Design of system

[Ashley]

Can anyone advise me of relevant and educated questions to put to a onsultant who has designed and sized a fridge plant. I am thinking of energy management and the fact that most systems are designed for full load conditions.

I have had experience in the past of a consultant specing air compressors up to 3 times actual max load and 5 times minimum load, but refrigeration is a new ball game to me. I did have an issue once where we were running 3 fridge units for a customer, but because demand was so low on the fridge system then the units kept tripping out. Don't want this to be the case in this new warehouse. It will not be used often once it is filled, so not a large amount of traffic into or out of the coldoorm is expected.

I know in the boiler trade there were times that we had a row of boilers coming in as and when reqd, but this preliminary design just shows a duty and standby unit.

Appreciate any help I can get

[hendry]

Ashley
energy efficiency is depends on which side you are representing.
these days, clients do not provide good data for any specialist to work on the design.
assumptions are the only fall back a specialist has; that explain the oversizing of plant.
i would not call it 'oversizing' anyway.
it is pretty much a bad gestimation on client's data & his operations.
sometimes, client wants quick pulldown but forget about redundancy in low peak seasons.
i hope this has answered your quest.

[US Iceman]

1. Is the system designed for efficient operation at minimum load?
2. If so, what have you done and how is it implemented?
3. During full load operation what have you done to minimize the demand charges for electricity?
4. How have you accounted for redundancy?
5. What have you done to minimize refrigerant charge?

These should get a real good conversation started!

[samiam]

Hi Ashley,

I assume there are manu coldrooms and additional rooms connected to the refrigeration system.

Ask him waht Diversity factor he has used?

Regards,

[Andy]

Can anyone advise me of relevant and educated questions to put to a onsultant who has designed and sized a fridge plant. I am thinking of energy management and the fact that most systems are designed for full load conditions.



Appreciate any help I can get

Hello Ashley

I have used inverter drives on large coldstores to match the load to duty during part load operation.

Questions to ask

1/What are the summer design conditions and COP for these
2/What are the winter design conditions and what is the COP at these conditions
3/What options can I add to reduce energy consumption and what is the payback on these
4/what is the estimated annual running and maintenance costs for the plant proposed.


I would go for one central plant not a run and standby plant to increase part load efficiency.

Kind Regards Andy

[Ashley]

1. Is the system designed for efficient operation at minimum load?
2. If so, what have you done and how is it implemented?
3. During full load operation what have you done to minimize the demand charges for electricity?
4. How have you accounted for redundancy?
5. What have you done to minimize refrigerant charge?

These should get a real good conversation started!

Thanks for that. Sure seems like that could stimulate conversation!!!! and I know 'experts' HATE being challenged, however at my CEng interview, the guy told me to continue challenging what we are presented.. even if it annoys people!!!!

Appreciate you taking time to respond.

[Ashley]

Hello Ashley

I have used inverter drives on large coldstores to match the load to duty during part load operation.

Questions to ask

1/What are the summer design conditions and COP for these
2/What are the winter design conditions and what is the COP at these conditions
3/What options can I add to reduce energy consumption and what is the payback on these
4/what is the estimated annual running and maintenance costs for the plant proposed.


I would go for one central plant not a run and standby plant to increase part load efficiency.

Kind Regards Andy

Andy,

I did wonder about invertors and if they would work. Thanks for the other questions too.

As regards duty/standby, the company stipulates that we must have it due to the product being held.

PS... Am I allowed to ask who you work for? Might we cross paths in our life?? Wee Norn Iron is a small place when it comes to work life!!!!

[Samarjit Sen]

Mike & Andy,

Your questions have provided me food for my thoughts. These are questions worth considering.

Thanks to both of you.

[US Iceman]

...the guy told me to continue challenging what we are presented.. even if it annoys people!!!!

That's what I call my imitation of a 2 year old child. If you have kids you know what I mean.

What do they always ask?

WHY?

Fairly soon you get under a persons skin who has trouble defending their position.

I also wanted to comment on the use of inverters. I really like them. We have completed several jobs with excellent success. These really help with part load capacity control.

Andy had another good comment I want to expand on; the central system concept. That is the only way to do this in my opinion. At part load conditions the excess condenser surface helps to decrease discharge pressures and if you float the suction pressure upwards you can get a significant boost in compressor capacity and COP. Not a bad thing to offer a client.

Good luck with your project.

[Ashley]

That's what I call my imitation of a 2 year old child. If you have kids you know what I mean.

What do they always ask?

WHY?

Fairly soon you get under a persons skin who has trouble defending their position.

I also wanted to comment on the use of inverters. I really like them. We have completed several jobs with excellent success. These really help with part load capacity control.

Andy had another good comment I want to expand on; the central system concept. That is the only way to do this in my opinion. At part load conditions the excess condenser surface helps to decrease discharge pressures and if you float the suction pressure upwards you can get a significant boost in compressor capacity and COP. Not a bad thing to offer a client.

Good luck with your project.

Yes... parents always told me that I was a 'why' person, however if you are guts enough to admit when you have reached the limit of your knowledge, then I admire that, but keep spouting out crap and I love to trip people up in it... until they are embarassed. Was anyone ever shot for saying 'I don't know?'????????? I know I haven't been yet

Ok.. will float the invertors issue if they have not already thought of it. I like invertors as well for startup

As I mentioned to Andy, the company want 100% back-up in case of failure. We cannot allow this product to go out of temp range, otherwise there are too many questions to be answered. Your second part to that paragraph is 'black magic' to me... I am the poor client who is being offered the system.

I too hope the project goes well.

[US Iceman]

Your second part to that paragraph is 'black magic' to me...

OK, let's kick this around then...

Most designers and control systems want to shut off the liquid line solenoids when the spaces achieve temperature. At the same time they want the leave the evaporator fans running (which adds heat back into the room).

As the room temperature is approached you have two options in the control system and design of the refrigeration system.

Once again the use of inverters on the evaporator fans offers a two-for-one benefit. The inverters slows down the fan speed, which reduces the motor kW input and also reduces the heat added to the space. Since the motor heat is reduced the required compressor capacity is reduced. In a cold storage warehouse this can be significant.

Now if we allow the fan speeds to decrease we can also allow the evaporator pressure to increase slightly (resetting the suction pressure higher during low load conditions). This increase in suction pressure causes the compressor capacity to increase, while the COP also increases. This is what I meant with the floating the suction pressure comment.

We do a similar function with the condensers also. If you allow the system to unload (reduced capacity to meet lower cooling demands) and still utilize the effective surface of the heat exchangers, the compressor performance increases and the operating cost can decrease.

What you see as a final result is less compressor operation and lower energy use. With this type of operation is critical to match the cooling load profile to the available compressor capacity.

Therefore, for redundancy you might consider (3) 50% compressors. Two compressors offer the necessary capacity for 100% load requirements while the third compressor offers the 100% redundancy for either of the two compressors.

You also want to compare this load profile with the unloading performance of the compressor selected. Reciprocating compressors are good for this also.

I am the poor client who is being offered the system.

As a client it's important to recognize what you are buying and getting. It's your money and you should be able to tell someone what you want... not what you are willing to accept.

...however if you are guts enough to admit when you have reached the limit of your knowledge...

I quite agree. There is nothing wrong or embarrassing about admitting you don't know something. Saying this and also stating I will find the solution is much better than bluffing. That only works in poker or fights!

[Sergei]

Hi, US Iceman.
I agree with you, but can you explaine a similar function with condensers.

[US Iceman]

Well, it's not exactly the same with condensers. Using the inverter to slow down the condenser fans must be balanced with minimizing compressor energy. In some cases it is possible to use more fan energy to decrease the discharge pressure (than what you save on the compressors).

In some climates the condenser coils are valved off to prevent low discharge pressure operation (and cold weather start-ups).

If you can use all of the condenser heat transfer surface it helps to minimize the fan energy input requirements in cold weather.

If you still have some questions, I'd be happy to add further comments.

[Sergei]

I think, that it isn't good idea to valve off condensers. First, sometimes it can damage condensers in spring time if somebody forget to open these valve. Second, during operation, low condensing pressure can happen if direct strong cold wind blow through condensers. Simple wind shield can solve this problem. Cold weather start up has anothers better solutions. It is better for efficiency if all condenser surface is available.

[Samarjit Sen]

Hello US Iceman,

There are lots to learn from your comments and suggestion. Now by reducing the speed of the evaporator fans when the temperature is achieved, we add cost by providing an inverter for the fan, suction and discharge pressure regulator. This is a good idea, but then the customer here always sees the overall cost. Could you please tell me the overall effect of the price involved and the savings achieved. Further could you kindly be more specific in your explanation as I would like to learn more about the same.

[Sergei]

US Iceman mentioned about floating suction and discharge pressures to save energy.
Suction pressure. Base temperature is temperature in refrigerated space and it is constant. Floating suction pressure, will change approach between space temperature and suction temperature. As soon as refrigeration load down, approach can be reduced and suction pressure(temperature) can go up.
Different story for discharge pressure. Base temperature is wet bulb one. This temperature will float itself. If wet bulb T is 60 degF and approach is 15 degF, condensing temperature is 75 degF. If wet bulb T is 70 degF and approach is 15 degF, condensing temperature is 85 degF.

[US Iceman]

Different story for discharge pressure. Base temperature is wet bulb one. This temperature will float itself. If wet bulb T is 60 degF and approach is 15 degF, condensing temperature is 75 degF. If wet bulb T is 70 degF and approach is 15 degF, condensing temperature is 85 degF.

I agree. The one important item to consider is the amount of condenser capacity installed to affect the approach temperature used. You also have to remember this only occurs at full load also. At part load conditions, the approach temperature decreases as the heat rejection of the system decreases. Therefore the discharge pressure is reduced.

Sergei, I also agree with your comments about not valving off condenser coils. I dislike this practice, but some systems are so poorly designed this is one last resort that people use.

[US Iceman]

...the customer here always sees the overall cost.

I understand Samarjit. However, the owners need to recognize the affect of return-on-investment (ROI). An inverter does cost money. But, if you look at using an inverter only (no motor starters or by-pass contactors) some of the inverters available today are almost as cheap as motor starters.

Now, the cost upgrade is almost negligible. Having said that, the ROI is then quite good. You have to look at individual cases to see how this applies.

I'm not sure what you meant about requiring discharge pressure regulators. I don't use them on ammonia systems. The use of these keeps the discharge pressure higher, which increases the operating costs.

[Ashley]

I understand Samarjit. However, the owners need to recognize the affect of return-on-investment (ROI). An inverter does cost money. But, if you look at using an inverter only (no motor starters or by-pass contactors) some of the inverters available today are almost as cheap as motor starters.

Now, the cost upgrade is almost negligible. Having said that, the ROI is then quite good. You have to look at individual cases to see how this applies.

I'm not sure what you meant about requiring discharge pressure regulators. I don't use them on ammonia systems. The use of these keeps the discharge pressure higher, which increases the operating costs.

As regards cost and ROI, that is where I am a customer that takes a slightly different view. I am prepared to look at whole life costs to determine if it is worthwhile or not, rather than just take purchase price alone. My dad once had an old truck, Leyland in fact, parts were cheap, easy got etc etc. he bought a Mercrdes and thought that he woudl be crucified for price of parts... HOWEVER, parts were dearer, but he never put very many on, whereas he was drip feeding the old Leyland. Lesson, Lesson!!

Had meeting yesterday with supplier. What they are proposing is a bank of 4 compressors for duty and same again for standby, with 1 compressor being 'digitically' controlled, so when I pushed him on this he came finally came clear and admitted that the compressor motor and the evaporator fan motors are all constant speed, but the valve out of one of the compressors is stepped to achieve low run conditions.

[US Iceman]

I might recommend you spend a few pesos with Andy to come to your site and help you to evaluate the system. It sounds like what you are describing is a rack system and having a complete rack as redundant system seems an expensive proposition.