Results 1 to 50 of 93
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19-07-2008, 02:57 PM #1
Mass Flow Rate with Big Condenser
What will be the effect of Bigger Condenser in the system as compare to sub cooling and mass flow rate?
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19-07-2008, 03:41 PM #2
Re: Mass Flow Rate with Big Condenser
same charge?
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20-07-2008, 07:12 AM #3
Re: Mass Flow Rate with Big Condenser
Mass flow rate has to do with compressor and expansion device.
There are two opposing effects:
- Lower condensing improves compressor flow (less reexpansion).
- lower pressure differential at expansion device -> less refrigerant fed->lower suction pressure->higher specific volume at compressor suction.
What wins depends on compressort type, model, refrigerant, expansion device...
Subcooling should improve
david2008 has a point depending on how big the condenser is. Just imagine that inner volume is so much bigger that flow turns from turbulent to transitional or laminar, it would condense much worse!
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20-07-2008, 09:00 AM #4
Re: Mass Flow Rate with Big Condenser
The effect of lowering condensing pressure is much bigger than the lowering of the evaporating pressure.
We have some packs running in winter with less than 4 bar (58 PSI) DP over the Danfoss TEV's. We achieve COP's over 7 under these conditions!
Will subcool not be more dependent on the condenser coil construction? Real controlled subcooling in a condenser can only be done if the second-last bends in the coil are going upwards so that you make a small liquid collector.
Of course, the sooner the liquid is condensed - in a bigger condenser, the gases are already condensed somewhere in the middle of the coil - the more it will then subcool.
If we need a real subcooler, we install a supplementary coil after the liquid receiver or order a condenser with an integrated subcooler.
This is a good example of a real subcooler.
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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20-07-2008, 04:58 PM #5
Re: Mass Flow Rate with Big Condenser
Originally Posted by Peter_1
From a simple viewpoint, you want to keep the discharge pressure as low as you possibly can. This helps to lower the kW power input (the demand power) and the kWh.
If you need a larger condenser, then use one that offers a cost-effective solution to lowering the discharge pressure.
The mass flow will only change incrementally by using a larger condenser. The mass flow is determined by the operating conditions the compressor works in.If all else fails, ask for help.
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20-07-2008, 06:08 PM #6
Re: Mass Flow Rate with Big Condenser
Thanks a lot to ALL!!
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20-07-2008, 06:09 PM #7
Re: Mass Flow Rate with Big Condenser
Isn't it also that you need a minimum DP over a TEV and isn't 4bar a little low?
Also if you have very low discharge in winter, than liquid will also be subcooled very much... and it is possible that you don't have liquid-gas after expansion, but 100% liquid... no?
Also couldn't you have the problem of very low suction pressures in winter with a very low discharge? You need to keep a certain min, no??
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20-07-2008, 07:01 PM #8
Re: Mass Flow Rate with Big Condenser
Originally Posted by majo
Obviously, if the valve cannot operate at the low DP then the suction pressure would decrease. This is all based on the design of the system and the components used to achieve this lower operating conditions.
Originally Posted by majoIf all else fails, ask for help.
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20-07-2008, 08:27 PM #9
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20-07-2008, 08:31 PM #10
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20-07-2008, 09:52 PM #11
Re: Mass Flow Rate with Big Condenser
Originally Posted by David2008
A reasonable lower limit for DP is what Peter said. I have had some systems operating at less. And yes, I would use balanced port TEV's on ANY direct expansion system.If all else fails, ask for help.
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20-07-2008, 09:56 PM #12
Re: Mass Flow Rate with Big Condenser
Originally Posted by David2008
You must be kidding? Up means up and down means down. (sorry, couldn't resist)
The difference is what you call a liquid seal. If you drain downwards the liquid may not stay in the condenser to achieve any subcooling.
If you provide a liquid seal, the liquid has a place to collect so that it can subcool.
Condenser manufacturers handle this differently in their designs.If all else fails, ask for help.
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20-07-2008, 10:06 PM #13
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20-07-2008, 11:47 PM #14
Re: Mass Flow Rate with Big Condenser
A bigger condenser could have all of the following positive effects on a refrigeration system because it would have a lower condensing temperature.
1. A better refrigeration effect
2. A lower mass flow rate
3. A lower volume flow rate
4. A higher COP
5. A lower condenser heat of rejection
6. A lower condenser temp difference
7. A lower condenser split
8. A lower KW/ton
9. higher efficiency and a happy compressor and it's components
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21-07-2008, 07:44 AM #15
You need indeed a certain minimum Dp over the TEV but 4 bar is also given by Danfoss themselves.
And we have the proof it works (even 3.5 bar)
Look ones in the Danfoss tables and you will see that the capacity doesn't decrease that much with decreasing pressures.
You say that SC increases during winter which is true but this increases also the capacity of the TEV and compensates a little for the lower DP.
In theory, you can only have full liquid after the valve if you SC liquid to evaporating temperature.
But I think this won't give any problem.Last edited by Peter_1; 21-07-2008 at 08:02 AM.
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21-07-2008, 08:11 AM #16
Indeed, that's the correct expression, liquid seal
We had - perhaps 20 years ago - some condensing units running (Copeland with hermetic piston DCRQ's running), evaporating at 5°C.
The compressor could handle this perfectly but the condensers were ways too small. So we installed in line a second condenser and this decreased HP almost nothing. Only if we placed one in parallel, then it resulted in a serious HP decrease.
In line, what we made was a huge subcooler which didn't increase the condenser area, so HP remained almost unchanged.Last edited by Peter_1; 21-07-2008 at 02:04 PM.
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21-07-2008, 01:51 PM #17
Re: Mass Flow Rate with Big Condenser
You are absolutely right if the oversize is small, but here I get equipment designed for 60Hz working in 50Hz and if ambient temperature lowers or have two compressors with one off they inmediatelly start hunting.
So what will happen really depends on how the other system components are sized.
Even the type of expansion valve affects, (type of charge and balanced or not port).
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21-07-2008, 03:03 PM #18
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21-07-2008, 03:39 PM #19
Re: Mass Flow Rate with Big Condenser
Originally Posted by GXMPLX
If the equipment is balanced at 60Hz, the same should be expected at 50Hz with the only exception being the rated duty should be compensated for to achieve the desired results while operating at 50 Hz.Last edited by US Iceman; 21-07-2008 at 03:39 PM. Reason: fixed quote
If all else fails, ask for help.
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21-07-2008, 03:56 PM #20
Re: Mass Flow Rate with Big Condenser
Originally Posted by Peter_1
Refrigeration systems do NOT have to have high discharge pressures to work.
The only time you should see higher discharge pressures on air-cooled condensers (or evaporative condensers) is in the summer time (or during the hottest temperatures of you location).If all else fails, ask for help.
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21-07-2008, 04:01 PM #21
Re: Mass Flow Rate with Big Condenser
Originally Posted by jwasir
From my experience... the smallest condensers are used so that the installing contractor can have the lowest price.If all else fails, ask for help.
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21-07-2008, 04:42 PM #22
Re: Mass Flow Rate with Big Condenser
Usually, big condenser will improve system efficiency. Usually but not always. Greatly oversized condenser will use a lot of energy(fans, pumps). Sometimes, total energy use of the system with big condenser can be higher than for the system with smaller condenser.
For proper evaporator operation demand(refrigeration load) and supply(liquid supply) should be balanced. At lower head pressure supply can be reduced. However, demand can be reduced as well. If this happen simultaneously, system will be balanced.
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21-07-2008, 04:58 PM #23
Condenser size is has a lot to do with maximum ambient temp.
For example, I use very big condensers on milk tanks where the maximum ambient is 50°C, this is the only way I get condensate refrigerant in the receiver.
Head pressure is very high anyway at these conditions.
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21-07-2008, 08:24 PM #24
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21-07-2008, 08:31 PM #25
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21-07-2008, 08:37 PM #26
Re: Mass Flow Rate with Big Condenser
Not necessarily. You get TEV instability if the valve tries to operate below a curve called MSS (Minimum Stable Superheat) that graphs capacity versus valve superheat. By increasing subcooling you are moving this curve to higher superheat and will produce instability more often with TEVs that have fast acting charges.
TEV stability problems are the ones I find less understood in these posts, not an easy subject that needs some drawings and I'll post something on this when I'm ready.
Though the equipment remains in balance due to the fact that TEV works fine at 20% off nominal capacity, the compressor capacity is 20%less.
The evaporator and condenser are different because it depends if the manufacturer changed blade´s attack angle to compensate for lower RPMs or not.
The problem arises when you have low thermal loads the system becomes completely unstable and out of balance. The TEV is oversized and perform VERY poorly if thermal load drops 20% to 30% below the 50Hz value! Except for balanced port valves or absorber charges, that can control well as low as 20% off their rated capacity.
This is worse in some equipment that already had the TEV oversized for the 60Hz capacity (It was correctly chosen but a little oversized).
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21-07-2008, 08:40 PM #27
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21-07-2008, 08:49 PM #28
Re: Mass Flow Rate with Big Condenser
Cost is a BIG disadvantage.
No! Some compressor are optimized for high condensing other for low condensing, if you use the wrong compressor with a large condenser you will get less COP.
Consider that with this line of thought all condensers should be water cooled! Go ask Chemi_cool if this is possible!
Every application has some optimizing to be done. Little larger condensers ok, too large will cause problems and probably lower system COP.
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21-07-2008, 10:03 PM #29
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21-07-2008, 10:08 PM #30
Re: Mass Flow Rate with Big Condenser
This is worse in some equipment that already had the TEV oversized for the 60Hz capacity (It was correctly chosen but a little oversized).
I always prefer to have the valve slightly undersized and recommend the balanced port valves because I have had such good luck with them.If all else fails, ask for help.
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21-07-2008, 10:41 PM #31
Don´t make me dig on semantics here if you need X, anything bigger is oversized.
Oversize is just another way of saying "safety factor".
Your design objectives give you the right safety factor.
Usually economical decisions let you choose the "right" condenser (No, US_Iceman please don't comment this on me! You simply have to fit in!)
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21-07-2008, 10:43 PM #32
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21-07-2008, 11:05 PM #33
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21-07-2008, 11:51 PM #34
Re: Mass Flow Rate with Big Condenser
X = 10 deg TD.
Hows that for starters.
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22-07-2008, 12:10 AM #35
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22-07-2008, 12:19 AM #36
Re: Mass Flow Rate with Big Condenser
Many times I have read absolute remarks in these posts, the problem is that most of them are right under certain conditions and maybe that's what makes people disagree.
I agree that oversized condensers improve COP if they are little oversized in my case with 60Hz 50Hz problems see them 40% oversized and up.
X in condensers stand for heat of compression+absorbed heat in the evap at the maximum operating suction pressure.
This means that if you operate a system below its operating suction pressure the condenser is oversized and you won't see me pulling my hair (sorry scarce hair) having an oversized condenser.
Low suction pressures usually (remark usually) means that the compressor COP goes down and it is the most important device of the system so the system should lower its COP
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22-07-2008, 01:04 AM #37
Re: Mass Flow Rate with Big Condenser
Originally Posted by GXMPLXIf all else fails, ask for help.
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22-07-2008, 01:07 AM #38
Re: Mass Flow Rate with Big Condenser
Why do not control suction pressure? Bigger condenser will lead to lower condensing pressure and better compressor COP. What about system COP? This is the different question. To run bigger condenser we have to use more energy(fans, pumps). Sometimes additional condenser energy is greater than compressor energy savings from lower condensing pressure. Another issue, every refrigeration plant has minimum allowable condensing pressure. What is this pressure? Certainly, it is better if we can reduce this minimum. How can we do that? Sorry, but have many questions.
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22-07-2008, 01:11 AM #39
Re: Mass Flow Rate with Big Condenser
Originally Posted by Sergei
Larger condensers improve the COP of the refrigeration cycle.
If the fan or pump power is higher than the compressor energy saved, you lose.
The condenser is the correct size, if the heat rejection capacity is equal to that of the compressor(s).
The correct condenser selection is one that balances all of the above in a cost-effective manner.
We could continue to say absolute statements or settle into semantics. I'm quilty of this too. Therefore, if we look at the underlying logic of our statements I think we can all agree.
These are not directed at anyone specifically, just in general...
Can we shake hands on that?Last edited by US Iceman; 22-07-2008 at 02:04 AM. Reason: edit
If all else fails, ask for help.
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22-07-2008, 01:36 AM #40
Re: Mass Flow Rate with Big Condenser
Sergei, What GLMPLX is saying is absolutely correct If the condenser is too big then you can have some of the problems suggested by US Icaman and GLMPLX. That's what makes this forom so valuable is the comments and discussions about problem solving. I.m sorry if I was incomplete with my statement.
The thing you need to know and understand with refrigeration is that all the components will do a good job (within certain perameters) you can't have too much and you can't have too little that's why we have to understand the complete refrigeration cycle and what all the effects that will happen when we make any changesLast edited by wambat; 22-07-2008 at 01:47 AM.
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22-07-2008, 02:03 AM #41
Re: Mass Flow Rate with Big Condenser
Originally Posted by wambatIf all else fails, ask for help.
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22-07-2008, 02:19 AM #42
Re: Mass Flow Rate with Big Condenser
I think that X is optimum size of condenser. What is the optimum? The optimum is the size of condenser when total(compressors, condensers, evaporators) power use per unit of refrigeration is minimum. If condenser is oversized we should reduce capacity(and energy use) of this condenser. So capacities of compressors and condensers should balanced to keep total power use at minimum level. This balancing should be done at different ambient conditions and different refrigeration loads. Sometimes optimum condensing pressure is low and we have a barriers to keep it low. However, every barrier has a solution. The longer we run plant at optimum condensing pressure, the better efficiency of this plant.
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22-07-2008, 04:34 AM #43
Re: Mass Flow Rate with Big Condenser
Didn't know "X" would make such a fuzz!
Optimums are hard to get at, when we are more like searching for feasible solutions! ... like not having our fuses blown!
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22-07-2008, 05:06 AM #44
Re: Mass Flow Rate with Big Condenser
I certinaly don't think anyone is getting their fuses (or fuzzes) blown.
What we see with this discussion is the many different ways people look at topics and how they attack them to develop solutions.
It is also important to recognize that any topic will have differing viewpoints because the posters all have different skill sets and experience levels.
So...a simple question on over-sizing condensers went from theoretical to optimization to practical in the span of only several posts. How cool is that?!If all else fails, ask for help.
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22-07-2008, 06:45 PM #45
Re: Mass Flow Rate with Big Condenser
Ok, didn't think it was able to work at these low pressures, I'll take a look.
But If you have such a huge condensor with pressures as low as these. When working in a chamber at e.g. 10°C, with outside temps of -10° for example, you could have major problems with head pressure and need to keep your pressures up, no?
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22-07-2008, 07:25 PM #46
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22-07-2008, 08:01 PM #47
Re: Mass Flow Rate with Big Condenser
Sorry I didn't have enough time to answer these, my way.
Thanks wambat I think this is a wise post! And it was in response to Sergei's below.
The only problem with it is that it implies that an optimum is somewhere in the middle and if I interpret Sergei right it's what he's after.
Unfortunately I've never seen a system designed so that it minimizes power per unit refrigeration.
The big problem is this is impossible to do in the field you can calculate it and we better do, but it is done after the system is built not before.
This is exactly what people in NASA should do with good computer models and crystal clear objectives and lots of computing resources, and maybe then build the system.
YOU ARE ABSOLUTLY RIGHT WANTING THIS OBJECTIVE but I'm afraid we are far from it!
Yes, the balancing is done depending on ambient and cold room conditions but with a different objective an that is comply with demand.
The primary objective of the system is to do the cooling what it is supposed to. All optimizations that I know are done are on component basis, not on system basis because it is impossible to stretch the condenser 5 inches to reach a better working condition.
What you do is once you have the system (or at least selected componets) you do all optimization possible like adding capacity control maybe add a VFD but your system may be far from working at any optimum.
A big one and very cost effective is heat recovery, that's why it has become a must with all big brands!
Some system designers have programs to work in this direction but their own components cannot be assembled in such a way that you can make enough combinations to get even close to an energy use optimum like you want, THAT I KNOW OF!
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22-07-2008, 08:26 PM #48
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22-07-2008, 08:55 PM #49
Re: Mass Flow Rate with Big Condenser
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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22-07-2008, 11:10 PM #50
Re: Mass Flow Rate with Big Condenser
Originally Posted by GXMPLX
There is an optimum for each operating condition. The real trick is to find these operating points and match the system response to these with the most cost effective method of operation.If all else fails, ask for help.
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