Undersized evaporator?

[crocens]

Hello all great people :)

Today i had a danfoss training on Electronic Expansion Valve. Nice mister said, that with that EEV we are able to get condensing temperature quite low (for example in winter). In resault we get more cooling power from compresor and lower power (electrical) consumption.

I agree with both of them. But...
I understand that power conumption in lower. i agree that we have savings here.
I asked how can we get use of that higher compressor cooling power when we have still thesame evaporator. Nice guy told me, that it does not matter and compressor will be running for shorter time. I cant understand it in any way. After all day of thinking i'm a little bit confused with all that.

Can any one please tell me is it true, that compressor will work shorter time or it is just a big bullsh.. ?

Thanks for replay,
Marcin Kroczek

[monkey spanners]

Say your tev system is evaporating at -5c and condensing at 35c, before the refrigerant can start cooling the pipes in the evaporator, it has to cool itself down from 35c to -5c.

But in a system with a eev still evaporating at -5c and now condensing at 20c (due to winter weather) the refrigerant only has to cool itself from 20c to -5c, which means the wasted work done in the first system can do useful cooling in this system.

This is my understanding of it.

Jon :)

[Tayters]

Hello all great people :)

Today i had a danfoss training on Electronic Expansion Valve. Nice mister said, that with that EEV we are able to get condensing temperature quite low (for example in winter). In resault we get more cooling power from compresor and lower power (electrical) consumption.

I agree with both of them. But...
I understand that power conumption in lower. i agree that we have savings here.
I asked how can we get use of that higher compressor cooling power when we have still thesame evaporator. Nice guy told me, that it does not matter and compressor will be running for shorter time. I cant understand it in any way. After all day of thinking i'm a little bit confused with all that.

Can any one please tell me is it true, that compressor will work shorter time or it is just a big bullsh.. ?

Thanks for replay,
Marcin Kroczek

Hi Marcin,
My understanding of the EEV is this. With a TEV you need a pressure differance across the valve for it to work. Say the system is like Monkeys example condensing at 35*C, evaporating at -5*C everything works. If the high side pressure is now lowered, now it's 20*C, then it follows that there is less pressure difference across the TEV and it wont work as designed. It wont pass as much refrigerant and the evaporator will be starved. Some call this over-condensing. Not the right term. I think Danfoss TEV's need about 130psi across them.

With a EEV it doesn't need this pressure difference so the high side pressure can be lower as a result.

I think that may be what the instructor meant.

Cheers and Do widzenia,

Andy.

[Tradewinds]

Hi Crocens,
In think its because you can get finer adjustments out of an EEV and have better control over your evaporating temperature. By raising the evaporator temp your will be increasing the refrigeration effect and subsequently get more cooling out of the system.

[Aik]

Hi Marcin!
You are right, if you have one compressor without capacity control and one evaporator then you can't use thise energy saving technology. But if you have for example multicompressor plant and many evaporators such as in supermarkets you can save up to 25% energy input.

[Peter_1]

I agree completely with Aik, the savings due to the lower condensing pressure are partly lost again due to the lower evaporating pressure. The penalty of lower evaporating pressure is bigger then the same pressure loss (same DP) on the condenser side.

You can only see what the final result will be by plotting it on an equilibrium chart and see what the new equilbrium point will be. That's how Danfoss is selling this product and not telling the whole story.

But Aik, you"'re 100% right about the savings but this statement is also valid for TEV's, most Danfoss TEV's can work with a DP of 4 bar, so evaporating at -10°C and condensing at 20°C is possible. We have a small pack running this way and it runs fine in winter and summer.

Increasing the evaporating temperature - if possible of course - gives you the biggest savings.

[crocens]

Hi Marcin,
My understanding of the EEV is this. With a TEV you need a pressure differance across the valve for it to work. Say the system is like Monkeys example condensing at 35*C, evaporating at -5*C everything works. If the high side pressure is now lowered, now it's 20*C, then it follows that there is less pressure difference across the TEV and it wont work as designed. It wont pass as much refrigerant and the evaporator will be starved. Some call this over-condensing. Not the right term. I think Danfoss TEV's need about 130psi across them.

With a EEV it doesn't need this pressure difference so the high side pressure can be lower as a result.

I think that may be what the instructor meant.

Cheers and Do widzenia,

Andy.

Dzień dobry Andy :) And hi to all. I understand difference betwen EEV and TEV. I just dont understand how we can get "bonus" from lower condensing. I know that compressor will have greater cooling capacity and i agree, but we still have the same evaporator so how the compresor can work for shorter time? Lets say we have 10kW compressor and 10kW evaporator at -10/+40. Now we have -8/+20 (EEV) and we get 13kW compressor capacity but still about 10kW evaporator capacity. The nice guy show me on log(p)-i (log(p)-h) chart that there is ingress in capacity when condensing is lower. And I agree!, but i think we are unable to use it becouse of undersized (in -8/+20) evaporator.


Hi Crocens,
In think its because you can get finer adjustments out of an EEV and have better control over your evaporating temperature. By raising the evaporator temp your will be increasing the refrigeration effect and subsequently get more cooling out of the system.

And that i understand and i agree.

Hi Marcin!
You are right, if you have one compressor without capacity control and one evaporator then you can't use thise energy saving technology. But if you have for example multicompressor plant and many evaporators such as in supermarkets you can save up to 25% energy input.

Ok, but we will be reduce capacity of comressor to fit evaporators capacity. that will save a lot of energy for sure, but it wont cut the time the compresoor was running.
The problem is that the nice guy said, that single compressor with no capacity control will work shorter time on single evaporator with EEV becouse compresor capacity will rise. For me it will be just to big and evaporator will be undesized in that moment. I think this can be only a problem...

I entice you to dialog over that problem :) Is the danfoss guy right or not...

[crocens]

I agree completely with Aik, the savings due to the lower condensing pressure are partly lost again due to the lower evaporating pressure. The penalty of lower evaporating pressure is bigger then the same pressure loss (same DP) on the condenser side.

You can only see what the final result will be by plotting it on an equilibrium chart and see what the new equilbrium point will be. That's how Danfoss is selling this product and not telling the whole story.

But Aik, you"'re 100% right about the savings but this statement is also valid for TEV's, most Danfoss TEV's can work with a DP of 4 bar, so evaporating at -10°C and condensing at 20°C is possible. We have a small pack running this way and it runs fine in winter and summer.

Increasing the evaporating temperature - if possible of course - gives you the biggest savings.

Ok, all in all mounting only EEV when we dont have a compressor with capacity control (or multicompressor), or we have no posibility to increase evaporator capacity (for example by haveing two speed funs) does not have any sens and there is no point in useing it?

[Tradewinds]

6566

If we raise the SST by way of better control with an EEV, we will reduce the amount of Work Done (WD) (saves electrical power) by the compressor and increase the Refrigeration Effect (RE) (more cooling capacity) from the evaporator.

The same happens on the high side. If we lower the SCT we reduce the amount of WorK Done by the compressor (saves energy).

He might be saying that if you lower the SCT too much during the winter then an EEV is a better way of maintaining the SST because it is not limited so much by pressure drop and it can give finer control.

[Aik]

But Aik, you"'re 100% right about the savings but this statement is also valid for TEV's, most Danfoss TEV's can work with a DP of 4 bar, so evaporating at -10°C and condensing at 20°C is possible. We have a small pack running this way and it runs fine in winter and summer.

Without any doubts you are right, you can use TEV's but if you choice TEV on +30°C then at +20°C TEV may be to small to work well. Acording EEV, one EEV has more wide range and you can sellect it whithout any problems.

Is the danfoss guy right or not...
IMHO, he was mistaken (about energy saving)...

[Peter_1]

Dzień dobry Andy :) ..... I know that compressor will have greater cooling capacity and i agree, but we still have the same evaporator so how the compresor can work for shorter time? Lets say we have 10kW compressor and 10kW evaporator at -10/+40. Now we have -8/+20 (EEV) and we get 13kW compressor capacity but still about 10kW evaporator capacity. .



Well, your Te will drop so the DT over your evaporator will increase (Q= m.c.DT) so the capacity of the evaporator will increase also.

[Aik]

Well, your Te will drop so the DT over your evaporator will increase (Q= m.c.DT) so the capacity of the evaporator will increase also.
What about electrical energy saving?

[mad fridgie]

You need to go back one step, when a heat exchanger is sized, in this case the evap, it is sized at a set of known conditions, for example a 6Kw evap at a 6C TD, could be also stated in general terms a 1Kw/C so lowering the evap temp increases the TD of the coil, and increases the duty of the evap.
By lowing the cond pressure, you increase comp duty at design SST and use less power, but to reach equalibrium the SST must drop, this will reduce the compressor duty but increase the evap duty. If on a single comp, cooling load will be reached quicker, less run time, so less power and when running lower power draw for net cooling obtained.
The EEV manufactures DO overstate performance for single comp systems, but can be more realistic for systems that have the ability to unload without any loss in compression efficiency (screw on VSD, multi compressor)

[crocens]

ok, i start to understand that problem, correct me if i am wrong:

Hipotetical situation:
We have cold room where we want 0*C
We choose compressor 20kW at -10 / 45
We choose 20kW evaporator
We choose EEV to control injection of liquid to evaporator.

Summer:
Condensing temperature 45*C
EEV is opend in... lets say 60%
We have compressor capacity 20kW
We have evaporator capacity 20kW

Winter
Condesing temperature :20*C
compressor capacity rises due to lower condensig temp., lets say to 26kW
what EEV is going to do, will it open more or rather close in those conditions? We are not changing anything in evaporator.

[Peter_1]

Your EEV will adapt its opening/injection so that the preset SH is maintained at 10K (you choose 10K as SH) You can simulate all this with Coolpack (for free to download on DTU)

[crocens]

ok, i put wrong question, a should as like this:
will the evaporating pressure drop (as i think) or rise a little big (as danfoss guy sais about 2*C) when compressor capacity will rise from 20 to 26 kW?

[Aik]

At equilibrium evaporator capacity is equal to compressor capacity. Evaporator capacity linearly depends on differential between cooling air and evaporating temperature - more differential, more evaporator capacity (very simple). For equilibrium if compressor capacity rise then evaporator capacity also must rise, but evaporator capacity may rise if evaporating temperature decrease (differential rise). Therefore if condencing temperature decrease compressor capacity increase but at the same time evaporator capacity also increase. As result capacity of your system, when condencing temperature decrease, will increase but not so much like if you campare with single compressor with constant suction pressure and decreasing condencing temperature, because suction pressure in system will also decrease.

[crocens]

So what do we have so far:

Lower condensating temp: more capacity
lower evaporating temp: less capacity
all in all we get more capacity, but not as much as danfoss guy said.

Next thing is, that when we have lower evaporating temperature, we can get faster evaporating iceing up and more frequent defrosting. If i am right then we have another point against using EEV with compressor without capacity control...

[Peter_1]

ok, i put wrong question, a should as like this:
will the evaporating pressure drop (as i think) or rise a little big (as danfoss guy sais about 2*C) when compressor capacity will rise from 20 to 26 kW?

Without any doubts: TE will drop and if Danfoss says otherwise, then I strongly suggest that this guy learn to plot this on an equilibrium graph chart.(= described also in the Dossat book)

[Peter_1]

So what do we have so far:

Lower condensating temp: more capacity
lower evaporating temp: less capacity
all in all we get more capacity, but not as much as danfoss guy said.

Next thing is, that when we have lower evaporating temperature, we can get faster evaporating iceing up and more frequent defrosting. If i am right then we have another point against using EEV with compressor without capacity control...


You're 100% right about this, and not to forget..changing RH levels in the cold room