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Thread: Liquid receiver
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14-04-2006, 10:23 AM #51
Re: Liquid receiver
Yes if evaporating temp. stais at -10C.But i think (maybe wrong dont atack me emediatly) that increase in subcooling would change evaporating temperature.Then new rules are in game.
Renato
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14-04-2006, 10:36 AM #52
Re: Liquid receiver
And increase in subcooling will decrease the evap temp, which will further decrease the TEV performance, all other data the same. Anyway a TEV must be selected for dp, not just for Te.
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14-04-2006, 11:48 AM #53
Re: Liquid receiver
Originally Posted by wambatIt's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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14-04-2006, 12:33 PM #54
Re: Liquid receiver
Originally Posted by NoNickName
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14-04-2006, 01:10 PM #55
Re: Liquid receiver
The reason for this correction factor is just because the TEV is sized for 'normal' liquid temperatures.
When this liquid is subcooled, it will overfeed the evaporator because on that moment, the valve will be too big and hunting will occur.
The correction factor has nothing to do with a reduced evaporator capacity but with an increased valve capacity.It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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14-04-2006, 01:12 PM #56
Re: Liquid receiver
Originally Posted by NoNickNameIt's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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14-04-2006, 01:16 PM #57
Re: Liquid receiver
Originally Posted by Peter_1
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14-04-2006, 01:19 PM #58
Re: Liquid receiver
Originally Posted by Peter_1
... roll of drums....
lower evaporating pressure and lower performance.
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14-04-2006, 04:08 PM #59
Re: Liquid receiver
Originally Posted by Peter
In these systems the liquid refrigerant in the pump receiver is at the saturation temperature of the lowest pressure.
An example:
We have a pumped overfeed system with three temperature requirements; -12.2C (10F), -6.6C (20F), and 0C (32F).
All evaporators are connected to the suction line going to the pump receiver operating at -12.2C. The higher temperature evaporators (-6.6C & 0C) are controlled by back-pressure regulators.
Several evaporators are piped directly into this suction line; no back-pressure regulators. Their evaporating temperature will be -12.2C with no allowance for pressure losses.
The other evaporators use back-pressure regulators to control the saturation temperature/evaporating temperature at -6.6C, and 0C.
The refrigerant pumps are supplying liquid into all of these evaporators at -12.2C.
On the -12.2C evaporators all is well, since the liquid temperature is -12.2C and the evaporating temperature is -12.2C, so any heat added to the refrigerant (by the evaporator) causes the liquid to boil.
The evaporators operating at -6.6C and at 0C also have liquid entering the coil at -12.2C. The liquid is colder than the evaporating temperature, so it does not boil. It just flows through the evaporator picking up sensible heat at the evaporating pressure.
Since the evaporator pressure is higher (due to the back-pressure regulator), the liquid remains subcooled below the saturation temperature. When the liquid has warmed up to the saturation temperature of the evaporator pressure, it begins to boil.
When the evaporator has to warm the liquid up for it to boil, the evaporator has lost some capacity due to the sensible heat transfer, rather than a phase change.
Same thing can happen in a DX system. If the liquid refrigerant is colder than the evaporating temperature, the liquid will not boil in the evaporator.
The main concern with all of this is: What is the evaporating temperature and what is the liquid refrigerant temperature?
Does that help explain my earlier comments?
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14-04-2006, 04:21 PM #60
Re: Liquid receiver
Originally Posted by Peter
The more we feed liquid at evaporating temperature, the less flash gas will form behind the TEV, the more energy remains in the liquid for a given mass flow, the more surface there will be covered with liquid.
The total mass flow has not changed we are just using the mass flow in circulation more efficiently.
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14-04-2006, 04:44 PM #61
Re: Liquid receiver
If I try to translate and understand the Danfoss data posted, the example says the evaporator capacity is 40 kW / 11.4 Tons.
Using the same example for R-22, the correction factor for 4K of subcooling is 1.0. If the subcooling is no greater than 4K, no correction to the valve capacity is required.
Further, the example uses a 15K value for subcooling that will be provided by something. It does not say where the subcooling comes from, only that 15k of subcooling will be available.
The correction factor for this is 1.11.
Using the example evaporator capacity of 40 kW, the valve capacity required with 15k of subcooling is now, 40 kW divided by 1.11 = 36 kW.
Th evaporator capacity is not reduced 11%. The valve capacity has increased 11% due to the additional subcooling.
What this example is stating is the TEV valve capacity has increased 11% with 15k of subcooling instead of the same valve capacity with 4k of subcooling.
Therefore you select a TEV with a capacity of 36 kW. When the 15k of subcooling is included, the valve will have sufficient capacity to operate a 40 kW evaporator.
Sorry my friend, you are reading the information incorrectly.
EDIT+++
Originally Posted by PeterLast edited by US Iceman; 14-04-2006 at 04:50 PM. Reason: added text & quote
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14-04-2006, 04:56 PM #62
Re: Liquid receiver
Originally Posted by Renato RR
You have to base it on the worse point of operation, which means the lower subcooling value. This provides sufficient valve capacity during the summer when the lower subcooling is available.
If the subcooling increases, the valve capacity also increases.
The increased subcooling in the winter is also another reason the refrigeration systems do not run as long during the winter. The systems are more efficient due to the lower liquid enthalpy entering the TEV's.Last edited by US Iceman; 14-04-2006 at 05:05 PM. Reason: fixed quote
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14-04-2006, 05:05 PM #63
Re: Liquid receiver
Originally Posted by wambat
If the subcooling is increased after the system (which was designed for little or no subcooling) has been installed, the system run time will be greatly reduced due to the increase in capacity from the additional subcooling.
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14-04-2006, 05:12 PM #64
Re: Liquid receiver
Well, I won't insist any longer, everyone is convinced of its own opinions. But the arguments on this thread would be useful to me when training our engineers. I didn't think that so many engineers would be convinced that overcharging a chiller is the right thing to do in order to increase COP.
I will include the caveats in the installation manuals.
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14-04-2006, 05:14 PM #65
Re: Liquid receiver
You should consider the ratio of liquid and vapour.Ratio should be 80:20. 80 percent liquid and 20 percent vapour.
Regards
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14-04-2006, 05:42 PM #66
Re: Liquid receiver
Originally Posted by NoNickName
Obviously we must be loosing something in translation.
If the system is designed to use a large amount of subcooling you can take advantage of the benefits with less compressor displacement and lower required mass flows. The evaporating temperature will not decrease, since it is fixed by the compressor displacement and evaporator surface area.
If a system is designed for a very small amount of subcooling, and later a lot more subcooling is provided (by whatever means) after the system has been designed and installed, it is possible some interesting things could occur. This I believe is your point if I am not mistaken.
It did provide for a very good discussion though.
Best Regards,
US Iceman
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14-04-2006, 05:49 PM #67
Re: Liquid receiver
Originally Posted by US Iceman
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14-04-2006, 06:02 PM #68
Re: Liquid receiver
Convinced or not, that's not the point.
I asked the opinion of Helpman and Goedhart and Danfoss. As soon as they gave me an answer, I will post their view on this issue.
Sometimes, things are difficult to explain, especially in another language, I mostly feel this on my elbow.
I will try to explain it in another way, I hope you can follow me with this one: if you inject refrigerant in a coil at -10°C or you feed glycol of -10°C in a coil, the cooling capacity of the glycol coil will be a lot higher and there is even no phase change with the glycol, only a very good contact of the glycol with the inner walls of the copper.
The reason why the refrigerant has to boil completely to vapor in an evaporator is that the compressor can't handle liquid.
I personally think that pure thermodynamically seen, it's better that the refrigerant remains as long as possible at its coldest temperature.
I'm not saying my view is the right one, just throwing some suggestions in the ring again.It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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14-04-2006, 06:04 PM #69
Re: Liquid receiver
Originally Posted by AMARNATH JHA
You can calculate this but it's determined by evaporating and liquid pressure and liquid temperature.
Not excatly 80/20...theoretically, it can be 100/0.It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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14-04-2006, 06:46 PM #70
Re: Liquid receiver
Hi Peter,
This is discussion is developing into a long one.
... you feed glycol of -10°C in a coil, the cooling capacity of the glycol coil will be a lot higher and there is even no phase change with the glycol, only a very good contact of the glycol with the inner walls of the copper.
The glycol heat transfer is dependent on the glycol flow rate and temperature differentials.
On a refrigerant evaporator, the capacity depends on the mass flow and enthalpy difference between liquid and vapor. This one is a phase change, where the glycol is a sensible heat transfer process.
I think we can agree on those points.
The tricky part with refrigerant feeding into an evaporator is the condition of the refrigerant; subcooled or saturated.
Plot this on a PH diagram. (example attached)
If the liquid exists at a pressure equal to the saturation temperature of 0C for example. If you subcool the liquid more, the line at 0C is extended to the left. If the liquid is cold enough when it flows through the TEV, the pressure drops straight down on the PH diagram.
If the liquid temperature is colder than the evaporating temperature the liquid will not begin to boil until it has warmed up to the saturation line (bubble point) of the refrigerant at that pressure.
In other words, the liquid is subcooled below the saturation temperature of the evaporator pressure.
On a liquid overfeed system using back-pressure regulators it is the same. If we pump liquid refrigerant from -10C up to a pressure equal to 0C, when the liquid enters the hand expansion valve the liquid temperature is still below the evaporating temperature and will not boil. The liquid is still in the subcooled region of the PH diagram.
After the liquid begins to warm up to the saturation temperature of the evaporator, the liquid begins to immediately boil. So you do have increased heat transfer with more useful liquid, and less flash gas.
The problem is we have to use part of the evaporator surface to warm the liquid up. This may not be very much, but it does happen.
Most evaporator manufacturers will say the liquid temperature must not be lower than the evaporating temperature. This is the reason why.
Liquid temperature above the evaporating temperature are OK.
The reason why the refrigerant has to boil completely to vapor in an evaporator is that the compressor can't handle liquid.
Does that help explain my comments?
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14-04-2006, 07:56 PM #71
Re: Liquid receiver
Originally Posted by Peter_1
A given evaporator will transfer a certain amount of kJ/s at a design condition. Increasing the enthalpy in kJ/Kg, given a fixed kJ heat transfer capacity, the mass flow in Kg/s must reduce to keep things balanced.
In order to achieve this result, the TEV must throttle back, with a net result of a lower evaporating pressure (same surface, less mass flow) for the same SH.
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14-04-2006, 09:11 PM #72
Re: Liquid receiver
Originally Posted by US Iceman
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14-04-2006, 09:40 PM #73
Re: Liquid receiver
hi wambat,
I am not disputing any of the information you provided. In fact I agree with the logic behind the statements.
This thread originally started off as a simple question of how to size receivers and took a turn somewhere about the real and perceived effects of subcooling and the effect on the system.
Now it's taken another direction about refrigeration thermodynamics.
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14-04-2006, 10:59 PM #74
Re: Liquid receiver
Well, these discussions forces you think a little bit further then you normally do, then most techs do, and you soemtimes can learn a lot of it.
It also pushes you to go back to the basic and sometimes, you need to grab a book again.
Will read it tomorrow morning, it's to late now (midnight) and someone's waiting upstairsIt's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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15-04-2006, 12:03 AM #75
Re: Liquid receiver
Peter...Lucky! :>)
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15-04-2006, 07:57 AM #76
Re: Liquid receiver
Originally Posted by NoNickName
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.It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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15-04-2006, 08:03 AM #77
Re: Liquid receiver
Originally Posted by NoNickNameIt's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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15-04-2006, 08:26 AM #78
Re: Liquid receiver
Originally Posted by US Iceman
I agree wih you on lest's say everything, just the above extract out of your post needs some clarification for me...
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.
I see this as a positive thing, heat is been drawn away from the space...
There 's no necessity to reach the phase change (unless for the compressor of course in a common DX system)
Perhaps, that was what you were trying to explain say and something was lost due to the language barrier.
It became here of course more and more a hypothecial thesis.
There's no necessity that we reach the boiling point. 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.
And we haven't reached yet the phase change status.
If we feed that coil with liquid at evaporating temperature, then this coil will perform much worse then the first one.It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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15-04-2006, 11:21 AM #79
Re: Liquid receiver
Originally Posted by Peter_1
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15-04-2006, 11:49 AM #80
Re: Liquid receiver
Small to your standards???
That's a chiller of +/- 130 kW and fits perfectly in Montair's higher capacity products.It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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15-04-2006, 12:14 PM #81
Re: Liquid receiver
130kW cooling capacity is one of our smallest chillers, Peter.
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15-04-2006, 12:39 PM #82
Re: Liquid receiver
Can yo build those in a factory of only 6000 sqm?
My place is already 4000 sqm.It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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15-04-2006, 03:21 PM #83
Re: Liquid receiver
Originally Posted by Peter_1
There's no necessity that we reach the boiling point.
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.
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?
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15-04-2006, 04:33 PM #84
Re: Liquid receiver
US Iceman,
I will have to study a litlle bit on the liquid overfeed systems and the phase change designed coils.
I'm really impressed about the knowledge of you.
I think Wambat and Josip can add also something more regarding this issue.
What literature do you recommend?
It's not that I do'nt believe you but I want to read something more about all this.
Searched in Stoecker bu didn't found anything usefull.
Wambat, or do I have to say Sh..un pointed me to Dossat. I will try to make some time to read that specific chapter.It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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15-04-2006, 04:57 PM #85
Re: Liquid receiver
It's not that I don't believe you but I want to read something more about all this.
It is a lot of information to try to post and explain. There is another thread in the industrial section for liquid overfeed systems perhaps we should move over to that thread to keep it together??
I will look for some information also.
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15-04-2006, 05:04 PM #86
Re: Liquid receiver
This is perhaps one USIceman
http://www.wlv.com/products/databook...ta/db3ch15.pdf
Perhaps the Mod of this section can cut and paste them in that new sectionIt's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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24-04-2006, 02:35 AM #87
Re: Liquid receiver
surge receiver or through recriver
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24-04-2006, 02:36 AM #88
Re: Liquid receiver
surge receiver or through receiver
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24-04-2006, 07:04 AM #89
Re: Liquid receiver
Commisioned last Friday a Blue Box chiller with...of course...a Carel unit in it.
Very nice machine.
Peace of cake of courseIt's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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01-05-2006, 05:50 PM #90
Re: Liquid receiver
When the evaporator has to warm the liquid up for it to boil, there will be a difference between coefficients of heat transfer between the refrigerant and tube wall. In approximate numbers, the heat transfer between well-turbulented liquid and tube wall will be about 2000-3000 W/(m2.K). But in the case of boiling, this coefficient will be 2-3 times greater - 6000-8000 W/(m2.K). Buuut, when the refrigerant step into the liquid/gas ratio of about 30% and lesser (due to it's boiling), the effective coeficient of heat transfer becomes lesser - due to decreasing area of contact between (boiling) liquid and tube wall and thus increased contact area between gas and tube wall (and its greately lesser heat transfer coefficient)
alpha_liquid_tube ~ 2000-3000 W/(m2.K)
alpha_boilingliquid_tube ~ 8000 W/(m2.K)
alpha_gas_tube ~ 50..100 W/(m2.K)
Tube wall thickness is about 1mm, copper, the resistivity to transfer the heat is about 0.4 (m2.K)/W
Total heat transfer resistivity of tube wall itself and tube inner surface (contact with refrigerant) is a sum of h.t.resistivities. Let's look onto differences:
R1=0.4+1/8000=0.400125 (case of boiling refrigerant, 100% liquid)
R2=0.4+1/100=0.41 (case of 100% gas)
Difference between R1 and R2 is only about 3%.
We can see that most of resistance comes from tube wall (it's thickness and material), and that boiling process brings only a small effect to the heat transfer.
But also we should consider the hydrodynamics in the tube and the lesser specific heat capacity of gaseous refrigerant rather than liquified due to it's lesser density and ability to faster get outer temperature.
That's the explanation I was looking for and it makes sense: the difference in heat transfer coefficient between boiling and liquid state.
Is your conclusion that it doesn't make only a small negatiev difference in evaporator capacity if we feed seriously subcooled liquid?
May I ask you, where have you found this literature?
These figures were derived from tests in a laboratory environment I suppose?
Second question, where can we see/buy your work?
Wow, two posters (you and the other Chinese poster) in one day with the professional competence of programming and the needed thermodynamic knowledge to convert it in a software program.
As said with the other poster, I'm impressed.Last edited by Peter_1; 01-05-2006 at 08:16 PM.
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01-05-2006, 06:50 PM #91
Re: Liquid receiver
WOW. That was a great explanation mcgru-.
I would like to add; any improvement in the tube side heat transfer makes very little difference since the air film coefficient may the controlling factor. The refrigerant heat transfer (whether it is phase change or sensible) is almost always greater than the air film coefficient.
In effect, we can improve the refrigerant film coefficient using turbulators, spray nozzles or other mechanisms in the phase change or single phase regime. However, since the air film coefficient is so low to begin with any improvement in the overall U value due to the refrigerant heat transfer is negligible.
As you suggest, the wall resistance may be the easiest factor to deal with.
You are correct. I was not allowing for the wall resistance in my discussions. My thoughts were only based on:
alpha_liquid_tube ~ 2000-3000 W/(m2.K)
alpha_boiling liquid_tube ~ 8000 W/(m2.K)
a) processes inside the reciprocating compressor,
b) dynamic heat transfer between tube with refrigerant flow and environment
Welcome to the RE forum.
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01-05-2006, 08:19 PM #92
Re: Liquid receiver
Mcgru and USIceman and other readers, I made just a terrible mistake.
I edited this marvelous post instead of quoting it.
DAMN ME.
Can someone please restore my fault asap. with teh original post.
PLEASE!!!!!!!!!!!!!!!!!
1000 times SORRY for this.It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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01-05-2006, 08:44 PM #93
Re: Liquid receiver
Peter,
I'm not sure who might be able to do this except the original poster or WebRam.
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01-05-2006, 08:44 PM #94
Re: Liquid receiver
This was the post I should have been posted:
That's the explanation I was looking for and it makes sense: the difference in heat transfer coefficient between boiling and liquid state.
Is your conclusion that it doesn't make only a small negatiev difference in evaporator capacity if we feed seriously subcooled liquid?
May I ask you, where have you found this literature?
These figures were derived from tests in a laboratory environment I suppose?
Second question, where can we see/buy your work?
Wow, two posters (you and the other Chinese poster) in one day with the professional competence of programming and the needed thermodynamic knowledge to convert it in a software program.
As said with the other poster, I'm impressed.It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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01-05-2006, 08:46 PM #95
Re: Liquid receiver
Mike (or others), don't you have the original post in your private email box as send to you automatically when someone post a reply on a thread you subscribed?
What a stupid mistake.
I will contact Webram immediately now.It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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01-05-2006, 08:50 PM #96
Re: Liquid receiver
Peter,
I changed my email notification. I was getting so many emails every day I changed the setting to NO EMAILS since I visit the site every day.
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02-05-2006, 06:35 AM #97
Re: Liquid receiver
Originally Posted by Peter_1
It's ok in quoted state.
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02-05-2006, 06:50 AM #98
Re: Liquid receiver
McGru,
If there's one thing you shouldn't care about, then it is how you write English here on RE.
Some have difficulties with my English but who cares..
If you use IExplore, then you can instal a free, goodworking English spell checker for it at http://www.iespell.com/
I bet you speak better Russia and other languages then I speak English, so...
The purpose of this site is providing information to techs all over the world with the same interest, let's say for some... the same passion.
What about the possibility we can see your work? I'm very interested in it and others too.
Do you mind if I try to restore your original post?
Kind regards.It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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02-05-2006, 07:44 AM #99
Re: Liquid receiver
Originally Posted by Peter_1
So, i think that there are a lot of better books, but for my _estimative_ calculations in many cases the Polmann-handbook is enough.
Complicated hydro-dynamic simulations are carried out using very complicated software. It's not in my force to create such one by myself only in a foreseeable future. So my way is an estimation now. Sorry
OK, the source of my estimative data on boiled liquid heat transfer coefficients is....
1. Turbulent flow of water in a tube
1.a (by Sender and Merkel)
h_ci = 2040*(1+0.015*t)*(w^0.87)/(d^0.13) [W/(m2.K)]
1.b (by Schack)
h_ci = 3370*(1+0.014*t)*(w^0.85) [W/(m2.K)]
Let we have a tube of 12mm diameter (10mm inner), the speed of liquid about 1m/s. So, for water, estimative h_ci will be:
h_ci_1a = 2040 * (1^0.87)/(0.01^0.13) = 3712
h_ci_1b = 3370 * (1^0.85) = 3370
[W/m2.K]
Imagine that instead of water we have the liquid subcooled refrigerant. Heat capacity of water is 4.18 kJ/(kg.K), and the h.c. of liquid refrigerant is about 1.2-1.6 kJ/(kg.K), about 2.5 times lesser.
There is no linear proportion, but somehow the heat capacity of liquid depends heat transfer coefficient.
So, if we divide that values (3712 or 3370) onto, for example, 1.5-2, we'll get those estimations of heat capacity coefficient for turbulent flow of refrigerant: (3370..3712)/(1.5..2)=1685..2474.
Difference from estimation of 2000-3000 goes from... tubes may have outer diameter of 10mm (8mm inner), or speeds are greater than 1m/s.
2. Boiling/Condensation
2.a (Condensation, according to Nusselt, for horizontal tubes)
h_c = 8900 * 1 / (d*dt)^0.25
estimation (for water): h_c = 8900*1/(0.01*10)^0.25 = 15800 [W/(m2.K)]
here, dt is a temperature difference, so this dependance is non-linear on temperature.
To make 1kg of water boiled, we need [s]333[/s] (Sorry! my mistake it is about 2200) kJ. The usual enthalpy differences of refrigerant in subcooled applications are 100..150 kJ/kg - as we can see the ratio is 14..20 times.
Let's divide... 15800/(14..20)=790..1120 [W/(m2.K)] - the value for refrigerant.
(yeah, the condensation has lesser coefficients than in boiling case due to lesser turbulences in droplets being created, laminar flow)
2.b (unknown author, for water)
h_c = 1.94 * q^0.72 * p^0.24, where q is heat flow density, W/m2, p - pressure
For evaporators we have heat flow desities of about 150kW/m2 (consider surface of not fins, but inner area of tubes!), pressures - 2..3 bars
So, estimation is: h_c = 1.94 * (150000)^0.72 * (2..3)^0.24 = 12200..13460 [W/(m2.K)]
it is a bit lesser, than of Nusselt for horizontal tubes, though.
(i am already rethinking my way, sorry)
2.c (diagram of unknown author, experimental values for R22)
It's a picture, i'll post it here later. For heat flow densities of about 100kW/m2, R22, the h_c is about 7500-12000 W/(m2.K) for pressures 0.39..2.15 bars respectively.
(But these data is experimental, and not mine so you may trust it )
(went for preparing coffee and that picture)
Last edited by mcgru-; 02-05-2006 at 08:14 AM.
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02-05-2006, 08:20 AM #100
Re: Liquid receiver
if it will be any difficulties with translating russian symbols, i'll help you - just note it
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