Single stage refrigeration system optimizing diagram

[autt]

Hi everybody,

I have worked for optimization for a long time, but about optimizing refrigeration system, looks like it is seldom applied, maybe optimization is more complex, there is no mature and reliable optimizing method for systems like refrigeration, or other reasons as whole range search usually take unacceptable long time.

The optimization includes many aspects as factory cost, running cost, thermal performance, control performance, technology, structure, media as so on.

I should say not all of the aspects can be calculated, it is unrealistic to perform all optimizations in one step, the optimization here to talk is just the factory cost and running cost, thermal performance is used as calculating input. The other part of optimizing can be performed in other step, by experience or calculating.

I don't know other people how to do the optimization, here is my idea about it.

First picture is the pressure balance diagram, it shows how to get fan head by using upstream and downstream pressure calculating method. The fan head is used for fan selecting, and cost calculation.

The second picture is a basic single stage refrigeration system optimizing diagram.

Optimize object is sumed from each unit, used to evaluate system optimizing effect.

Optimize quantities is the selected parameter for optimize process to adjust. The optimize process modify these parameters slightly and do the system design calculation to get modification effect, and then modify parameters again and again by state of effect. These quantities have data range respectively to avoid unreasonably trying value.

Constrain quantities are usually the design requirements, some of them can just be inputed; the other are calculation output patameter, they can be selected together with optimize quantities, the optimizing process will check their data range after system design calculation finished, to estimate if the modification make sense.

Discrete quantities are usually the integer value, the optimizing method doesn't accept these quantities, so they are modified by hand after one step of optimizing calculation finished.

Anybody has ideas?

[Sergei]

First, you have to choose a criterion of optimization. What is your goal? (minimum initial investment, minimum operating costs or something else).

Sergei

[Josip]

Hi, Autt

I have worked for optimization for a long time, but about optimizing refrigeration system, looks like it is seldom applied, maybe optimization is more complex, there is no mature and reliable optimizing method for systems like refrigeration, or other reasons as whole range search usually take unacceptable long time.

This is true, for me refrigeration system is a living system changing inputs and outputs all the time (day, night, winter, summer, changing loads) what is not easy to optimize, but as you said at least trying to make a good design....

...optimizing process will check their data range after system design calculation finished, to estimate if the modification make sense.

and eventually make some modifications or make them possible.

But do not forget my signature...

Hi, Sergei

First, you have to choose a criterion of optimization. What is your goal? (minimum initial investment, minimum operating costs or something else).

Sergei

I do not believe there is much room for optimization on plants with minimum initial costs...

but....

Best regards, Josip

[autt]

Hi Sergei ,

The "optimize object" is criterion as you said, I think we use different words for same thing. It include "Cost" and "Running cost" in the diagram, "Cost" is the initial investment, "Running cost" is the operating cost as you said. The calculation works to find the minimum value.

Hi Josip ,

Our system use a different design method from you. First we select the "Design Point", this point is a critical point near the ambient of most hot and wet day state, The designer design a real system can work at the state of this point, for other state, use the control system to adjust to meet performance requirements.

To optimize the system with variable operating state, my idea is to add a coefficient to multiply the relevant parameter. For example the electric price, calculate the electricity consume hour by hour for one year, get the ratio of average power consume to that of critical point, then multiply the design lifetime or electric as the real parameter. It is the same as the Cu price, just multiply the increase coefficient of machining cost.

As I said before, if the optimize object is selected to "Lifetime cost", the factory cost may be rising, the HX size may be more large than before, but the system is good for energy saving. When this occurs, the decision depends on the boss .

Thank you two, keep post please, I will say more when I am back.

Regards,
Autt

[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

[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

[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]

Hi Sergei,

Both type of condensers will be used for same time per year. Perhaps you mean 150 psig condenser takes more heat than the one of 120 psig, so it runs less time? Not exactly, they have same capacity, that's why the low condense pressure one has larger size.

Autt

[autt]

This is true, for me refrigeration system is a living system changing inputs and outputs all the time (day, night, winter, summer, changing loads) what is not easy to optimize, but as you said at least trying to make a good design....

There are also other problems for optimize process. When one process finished, the result usually not the best, I have to set a button labeled "Run again" and press it once and once. Sometimes the result goes unreasonable, it is needed to check and modify some parameter by hand then run again, not everybody like this. This is by reason of the optim algorithm itself, maybe that's why the optimization not so popular as simulation.

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

[US Iceman]

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.

My best guess without spending any time evaluating this is, it is pretty close. My only concern would be the cost of the condensers to get the 85F (29.4C) condensing temperature with the entering wet bulb temperature at 75F (24C). That's a 10 degree F (5K) approach temperature, which is fairly ambitious.

If you don't mind paying for the condensers that is. Granted you did not say anything about money as a criteria, only design and benefits to operation.

[Sergei]

My best guess without spending any time evaluating this is, it is pretty close. My only concern would be the cost of the condensers to get the 85F (29.4C) condensing temperature with the entering wet bulb temperature at 75F (24C). That's a 10 degree F (5K) approach temperature, which is fairly ambitious.

If you don't mind paying for the condensers that is. Granted you did not say anything about money as a criteria, only design and benefits to operation.

Certainly, money is second criterion. To get acceptable payback, we should run this additional condenser capacity as long as possible. Two factors influence on this operation: refrigeration load and minimum allowable condensing pressure. Cold storages have significant load fluctuation between summer and winter. Usually, their winter load less than 50 % of summer load. However, production facilities have winter load 80 - 90 % of summer load.

[autt]

I need to do a simulation to compare two systems in future.

[lana]

Hi everybody,

Sorry for late contribution.
Interesting inputs.

Just a thought :
OPTIMIZATION is done in levels:
1- Design : make the design as realistic as possible and also optimize the design for energy consumption.
2- Control : when optimized design is put to work then control takes over. If control algorithm is good then operating cost will be optimized.
3- Maintenance : good service and maintenance would ensure that all design and operating parameters are within "optimized" limits.

For each level, tens of parameters must be taken into account and there are hundreds of research done.
My point is Optimization is not a single action. It is ongoing and complicated.
Cheers

[autt]

Optimizing covers so many aspects so I usually prefer to use "optimization" in the range of computable processes, mainly in design stage to express "optimized design", or "optimized control"(in dynamic simulating).

For system and control logic scheme selecting/choosing and things like these, I think they are engineers' work.

This is just my opinion of a classifing.

[US Iceman]

Just a thought :
OPTIMIZATION is done in levels:
1- Design : make the design as realistic as possible and also optimize the design for energy consumption.
2- Control : when optimized design is put to work then Control takes over. If Control algorithm is good then operating cost will be optimized.
3- Maintenance : good service and maintenance would ensure that all design and operating parameters are within "optimized" limits.

Now I believe we are getting down to the heart of the problem.

Optimization is traditionally viewed as the best point of operation. In most cases this is accomplished by leveraging the actual system design capabilities to achieve this optimal point.

The second area is being able to control the design capabilities to achieve the optimal operating point in any weather or any percentage of load.

The last (and probably just as important as the first two items) is maintenance. If the system is not maintained properly you potentially loose all capability of the first two criteria.

The definition of maintenance has many meanings like optimization has. I prefer to use the definition of maintenance in a manner equal to sustainability.

Sustainability is often used to describe "green design" or environmentally friendly or low impact to the environment.

If we think of maintenance as the ability to sustain the original design and control criteria, it tends to complete the circle as I see it.

And NO, I don't mean circular logic!

[nh3simman]

And NO, I don't mean circular logic!

Maybe it's fuzzy logic



[Sorry, couldn't stop myself here]

[US Iceman]

Maybe it's fuzzy logic

It could be that as it does get difficult to keep all of the details in clear focus.

[autt]

This diagram is used for program to optimize whole system thermal performance, so other influences are not illustrated.

Theraml optimizing usually use the static model. At the design point, control system can be thought as swith off. It works when real state diviate the design point. The control is usually looked as transient process, and optimized use transient model.

[TXiceman]

Optimizing a system is a very difficult thing to get a hand around. You have factors for the environment, system operability, longevity, maintenance, cost of installation and operating cost. all of these have to be considered at the desgn point as wel as off design operating points.

In order for an engineer to fully evaluate a system and provide the best system, the owner has to provide some design criteria for the engineer to use in his evaluation to seek the supposed optimum design. The owner needs to provide some information on cost of utilities for now as well as the foreseeable future, desired life of the equipment and cost of money. The engineer then needs to determine the load profile base on the use data, estimate a maintenance profile and cost, estimate the operating cost over the life cycle and also an installed cost. All of this has to be factored with the data from the owner to arrive at a single point cost of the equipment.

Trying to get some of the required information from the owner is nearly impossible. All to often, he leaves it to the engineer to come up with the numbers needed to fill inal of the blanks.

I do not know of a computer program that will simply let you plug in a few numbers and bingo, out pops the best design. This is where you start to get into "fuzzy logic" to make some soft determinations of which way the design should proceed. Do I put in a larger evaporator to raise the evap temp a degree and should I add condenser to lower the cond temp...both of which will lower the power input, but at the expense of first cost and installed cost. You can also put in slower speed compressors to improve realibility, but again, how much money can you spend in up front cost to save maintenance money and make the system run longer between major maintenance.

Trying to model this will still require the input of an experienced engineer to properly oversee the input and check the output. Remember GI-GO (garbage in- garbage out).

Ken

[kumaraudhesh]

ciat chiller one curcuit start & trip fom H.P and other plant one curcuit trip maximum overheating.

[autt]

Hi Ken,

I just talked about this at the New sub-forum thread by US Iceman, optimization covers so wide, we have to do it step by step, and use different tools in hand.

Integrating thermal and control optimization in same program will increase difficulty and make it impossible. To realize the optimization, processing them separately is a good way and efficient.

I'm trying to build the static simulate models to compare different design operates at non-design state, but there are still something wrong need to resolve.

Hi kumaraudhesh,

I'm not sure your words, you mean control logic?

[Ferdinand]

Hi All,

Im a new member, im working in a Semiconductor company as a reliability equipment tech. most of our equipment are using a refrigeration system, does any one from this room can help me to find a refrigeration system diagram w/ explanation.

[TXiceman]

Purchase a book on refrigeration by W. Stoecker. Excellent source of info. Also the first post onthis thread has two thumbnails attached that show a single stage Rankin cycle system.

Ken

[shooter]

initial cost is calculated on full power and worst conditions.
running cost is optimized by for example condensor power reduced to have a subcool of 5 celsius when higher a fan will stop. cheap but very effective.
most money win is at the expansion valve. please use electronic unit and try to put superheat as low as possible.

[alphi]

Hi Autt,

Optimization is a very wide angle to consider in terms of refrigeration as the Gentlemen said.

a. Design calculations is initially the start point wherein a lot of parameters need to consider as such,
1. variable loads
2. Atmospehic temperatures; high and low
3. Atmospheric impact and Quality of governing media
4. Laws and regulations (sanitation, etc.)
5. Available new technologies like automation, as such..BMS
6. Future development plans
7. Capital cost and Return of Investment
8 Project Management - time frame

"In my hometown(Phils.), I was a member of a Project Team to build the first State of The Art Ammonia refrigeration facility and certified as "AAA" food warehousing company. It was a successful product of workmanship and teamwork." The 8 points stated above were some of our refernce points.

kr,
alphi

[autt]

Hi,

Yes low superheat will increase evaporate temperature that can increase running efficiency, also a larger evaporator or condenser have same effect.

I had checked a condenser made in EU or USA, which installed on roof of building of beijing mobile telecom, has about 40% extra size than the design of normal 10 deg temp difference at criticle condition, this may reduce the condense temp but increase initial cost. As I know some small scale refrigeration plants in china, they produce condensers and evaporator with even 100% or more extra size for short time requirements, because they can't make accurate calculations and they have no time to test.

But how much extra size is the best? This is one of the optimization to do. For my optimization, many optimized values are located in recommended region, but not all. I think the thermal performance, initial cost and the running cost are the basic requirements, a system mismatch these requirements will not be economical, whatever how to control or maintain it.

Programs need conditions supported by designer, need the understanding of designer, or it may be harmful. But for a good designer, even a small program is better. I work for airborne thermal systems, they have same problems as everybody says, parts quality/select/arrangement, varible condition, control logic, airport sands, ventilating noise, personal ejector air speed, mixing air inside cabin..., so how should we do? We have to do, every gram is important, the key point is to create the criteria of "best". Only god can optimize everything, but we can devide the job to several parts and select the best combination to approach god, or waiting for god to do it.

Regards,