Mass Flow Rate with Big Condenser

[jwasir]

What will be the effect of Bigger Condenser in the system as compare to sub cooling and mass flow rate?

[david2008]

same charge?

[GXMPLX]

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!

[Peter_1]

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.
http://www.globaldensoproducts.com/images/condenser.jpg

[US Iceman]

We have some packs running in winter with less than 4 bar (58 PSI) DP over the Danfoss TEV's.


This is exactly what you want to have happen as often as you can (and as long as you can)!

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.

[jwasir]

Thanks a lot to ALL!!

[majo]

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!


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??

[US Iceman]

Isn't it also that you need a minimum DP over a TEV and isn't 4bar a little low?

[...]

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??


All of this depends on the TEV's ability to react to the lower DP, while controlling the evaporator superheat. Balanced port TEV's do this quite well.

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.



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?


It is conceivable the liquid would have a lot of subcooling. This would simply increase the Net Refrigerating Effect of the liquid and reduce the run-time required of the compressor. While the amount of flash gas would be considerably reduced, the TEV should still be controlling the evaporator superheat.

[david2008]

All of this depends on the TEV's ability to react to the lower DP, while controlling the evaporator superheat. Balanced port TEV's do this quite well.

Generally speaking, what is the number for low DP? 100psi? For low DP system, is the solution to use a Balanced port TEV?

[david2008]

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.


What is the difference between upwards and downwards? They have the same pressure, so the same function for TXV?

[US Iceman]

Generally speaking, what is the number for low DP? 100psi? For low DP system, is the solution to use a Balanced port TEV?


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.

[US Iceman]

What is the difference between upwards and downwards?


You must be kidding? Up means up and down means down. :p (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.

[david2008]

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You must be kidding? Up means up and down means down. :p (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.

Thank you. :D

[wambat]

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 :)

[Peter_1]

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??

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.

[Peter_1]

[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.
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.

[GXMPLX]

The effect of lowering condensing pressure is much bigger than the lowering of the evaporating pressure.

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).

[jwasir]

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 :)

If Bigger Cond is a WIN-Win situation, then why we dont design all the systems with BIGGER cond.

The initial cost will be high but system efficieny, COP, etc will increase drastically.

There must be some disadvantages......

[US Iceman]

...off they immediately start hunting.


Hhhmmm, usually hunting only occurs when the components are not balanced or going through some severe/strange transients.

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.

[US Iceman]

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.


These comments offer so much information that you do not typically learn in school. It would be well worth the time of other members to try to understand what Peter is describing and how it applies to refrigeration systems.

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).

[US Iceman]

There must be some disadvantages......


Well, there could be if the system is not designed to work under the lower discharge pressures. The refrigerant charge may increase a little, but the biggest problem is the additional cost of the condenser heat rejection capacity.

From my experience... the smallest condensers are used so that the installing contractor can have the lowest price.:mad:

[Sergei]

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.

[chemi-cool]

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.

[wambat]

If Bigger Cond is a WIN-Win situation, then why we dont design all the systems with BIGGER cond.

The initial cost will be high but system efficieny, COP, etc will increase drastically.

There must be some disadvantages......

They do make bigger condensers that is why the SEeR is going up every year, 15 years ago the seer was about 8 now it's 18 /19

[wambat]

Well, there could be if the system is not designed to work under the lower discharge pressures. The refrigerant charge may increase a little, but the biggest problem is the additional cost of the condenser heat rejection capacity.

From my experience... the smallest condensers are used so that the installing contractor can have the lowest price.:mad:

Whether the mfg likes it or not they will be mandated to increase to bigger condensers and the customer will have to pay. The return on investment will be lower KW use and maybe lower operating costs

[GXMPLX]

Hhhmmm, usually hunting only occurs when the components are not balanced or going through some severe/strange transients.

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.


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.

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).

[GXMPLX]

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.

Air conditioning equipment condensers are already oversized. Water cooled do this because of the fouling factor, air cooled condensers have a fouling factor too on the ouside (bugs, dirt, corrosion).

[GXMPLX]

If Bigger Cond is a WIN-Win situation, then why we dont design all the systems with BIGGER cond.

Cost is a BIG disadvantage.


The initial cost will be high but system efficieny, COP, etc will increase drastically.

There must be some disadvantages......

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.

[Sergei]

Air conditioning equipment condensers are already oversized. Water cooled do this because of the fouling factor, air cooled condensers have a fouling factor too on the ouside (bugs, dirt, corrosion).
Why do you think they are oversized? What is the right size?

[US Iceman]

This is worse in some equipment that already had the TEV oversized for the 60Hz capacity (It was correctly chosen but a little oversized).


No argument on that point at all.

I always prefer to have the valve slightly undersized and recommend the balanced port valves because I have had such good luck with them.

[GXMPLX]

Why do you think they are oversized? What is the right size?

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!)

[GXMPLX]

No argument on that point at all.

I always prefer to have the valve slightly undersized and recommend the balanced port valves because I have had such good luck with them.

It´s not luck they are well designed, good valves!

[Sergei]

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!)
This is my question. How big is the X? Why is it so big? Certainly, anything bigger than X is oversized.
Wambat mentioned that bigger condensers will improve efficiency of the plant. Who is right?

[Billy Ray]

X = 10 deg TD.


Hows that for starters.

[Sergei]

X = 10 deg TD.


Hows that for starters.
Deg. C or deg. F. Why is it 10, not 8 or 15? This is question for professionals.

[GXMPLX]

This is my question. How big is the X? Why is it so big? Certainly, anything bigger than X is oversized.
Wambat mentioned that bigger condensers will improve efficiency of the plant. Who is right?


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

[US Iceman]

It´s not luck they are well designed, good valves!


I did not mean I got lucky and they worked. I meant I had good results with the valve I carefully selected.

[Sergei]

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
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.

[US Iceman]

Who is right?


I think we are all dancing around the right answer.

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?:)

[wambat]

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. :o
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 changes

[US Iceman]

...that's why we have to understand the complete refrigeration cycle and what all the effects that will happen when we make any changes.


I think Sergei already knows based on some our past discussions. I also think what seems like divergent views are a lot closer than we might want to admit also.;)

[Sergei]

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.

[GXMPLX]

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!

[US Iceman]

I certinaly don't think anyone is getting their fuses (or fuzzes) blown.:D

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?!:cool:

[majo]

Look ones in the Danfoss tables and you will see that the capacity doesn't decrease that much with decreasing pressures.

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.

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?

[jwasir]

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?

Ofcourse, thats why we use low ambient package to maintain the min.design pressure differential across the TXV.

[GXMPLX]

Sorry I didn't have enough time to answer these, my way.


...
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 changes

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.


I .... 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 condense .

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!



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.

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!

[GXMPLX]

I did not mean I got lucky and they worked. I meant I had good results with the valve I carefully selected.

I hope jwasir forgives me for messing with his post in so many different directions like I know you would whack me for!

This reminded me of something ... but I´ll better post it in the new balanced port TEV post.

[Peter_1]

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?

Well, we then use a VFD or switch-off the fans.
But of course, you must take in account that the savings with running fans consuming power no longer match the savings they make.

[US Iceman]

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 what I do for a living. If you are lucky to do this before the system is built it is so much easier. If you have to do it after installation it is a lot of work.

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.

[jwasir]

I hope jwasir forgives me for messing with his post in so many different directions like I know you would whack me for!

No Sir,

I am so happy that So much information can be achieved with such a basic question.

THANKS TO ALL!!

[Sergei]

This is what I do for a living. If you are lucky to do this before the system is built it is so much easier. If you have to do it after installation it is a lot of work.

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.
Hi, Mike.
I agree with you.
There are 2 types of optimization. Optimum design and optimum operation. Optimum design is the foundation of optimum operation. However, optimum operation is the final goal of energy saving process. Two steps should be done for optimum operation.
1. Determination of optimum set points and operating strategies for different ambient conditions and different refrigeration loads. Very often we just guessing about these points and strategies. This is the most difficult part of optimization. However, how we can reach the goal if we don't know what is it.
2. Implementation of optimum set points and optimum operating strategies. Sometimes, we don't know how to implement these set points.

[GXMPLX]

This is what I do for a living. If you are lucky to do this before the system is built it is so much easier. If you have to do it after installation it is a lot of work.

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.

OK CONFESS!!!! How many evaluations (different component combinations) do you evaluate before building the system???

... Great! If you do this congratulations first I know of!

[US Iceman]

Hi, Mike.
I agree with you.


Hi Sergei. I sort of thought you might since we have talked about this before.:D

Optimization is completely different than balanced equipment selections.

[GXMPLX]

Hi, Mike.
I agree with you.
There are 2 types of optimization. Optimum design and optimum operation. Optimum design is the foundation of optimum operation. However, optimum operation is the final goal of energy saving process. Two steps should be done for optimum operation.
1. Determination of optimum set points and operating strategies for different ambient conditions and different refrigeration loads. Very often we just guessing about these points and strategies. This is the most difficult part of optimization. However, how we can reach the goal if we don't know what is it.
2. Implementation of optimum set points and optimum operating strategies. Sometimes, we don't know how to implement these set points.

Completely agree. But an optimum design is something I haven't laid hands on!

[US Iceman]

:off topic: OK, it's offical..you're whacked again. :p



OK CONFESS!!!! How many evaluations (different component combinations) do you evaluate before building the system???

... Great! If you do this congratulations first I know of!


Well...if you think I'm using calculus for this your crazy!:D

The way I approach this is once I know the loads, I look at the weather profiles. If the wet bulb temperature moves around like a bell curve I have a lot of hours available for low condensing temperatures. That's number 1.

Number 2 is determined by the lowest reasonable condensing temperature I can expect to run; somewhere around 45-50°F (7.2-10°C) during the low ambient condition. That's well within the margin for supplying hot gas for defrost.

Number 3 is to look at the load profile. If you don't know how the load acts, you can't design for it.

Next step is to determine the highest possible evaporating temperatures for each cooling load. AND, don't use back-pressure regulators...they are expensive to install, maintain, and pay for with operating costs.

Next step is to find the right mix (sizes and types) of compressors to match the load profile.

Next...talk to some guys who really understand control systems.

And lastly, get a contractor who can install it right.

Please bear in mind I'm talking about big ammonia systems, but the same logic applies to others also.

[GXMPLX]

:off topic: OK, it's offical..you're whacked again. :p

...

Please bear in mind I'm talking about big ammonia systems, but the same logic applies to others also.

Ammonia is great! ... only I don't like the smell, it gives me headaches!

Sorry but I call this GOOD design not optimum design in the sense I understand optimum!:(

[US Iceman]

Optimize
a) to enhance the effectiveness of something
to make something function at its best or most effective, or use something to its best advantage


b) write program concisely
to write computer programming instructions for a task in as few lines as possible to maximize the speed and efficiency of program execution

To me;

a = make sure the system is able to operate at any condition with the lowest energy use

b = follow the KISS principle

PS. I saw your note, but it disappeared on my last view. Send me a PM and I will see what I can do, OK?

[GXMPLX]

Optimize
...



Ok I admit my English is not nearly as good … but I don’t confuse optimize with optimum.

Optimize is the necessary process for reaching an optimum.

I’ll use calculus to explain it, in a typical scenario with your good practices you may have 4 TXVs to choose (2 types of charge) , 4 compressors (2 screw 2 recip), 3 condensers, and 2 evaporators.

The possible combinations are 4 x 4 x 3 x 2 =96.

With heuristics you can reduce them to half your experience to 1/3 but if you don’t check the remaining 32 combinations you may not be near an optimum.

You still need to check these at different working conditions.

For me an optimum design is yet to be seen and I’ve seen lot’s of big America’s main brands! If I can change one component to reduce energy consumption the design is not optimal.

It doesn’t mean they are bad! I'm pretty sure your designs are very good! ... or at least as good as mine!:D

[US Iceman]

Yeah, and if you run the permutations on a big ammonia system you end up having to use exponents to describe the number!

I think it's simpler than that, but it's difficult to put into words. For some reason I just do it in my head and it seems to be OK. The logic I offered earlier is about the simplest way I can explain it.

[GXMPLX]

Yeah, and if you run the permutations on a big ammonia system you end up having to use exponents to describe the number!

I think it's simpler than that, but it's difficult to put into words. For some reason I just do it in my head and it seems to be OK. The logic I offered earlier is about the simplest way I can explain it.

Heuristics! you use rules based on experience. It's a simple way to solve a humongous problem ... but not necessarily optimal.

Could you see in my CP the scans of the manual or do I have to send you a link?

[majo]

Well, we then use a VFD or switch-off the fans.
But of course, you must take in account that the savings with running fans consuming power no longer match the savings they make.


Ok so you let the fans switch of when they reach such low pressures.
Usually I don't let the fans run this long (DP 4-5bar), I cut them off earlier to maintain a little higher pressure.

Thanks for the enlightment :)

[Peter_1]

Majo, waar werk jij ergens als ik u dit vragen mag?
Your knowledge surprises me. Few at your age pose the questions you do. You're thinking further than most do.
Try also to understand what's the big benefit of lowering the HP.
I'm a teacher at Syntrawest (part-time evening classes) and I think I say this at least 10 times during the year: try to get those needles (HP and LP) as close tot each other as technical possible and allowable.

[majo]

Tuurlijk mag je dit vragen... ik werk nu bij ClimaTronix (gentse), verdelers van Thermotron environmental in de benelux. Ik heb sinds afgestudeerd aan nog niets anders gewerkt dan klimaatsimulatie, al was het de vorige jaren bij een ander merk :)

Kheb nog jong en heb nog veel te leren maar ik ben zeer geinteresseerd in het vak en vind dat er nog teveel mensen niet snappen hoe die koeltechniek echt in elkaar zit. Daarom dat ik ook graag lees wat mensen zoals jij te schrijven hebben. This is a great forum.

I understand about trying to get those needles as close as possible... but didn't ever thought about that close. 4bar is pretty new to me :D

sintrawest is dit antwerpen?

[Peter_1]

T..., al was het de vorige jaren bij een ander merk :)

I bet I know that company ..W...s:p
Look also once in our section CPU overclockers. This is something which may interest you, especially the autocascade systems.
Syntrawest in Kortrijk and Oostende http://www.syntrawest.be/FOLDERS/I000000534.PDF sorry, all in Dutch

[majo]

hmm indeed w...s

hmmz... nemen jullie ook de examens af voor de certificering dan?

[Peter_1]

I was the sole teacher for the first Flemish session for the certification, held in Kortrijk and the chairman for the first exams. All (10) succeeded but I doubt a little bit the usefulness of this new legislation, at least the way we're approaching this in Belgium.

I think you already read articles of me in Cool&Comfort. Look once in an old edition of C&C.

How I know W...s? We have a client in Bruges with a small environmental chamber and they gave price to install the condensing unit just outside the building. (because we proposed this to reduce internal heat load) I think they use/install golden tubes because we could do it for 1/3 of their price.
They needed a new liquid tank due to the longer lines: +/- 2 or 3 m 3/8 tube, new expansion device, new gas,....
Also another condenser, also due to the longer lines.

I was always interested in low temperature applications.
We made a cascade with ethylene and propylene, we service some Polycolds, serviced also in the past some environmental chambers at Siemens (can't remember right now the brand name),....

[US Iceman]

I understand about trying to get those needles as close as possible... but didn't ever thought about that close.


The highest you can get the low pressure is based on the temperatures required for cooling. That means you have a relatively fixed range the low side can operate in.

However, the high side can move a lot before you see problems. Peter is absolutely correct though. Get the pressures as close as you can to each other. That's the best advice...

[majo]

I will take a look, C&C is my favo magazine by far ;)

Hehe... w***s always uses golden tubes, very pricy indeed, a bit over the top!

Former Siemens is this the company in Oostkamp, near the E40? Could be EADS or Tyco, they have a large amount of environmentals.

[Peter_1]

That's the one, but I'm talking of many years ago.
Isn't EADS their military section?

[majo]

Yes indeed it's military :)
was it a w***s you serviced?

[Peter_1]

No Majo, it was a...xxx (the years, names are the first erased on my HD), there were as far as I remember 2 x 4 DWM/Copelands cascades in it (or 2 x 2), serving an enclosure of +/- 1.5 x 1.5 x 1.5 (3 x 3 x 3 ft)
The compressors were on the left side of the chamber.
I had some copies of the electrical circuit and refrigeration circuit but I don't know where starting to search for this.

[majo]

hmm...I've never seen a chamber of that capacity over there... maybe shredded.

[Poodzy]

:off topic: OK, it's offical..you're whacked again. :p



Well...if you think I'm using calculus for this your crazy!:D

The way I approach this is once I know the loads, I look at the weather profiles. If the wet bulb temperature moves around like a bell curve I have a lot of hours available for low condensing temperatures. That's number 1.

Number 2 is determined by the lowest reasonable condensing temperature I can expect to run; somewhere around 45-50°F (7.2-10°C) during the low ambient condition. That's well within the margin for supplying hot gas for defrost.

Number 3 is to look at the load profile. If you don't know how the load acts, you can't design for it.

Next step is to determine the highest possible evaporating temperatures for each cooling load. AND, don't use back-pressure regulators...they are expensive to install, maintain, and pay for with operating costs.

Next step is to find the right mix (sizes and types) of compressors to match the load profile.

Next...talk to some guys who really understand control systems.

And lastly, get a contractor who can install it right.

Please bear in mind I'm talking about big ammonia systems, but the same logic applies to others also.


Hey everyone have to chime in on this one. Well said Iceman. But there are really to many other variable that I have encountered here at the plant I work at. First they will always try and up the load hence your hp goes up. We have 6 Bac,4 Imeco, and 2 risto condensors atm and we need more. But even the best designed systems have problems some of are condensors take more of a heat load then the rest. Air in the system etc etc. The biggest problem is when we get to about 185lbs hp we start to lose 1st and second stage so nothing gets froze. So imo bigger is better. And as we all know its always the reffer guys fault.

[US Iceman]

But even the best designed systems have problems some of are condensers take more of a heat load then the rest. Air in the system etc etc.


That's not the condensers fault, but it could be the system designers. This is in part., two issues. The piping may not be correct which allows liquid to hang up in the condenser. This reduces the heat rejection capacity of the condensers. Air is a problem for the purgers to work on.

This sort of highlights a second issue; you can pick all of the right equipment, however the installation procedures can cause you to receive less benefit from the equipment.

[Peter_1]

That's sometimes frustrating: both companies are offering the same equipment but the final result can be so different.
Then try once to explain why your higher price is justified :mad:

[GXMPLX]

That's not the condensers fault, ...

May I also stress the importance of designing the pipework correctly, making sure all condensers get to condense, when different pressure drop ones are connected in parallel.

[US Iceman]

...making sure all condensers get to condense, when different pressure drop ones are connected in parallel.


Yes this is very true and it relates to this:



The piping may not be correct which allows liquid to hang up in the condenser. This reduces the heat rejection capacity of the condensers.


Sometimes the problem is not related to just selecting a bigger condenser, but making those you have work.

[GXMPLX]

Yes this is very true and it relates to this:



As Big Brother once said:... Here we go again!

No, I think they are absolutely different.

One thing is to flood the condenser with liquid and require a higher pressure to discharge it so you get a bumpy high pressure. You solve this with a siphon.

Another completely different is to divert more gas refrigerant through a condenser that has low pressure drop in parallel with another of higher pressure (until Pdrop on both branches equal) causing one small condenser not to be able to condense all refrigerant going through it and discharge superheated vapor. You solve this one with an INVERTED siphon.

Do you think they are the same because they are both pipework issues?

[GXMPLX]

Sometimes the problem is not related to just selecting a bigger condenser, but making those you have work.

Yes, I had a computer teacher (before personal computing era) that always said ... "It's not a matter of owning the latest technology but to learn how to use the one you have!"

Both wise words, I admit!

[US Iceman]

OK, let me try this a different way...

If the intention is TO flood the condenser for winter pressure control, this is something different.

If you have multiple condensers in parallel then the only way you can GRAVITY DRAIN them is to ensure the outlet pressures of EACH outlet branch connection exist at the same pressure. However you have to do this to make the condensers attain their rated heat rejection capacity is what is required.

And...more often than not, incorrect piping practices cause this.

[GXMPLX]

OK, let me try this a different way...



I was thinking of flooding as a problem not as a solution too.

In order to gravity drain in all condensers with different pressure drops across would need inverted siphons on the ones with less pressure drop.

Maybe you design your systems too good and never had these problems us mere mortals with smaller systems do.

[US Iceman]

In order to gravity drain in all condensers with different pressure drops across would need inverted siphons on the ones with less pressure drop.


How do you know which one will have the lowest pressure drop? This can change during operation if one condenser receivers colder air, or any other number of factors. Therefore to be safe you would have to determine what a maximum allowable pressure loss would be and design the liquid seal trap accordingly.

[GXMPLX]

How do you know which one will have the lowest pressure drop?

I like simple questions they lead to simpler answers: Not using the sense of smell, of course!.

If you want place a sight glass on each output.

I see you optimize too much, try a hypothetical problem: Installing an air cooled with one circuit in parallel to a cooling tower (refrigeration side of course), both with the same capacity, different pressure drop.

[US Iceman]

I see you optimize too much,...


No, I would rather have the system work properly the first time. That's why I look at all of the potential issues I know about.

I agree with what you're saying about the two different types of condenser. There the pressure drop could be considerably different between the two.

But... it's the same problem on evaporative condenser coils also. And there the low pressure drops found can cause excessive liquid back-up in the condensers and cause you to loos heat rejection capacity.

Same problem, just different devices...

[GXMPLX]

OK, fine for me. Check CP though.

[Peter_1]

How do you know which one will have the lowest pressure drop? This can change during operation if one condenser receivers colder air, or any other number of factors.
Or fans switching on and off. Installing them high enough and installing siphons is how we do t.
What do you mean with an inverted siphon?

[Peter_1]

Like this one with 1 condenser double the size as the other one.

[Peter_1]

hmm...I've never seen a chamber of that capacity over there... maybe shredded.

It was a Vötsch, perhaps 3 x 3 x 3 ft, it's to long ago. I think it was OEM made

[Peter_1]

The 2 parallel condensers were for a replacement of the old pack I made around 1989.
Perhaps for the young techs amongst us: no oil floats, no oil separators, no oil tanks, no interconnection between the compressors.

As long you respect the proper guidelines, you don't need all these fancy tricks.

We made several this way and this is the first one we replaced. The pack ran the last 5 years almost at 100% due to expansion of the company

The black ones (still the original Prestcold compressors) were chosen in a mathematical row 1 - 2 - 4 - 7.5 Hp. The PLC then could chose ramp up capacity in steps of 1 HP, sometimes 0.5 HP.
So no VFD needed and still able to regulate a preset LP very precise.

The two grey ones (DWM/Copeland) are for the freezer and those were replaced a year before the replacement.

[GXMPLX]

... no oil floats, no oil separators, no oil tanks, no interconnection between the compressors.

As long you respect the proper guidelines, you don't need all these fancy tricks.


Great congratulations! But I think you must count luck in your side too, I wouldn’t recommend this though obviously can be done!


…
So no VFD needed and still able to regulate a preset LP very precise.



Of this you don’t have to convince me!

DISCLAIMER: I am not talking against VSD/VFD in the following (they do have several advantages):

VFD/VSD vendors always compare their system application in a refrigeration system that is grossly oversized and it is obvious ANY form of capacity control would improve efficiency too!

Never seen a study of one compressor with VSD against a rack, or compressor with VSD against a compressor with capacity control. I don’t think these studies haven’t been done.

Not letting head pressure float or suction pressure float may be energetically inefficient. It does have other important advantages like simplifying control, correcting power factor, and in some situations save energy too, etc.

[Peter_1]

Great congratulations! But I think you must count luck in your side too, I wouldn’t recommend this though obviously can be done!


Luck?? At least 6 to 8 packs made this way, all before 1995 and all are still running, most even with the original compressors.
Does so much luck excist?

[GXMPLX]

Does so much luck excist?

Ask the lady who won 50 millon in the lottery!

I see many system with small hermetic compressors in parallel, and no equalization ... still wouldn't recommend to do this.

If you have the right maintenance guy anything can be done!