Single stage refrigeration system optimizing diagram

[Sergei]

Let's talk about condenser capacity. Additional capacity will reduce condensing pressure and, usually, save energy. How long we are going to use this additional capacity? Condensing pressure set point is 150 psig( 10 bars). Additional condenser operates 1000hrs per year. Condensing pressure set point is 120 psig( 8 bars). Additional condenser operates 2000hrs per year. Condensing pressure set point influence on operating time of additional condenser.

Sergei

No system operates at design conditions, they merely cross them at times.

[autt]

Hi Sergei,

To make sure how long the system runs, we need statistical data of one area.

No system operates at design conditions, they merely cross them at times.

I am not sure about your meaning, in my point, not only at design condition, the condenser will also be energy saving at other conditions, do I get it?

Regards,
Autt

[Sergei]

Hi Sergei,

To make sure how long the system runs, we need statistical data of one area.



I am not sure about your meaning, in my point, not only at design condition, the condenser will also be energy saving at other conditions, do I get it?

Regards,
Autt

Condenser itself is not saving energy. You need optimum set point of condensing pressure to save energy.

Sergei

[TXiceman]

Condenser itself is not saving energy. You need optimum set point of condensing pressure to save energy.

Sergei

Sergei, not sure what you are tying to say. The condenser sizing will directly effect energy consumption and allow going to lower condensing pressures sooner. Run the SEER number on a unit with an undersized condenser and then rebalance and go to a larger condenser and allow the head pressure to drop and see what the SEER number does.

Generally oversizing a condenser a bit is a pretty effective way to improve system efficiency.

Ken

[Sergei]

Sergei, not sure what you are tying to say. The condenser sizing will directly effect energy consumption and allow going to lower condensing pressures sooner. Run the SEER number on a unit with an undersized condenser and then rebalance and go to a larger condenser and allow the head pressure to drop and see what the SEER number does.

Generally oversizing a condenser a bit is a pretty effective way to improve system efficiency.

Ken

Assume that we have refrigeration plant with 2 condensers. Third condenser was installed. If we have condensing pressure set point 150 psig (10 bars), we will run this condenser only 2-3 months per year (summer time). If we have condensing pressure set point 120 psig, we will run this condenser 4-5 months per year. During the periods of cool weather old 2 condensers will keep condensing pressure at 120 psig.

Sergei

[autt]

Condenser itself is not saving energy. You need optimum set point of condensing pressure to save energy.

The condensing pressure correspond with the condense temperature, so can but and just set one is ok, they have same effect in calculation.

autt

[Andy]

No system operates at design conditions, they merely cross them at times.

We all design based on the warmest summer day, which the plant never runs at and we wonder why refrigeration and A/C is such a large consumer of energy

Kind Regards Andy

[US Iceman]

No system operates at design conditions, they merely cross them at times.

I think this gets to the heart of the problem.

An installed system is most alwyas designed for worse case operating conditions (summer & full load). This typically only occurs for a few hours of the year (8760 hours annual).

The rest of this time the system has a lot of capability (most notably the condensers). Therefore instead of complex algorithms, the optimization would consist of fully utilizing the system to produce the required capacity at the lowest possible cost (kW & kWh).

If you measure the system demand and energy use and use these as control setpoints, then you get into the need for good control logic.

Imagine if you had a totalizer on each of the condenser fan motors and compressor motors (for example kW). The best discharge pressure would be the one that results in the lowest energy and demand, not necessarily the lowest dicharge pressure, right?

The next step would be to implement VFD's on the condenser fan motors for better control, in addition to the energy savings produced.

You might also use VFD's on the air cooling coils. In this case, you need to monitor space temperature to ensure it is being maintained properly.

In the case of VFD's on heat exchangers you get a very good benefit, such as energy savings on the fan, plus... The heat transfer surface is better utilized so that lower approaches can be obtained with less energy input.

I maintain the first case for optimization is not after the system is installed, but rather when the system is being designed. This way you can take advantage of the increased performance and still get lower energy use.

It does NOT have to be difficult...

[Sergei]

I believe in 2 types of optimization.
1. Design optimization.
2. Operating optimization.
Let's talk about design optimization.
Ammonia refrigeration plant. Evaporative condenser. First, we have to make decision about createrion of design. I see 2 options:
- certain condensing temperature(condensing pressure).
- certain size of condenser.

Sergei

[US Iceman]

Let's talk about design optimization.
Ammonia refrigeration plant. Evaporative condenser. First, we have to make decision about createrion of design.

Absolutely. Several factors enter into this;

What do we use for the entering wet bulb temperature? If we look at the updated ASHRAE data, there are two wet bulb temperatures to select from....

What do we use for a design condensing temperature? 95F (35C) or 90F (32.2C) or lower...

How much surplus capacity do we include in the condenser(s) selection? 0% extra or more?

If recip. compressors are used, do we include de-superheaters or not? De-superheating coils can help with scale reduction on top row of tubes and improve total capacity. Although, very few systems seem to use these anymore and I question if anyone remembers these???

And a very important factor.... LOCATION of the evap. condenser(s)!

[Andy]

Absolutely. Several factors enter into this;

What do we use for the entering wet bulb temperature? If we look at the updated ASHRAE data, there are two wet bulb temperatures to select from....

What do we use for a design condensing temperature? 95F (35C) or 90F (32.2C) or lower...

How much surplus capacity do we include in the condenser(s) selection? 0% extra or more?

If recip. compressors are used, do we include de-superheaters or not? De-superheating coils can help with scale reduction on top row of tubes and improve total capacity. Although, very few systems seem to use these anymore and I question if anyone remembers these???

And a very important factor.... LOCATION of the evap. condenser(s)!

I normally use 34 deg c, so 35 deg c would e the one I would be thinking of.

On desuperheating, there is two ways I can think of. Desuperheating with water for heat recovery, free hot water, or desuperhaeting for plume minimisation, a coil above the condenser to stop the visible steam plume from a condenser.

Kind Regards Andy

[Sergei]

Assume that location is properly chosen. I estimated that optimum condensing temperature would be 85F(around 30C) for wet bulb temperature 75F(24C). Criterion is total(compressors+condensers) power consumption. This is for condensers with axial fans

Sergei