Mass Flow Rate with Big Condenser

[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.

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]

OK, it's offical..you're whacked again.

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!

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]

OK, it's offical..you're whacked again.

...

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!

[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

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
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]

OK, it's offical..you're whacked again.



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

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

[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!