Mass Flow Rate with Big Condenser

[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?

[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.