better COP at winter

[andersnielsen45]

When I hat my theoretical lectures on refrigeration we always set the condenser temperature to about 35 C. This makes sense on a summer day but on a winter day maybe 10 C is enough, resulting in much better COP. My question is:

• Is it possible to adjust the pressure on the condenser side “on the run” based on outdoor conditions?
• If yes what do I google to read more about this technique?

hop you can help
Anders Østergaard Nielsen (DK)

[NoNickName]

Yes, what you are looking for is called floating condensation. The limit is the lowest condensation pressure and highest subcooling for which the evaporator is able to exchange the excess capacity resulting. The excess capacity can easily result in a lowering evaporating pressure for which a low pressure alarm is likely to happen.

[icecube51]

do they not by-pass the low pressure alarm at the condenser, and read the low pressure at the compressor?

Ice

[NoNickName]

There is no such a thing like a low pressure in the condenser....

[Sandro Baptista]

I would do the following for R717, as example:

Condensing pressure set-point: 9 bar (+25ºC saturated temperature) still guarantee a good pressure difference to expansion valves feed the liquid separators. If the discharge pressure tend to be sufficient slower (depend of the differential) the condensing will reduce the ventilation to achieve the set-point.

On the summer the wet bulb is higher and for design condition at 100% load would achieve +35ºC (12,5 bar).

At winter the wet bulb is lower and many periods the equilibrium compressors «» condensers will natural be at +25ºC condensing temperature.

[Segei]

I would do the following for R717, as example:

Condensing pressure set-point: 9 bar (+25ºC saturated temperature) still guarantee a good pressure difference to expansion valves feed the liquid separators. If the discharge pressure tend to be sufficient slower (depend of the differential) the condensing will reduce the ventilation to achieve the set-point.

On the summer the wet bulb is higher and for design condition at 100% load would achieve +35ºC (12,5 bar).

At winter the wet bulb is lower and many periods the equilibrium compressors «» condensers will natural be at +25ºC condensing temperature.

I didn't get about natural equilibrium between compressors and condensers. Can you give us more information?

[Aik]

Condensing pressure set-point: 9 bar (+25ºC saturated temperature) still guarantee a good pressure difference to expansion valves feed the liquid separators.

Totaly agree with Sandro Baptista, before reduce condensing pressure you must examine how will work expansion valves.

[NoNickName]

If the discharge pressure tend to be sufficient slower (depend of the differential) the condensing will reduce the ventilation to achieve the set-point.

That's exactly what he does not want to achieve. He wants the fans at full steam always, to have the lowest possible condensing pressure.

[icecube51]

There is no such a thing like a low pressure in the condenser....

quite wright, never IN but at the condenser. some mount them before the 4way valve, others after. but always around the condenser.

ice

[NoNickName]

That's news to me. As far as I know, the LP switches are in or around or at the suction pipe.

[mad fridgie]

Just let the condensor pressure drop either use mutiple vales rated at different pressure drops (or dare I say an electronic valve) or just go LPA

[Sandro Baptista]

That's exactly what he does not want to achieve. He wants the fans at full steam always, to have the lowest possible condensing pressure.

But in that way it can compromise the liquid feeding and besides that in some cases it can expend more energy because the reduction of discharge pressure is small (when the condensing temperature and the wet bulb are almost the same) and the COP of the compressors already doesn't change significantly. The mechanical power of the fans is proportional to the cube of rpm (N^3).

[Sandro Baptista]

I didn't get about natural equilibrium between compressors and condensers. Can you give us more information?

Segei I was try to say as an example that at a certain wet bulb at winter (low wet bulb temperature) the designed condenser with CT=+35ºC will give a lot of rejection heat so the equilibrium will be the decrease of the condensing temperature to +25ºC, as example.

[Segei]

We should talk about COPS(system COP). To optimize condensing pressure, total(compressors+condensers) energy use should be minimum. For wet bulb temp. 30C optimum condensing temp. can be 35C. To get 25C condensing pressure, wet bulb should be 20C or lower. However, optimum(minimum total energy use) condensing pressure will be lower at low wet bulb temp. For 10C wet bulb it can be 15C. Usually, a refrigeration plant has several barriers to operate at such low cond. pressure. These barriers are liquid supply(expansion valves), hot gas defrosting, oil carry-over, liquid injection oil cooling and etc. However, every barrier has a solution. I think that this is our job as refrigeration engineers to solve these barriers.

[Sandro Baptista]

However, every barrier has a solution. I think that this is our job as refrigeration engineers to solve these barriers.

Not every barrier...but almost
In general I agree with you

[Segei]

Not every barrier...but almost
In general I agree with you

Tell me a barrier that can't be overcomed.

[Sandro Baptista]

Tell me a barrier that can't be overcomed.

for example liquid injection oil cooling (LIOC system) when you have a evaporating temperature of....-5ºC for example... and you condensing at +15ºC.

[Segei]

for example liquid injection oil cooling (LIOC system) when you have a evaporating temperature of....-5ºC for example... and you condensing at +15ºC.

Solutions are 2 TEVs, EEV, LPA, maximum load of the compressor to 80-90%(not 100%) and etc. Choose the solution that is the best for your plant.

[Sandro Baptista]

okay men...you win...I'm tired to think on another barrier...maybe one day we talk again about this when happen an opportunity.

Wait...try this barrier: one R717 refrigerant plant without R717 or any other kind of refrigerant...how can it refrigerate??

[Segei]

okay men...you win...I'm tired to think on another barrier...maybe one day we talk again about this when happen an opportunity.

Wait...try this barrier: one R717 refrigerant plant without R717 or any other kind of refrigerant...how can it refrigerate??

This is not matter of winning or loosing.
My point is that energy savings is complicated issue. For many years we were focused on designing plants with minimum capital investments. The goal was to keep required temperature at maximum refrigeration load(design conditions). Surprisingly, even today many engineers estimate energy savings at design conditions. Plant operate at maximum refrigeration load less than 1% of operating time. How to run this plant for the rest 99%? So people start thinking about designing a plant that can operate more efficient during this 99% of operating time. This is just first step. To get maximum energy savings, plant operation should be optimized. It means optimum operating set points should be determined and implemented. Many companies in North America have given this optimization to the operators. They have achieved just partial optimization. In energy savings, it is easy to do first step, but every next step is harder and harder.

[Sandro Baptista]

This is not matter of winning or loosing.

I was joking...it was an "expression".

Good post Segei.

[NoNickName]

Funny enough, cars are operated at their rated power for less than 1% of the time. It didn't take 100 years to realise that, and they are designed for running economy at much less power than their maximum one.
Yes, time to revise designing criteria of refrigeration and a/c plants.

[Peter_1]

For an example, the Danfoss TES2, wit Orifice 6 at TE -10°C, gives 10.26 kW at 16 DP and 8,23 kW at 4Dp.
So the capacity is very close to each other despites a 12bar DP which is huge.
What we do: we select for a DP of 4 to 5 and we can drop the HP as low as we want, the TEV will follow, winter and summer. Most of our packs are running this way.

[AlexKram]

What we do: we select for a DP of 4 to 5 and we can drop the HP as low as we want, the TEV will follow, winter and summer. Most of our packs are running this way.

How can you do this? Is it with a variable speed compressor motor? Can you select the DP by adjusting the TEV?

The theoretical maximum Carnot COP of a refrigeration system increases as the condenser temperature is reduced.

I want to know if the COP of a real refrigeration system can be increased as much as the theoretical maximum Carnot COP change would indicate. For example, if the Carnot COP is doubled by reducing the condenser temperature, could a real system COP be doubled as well? Perhaps through the use of a variable speed compressor, and electronic expansion valve?