there is no vapor formed. Then you would not need a compressor.

DX evaporators and liquid overfeed evaporators are similar in construction and purpose. Both boil liquid refrigerant.

A DX coil adds superheat by using a TEV to protect the compressor. 100% vapor leaves the coil.

A liquid overfeed coil has zero superheat for higher efficiency. 25-30% of the refrigerant leaving the evaporator is vapor, the rest of the refrigerant (70-75%) is liquid (on a mass basis).

Does that help to explain it?

[Lc_shi]

It's very good discussion
Many basic concepts are coming out from fuzzy pictures.
Enough subcooling can prevent flashgas and then affect the valve capacity.
It should be said that: subcooling prevent valve normal capacity not decrease

really appreciate master's opinions!
Wish there's some day to come together to have face to face talk and argument

rgds
LC

[Renato RR]

Yes,subcooling increase valve capacity.But how can eneywone predict the amount of subcooling for all year condition.During summer subcoling can be 5 K but in the winter 25 K.
I have to reed some Sporlan bulletin to continue, scuse me.Untill then keep this discusion live.

Renato

[wambat]

Dossat/Horan say, When liquid refrigerant is subcooled before entering the metering device, the refrigeration effect per unit mass is increased. The example given is the heat removed (6.96btu/lbm). for the saturated cycle, in the example, the refrigeration effect per unit mass is 60.38btu/lbmas shown in a proof11.11 in section 11.6. the proof states for the sub cooled cycle, the refrigeration effect per unit mass, is 67.34btu/lb, a difference of 11.53% This is for actual refrigeration cycle
This in turn reduces mass flow rate by 10.27% and since the mass flow rate is less for the sub cooled cycle, it follows that the volume of vaporthat the compressor must handle per unit of capacity will also be less. Going on, the compressor displacement required for the sub cooled cycle is smaller then that required for the saturated cycle because the volume of vapor per unit of capacity is less. so the COP will increase by 11.5% in the example.

[NoNickName]

... so the COP will increase by 11.5% in the example.

This is the example of how books should not be read.
In real life we already have a compressor, a TEV and an evaporator, the three chosen for a given SC.
In case SC is increased, there will be no COP increase and no different mass flow, because the components are still the same.
The net enthalpy gain will become lower evap. pressure, which in turn becomes as a lower compressor suction pressure, and a lower COP.

If the manufacturer declares data with 3°K SC, please stay with that. Any increase in SC will result in poorer performance for a given chiller.

[Peter_1]

....Dossat/Horan say, ..... so the COP will increase by 11.5% in the example.

That's what I thouht and was trying to explain with no so many words.

[wambat]

Gee when you write your book ...please let us know, I can always use a good laugh