by a negligible fraction of the TEV design capacity.Quote:
Originally Posted by US Iceman
I'm afraid that we have to agree to disagree on this matter.
But I'm proud to read opinions from such competent forumists.
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by a negligible fraction of the TEV design capacity.Quote:
Originally Posted by US Iceman
I'm afraid that we have to agree to disagree on this matter.
But I'm proud to read opinions from such competent forumists.
Thank you :D
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:D
really appreciate master's opinions!
Wish there's some day to come together to have face to face talk and argument:p
rgds
LC
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
Mike,Quote:
Originally Posted by US Iceman
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.
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.
Quote:
Originally Posted by wambat
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.
Gee when you write your book ...please let us know, I can always use a good laugh
Ok, Let's publish some producers data. I attach the image below. sorry, the catalogue is in Italian, so I will translate for you.
The capacity of the evaporator must be corrected if the subcooling is different than 4°K. the corrected capacity is obtained by DIVIDING the evaporator capacity by the correction factor.
Eg. 15°K 40kW R22, the new evaporator capacity is 40:1.11 = 36kW
So the evaporator capacity DECREASES as subcooling increases.
My intuition told me there's some misunderstanding. I'll think it over and come back.
thanks for every one endeavour to make it clear.
rgds
LC
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
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.
That's what I thouht and was trying to explain with no so many words.Quote:
Originally Posted by wambat
Everybody please have a look at the table attached in the previous page.Quote:
Originally Posted by NoNickName
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.
Table in previous page, I don't find any table in the previous page.Quote:
Originally Posted by NoNickName
http://www.refrigeration-engineer.co...1&d=1145004047Quote:
Originally Posted by Peter_1
Well, which is the same seen under a different POV. Infact overcharging (please read here: higher subcooling, or flooded condenser) will result in a higher feeding (thanks to higher enthalpy) into the evaporator andQuote:
Originally Posted by Peter_1
... roll of drums....
lower evaporating pressure and lower performance.
A lot of pumped overfeed systems use back-pressure regulators on the evaporators. In these systems there are usually several evaporators that are operating only at the pressure of the pump receiver. Some evaporators are at high pressures, some are at low pressures.Quote:
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?
I quite agree with you. The flash gas is the "cost" of achieving the lower temperature liquid at evaporating pressure. The percentage of flash formed is due to the condensing and evaporating temperatures.Quote:
Originally Posted by Peter
Yes. The colder the liquid is before it flows into the TEV, lower amounts of flash gas will form, which increases the NRE (net refrigerating effect). This means you will have a higher percentage of mass available for useful cooling.Quote:
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.
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+++
I agree with Peter. He had the correct answer the first time.Quote:
Originally Posted by Peter
A good question Renato.Quote:
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.
This I can agree with. When subcooling is incorporated into the design calculations, the compressor can be smaller as the mass flow required is less with the additional subcooling.Quote:
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.
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.
You should consider the ratio of liquid and vapour.Ratio should be 80:20. 80 percent liquid and 20 percent vapour.
Regards
We are not suggesting any system be overcharged and that by overcharging a system you increase the COP.:(Quote:
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
Yes, it is exactly my point. ;)Quote:
Originally Posted by US Iceman
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.
You mean after the TEV?Quote:
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.
Hi Peter,
This is discussion is developing into a long one. :D
If a colder glycol is used, the coil capacity will certainly increase due to an increase in LMTD. If the glycol temperature is warmer, the coil capacity will decrease since the LMTD is now smaller. I agree with you the heat transfer is better with the liquid; glycol or liquid refrigerant. Heat transfer with refrigerant vapor is not very good.Quote:
... 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.
Very true on a DX system. On liquid overfeed system the pump receiver protects the compressor. On a DX system you must have suction superheat to protect the compressor.Quote:
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? ;)
That makes perfectly sense. But in a real chiller, things can't be seen under a purely thermodinamic point of view. Components are chosen and installed based on a design (essentially cost-driven).Quote:
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.
Either way, weather it,s designed for more subcooling or not the fact remains that all that I quoted remains true, presonnally I trust the authors credentials and for the life of me I don't understand why this is so confusing. All was written as operating under actual conditions. That is to say that given some operating condition, if the conditions were to change and more sub cooling were made avialible then the results as I quoted would result. :confused:Quote:
Originally Posted by US Iceman
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.
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 upstairs :)
Peter...Lucky! :>)
Only the big consumers have a kvarh meter.Quote:
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.
I think that you should say....(same surface, less mass flow...but a mass flow with a higher enthalpy)Quote:
Originally Posted by NoNickName
Long discussions learns me the most.Quote:
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...:confused:
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.
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).Quote:
Originally Posted by Peter_1
Small to your standards???
That's a chiller of +/- 130 kW and fits perfectly in Montair's higher capacity products.