Need help with a dual compressor display case

I work for a small custom display case mfr. We have a client who wants several units but they are very specific in the sizes and design. The desired unit is an 8 foot long X 78" high, medium temp air screen. we build the same unit only up to 6-feet due to space limitations in the mechanical compartment. It must be self contained and use 404A refrigerant. We have built several with dual 3/4 hp condensing units feeding seperate coils and they work but ww need to improve the control circut. Any suggestions wil be appeciated. I f you need more info please use e-mail Thanks, Kengineering .......

HI kengineering,

If the only actual change in performance requirements is due to the increased length, why not scale up the refrigeration system to match the new length?

8/6=1.33, so the other performance data should be bumped up, say 33%.

The evaporator circuit lengths will grow, which may require a different coil circuiting arrangement to lower the circuit pressure drop. But then again, if the coil circuit lengths only change a little, the pressure drop may not be excessive for medium temperature operation.

The condenser (I'm assuming air-cooled) would also have to be increased, but I think a custom condenser coil (maybe a lower profile) could be cost effective, versus two separate refrigeration circuits.

Scaling the system up for a slight increase in performance might be easier than revamping the control methodology.

I think this falls into the category of: the Laws of Similitude.

http://www.humes.com.au/products/Sto.../FieldData.htm

This link has a very interesting discussion on the use of this "law". You do not have to be designing storm water systems, but the principles are the same. A similar thing is done with pump and fan design.

Some thoughts for you to consider.

I hope this is useful in your endeavor.

Best Regards,
US Iceman

Thanks for the reply This is the approach I would like to take and it's sound advise indeed. I am having trouble convinsing my supeiors. Seems they know just enough to be dangeorus but I'm working on them. I have also proposed that we reposition the coil to gain the mechanical space we need but this would change many things and disrupt the pricing I'm sure. But anyway; Presently we run Two 3/4 hp condensing units feeding identical evap through a 1/2 ton valve. separate electrical circuts but a common control circut and defrost. Off cycle defrost timer energizes common Penn A-19 T-stat controling a single Contactor breaking one leg of each 230 volt cond. unit. T-stat probe in front of the rightcoil. Our most common service call is Left coil frozen. Any more ideas? thanks again, Kenginering

A schematic of the electrical set-up which seems very basic, a picture of the case, should say more then 1000 lines of explanation.

Why not chose the OEM version of the Carel range for your product?

why not use eliwell control they are good reliable and use it on a pump down system

Ever looked t othe product range of Carel?

They're the only one who can control for example a pack direct with VFD controlled compressers based on a LP probe (you can even display evaporating temperature instead of pressure by setting the proper refrigerant used), insert capacity regulation with lifting cilinders together with the VFD, VFD controlled condenser fans, communicate in a network (supermarket), even over internet, set PID as you like, more then hundred setable parameters ... and for a price of +/- 500 € for the unit, 50 € for a pressure probe.
You can throw all the other stuff including PLC's in the garbage litter.

Dixell now introduced teh modbus protocol in their regulators so that it's possible now to communicate direct with a PLC for example. You can receive this protocol for free.

Where is Eliwell? I must admit, we still install most of the times Eliwell because I use them now already 20 years and they were the real pioneers of the rather cheap and reliable electronic controls. I kow most of the prameters out of my head.
We gonna change now to Carel.

kenengineering,

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Seems they know just enough to be dangerous but I'm working on them.

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This seems to be the common problem with management types. The simple, first try it seems they used almost always leads to additional problems, because...

Someone did not think everything through.

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Our most common service call is Left coil frozen.

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As you can probably guess, there is something else going on, which prevents the original concept from working.

Without a wiring diagram and more information/pictures, any additional help is all speculation.

I'm not sure how to post the pictures I will give it a go as soon as I get one.. Thanks for all your help so far..Being stuck in a production shop I don't get to see some of the new stuff out there. Where can I find info on the units you spoke of? K

Kenengineering,

If you want to post pictures, scroll down on the Reply to Thread page. There you wll see a button "Manage Atachments". From the pop-up window you can browse your PC and upload the files. Notice the file limit size though.

On the controllers the other members were discussing, do a web search for the company name. That should do it!

I used to work in a similar environment (shop fabrication). Ask your supervisor or one of the engineers if you can read their magazines and other subscriptions when they are done with them. You can also spend a lot of time doing various web searches for something you find interesting.

Keep trying and don't ever give up.:D

My supervisors look at each other when i ask questions on how they want things. That way I'm not responsible whhen things go wrong. I am trying to make a name for myself here and solving some of the majr service issues will go a long way. I dion't think they read any of the publications if they get them. That's how I gat the handle kengineering since I'm the closest thing. K

Hi,

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Originally Posted by kenengineering

My supervisors look at each other when i ask questions on how they want things.

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Definitely not a good sign.:D

You can learn a LOT by taking on the tough tasks that no one can solve (or wants to do). Sooner or later someone important will notice.

Just don't rub their noses in it, when you fix it. :p

Kengineering, did you ever get your controls selected for your project? I am curious what you used. As suggested, the Carel products would be a great choice.

I did ship the project after chooseing a simple Penn A-19 Electronic T-stat. Actually two of them, The low pressure controls that were standard copeland issue were used on the saftey side. I would like to use the Carel line but that would require a bit more time. Lots of red tape in this place. It used to be I told the purchaser what I needed and had it in a day. Now there forms to fillout and YUCK... Hey I hope to get a sample any day now.. I got the ball rolling anyway.

They hate you if you're clever and dispise the fool..Working class hero. J. L.

Ken

Hmmm. I wrote this without reading all the more recent responses. I will send it anyway, since it has things that were not discussed in the later posts.

If your left coil is freezing up, you could have many issues. But the first solution is to have 2 termination thermostats connected in series to the defrost timer. Don't terminate a defrost until both thermostats sense a 55 deg F discharge air temperture in the middle of the discharge airstream.

When you get to this size of merchandiser, the equipment should be remotely refrigerated. Energy, maintentance, temperature control, noise levels are all higher than is advisable, and even looking at a one year payback, Your salesperson should be able to show that going remote is a wise choice. Most convenience stores with 8-foot dairy cases use a 2 or 3 hp remote condensing unit. 7-11 used a pair of 4-foot Sanden sandwich cases which they speced out at 1 hp each because of temperature codes. Having two cases, self-contained makes some sense. At least one is working when the other fails. But an 8-foot self-contained sandwich case makes no sense unless you have a divider to separate the section. Otherwise, if one side has a problem, the entire case is compromised.

And if I understand you, you only have 1-1/2 hp trying to refrigerate an 8-foot multideck case. I don't see this working out.

Where are the pictures Ken? They're gone nnow.
Posting pictures can add so much more to this forum?
Remember, a picture says more then....;)

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Where are the pictures Ken? They're gone nnow.
Posting pictures can add so much more to this forum?
Remember, a picture says more then....

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Sorry I will try to get some shots of the Octagon display case I will soon be working on as well as the Sushi Ice Pan which is finally coming to production. It always seems the planing dept gets all the time in the world but production is on a tight deadline.

To Dan, I have been building similar cases for years and it is the self contained feature the sells the unit. Customers like that it just plugs in... We always recomend remote and floor drains for the best results but it's only a small percent of our producton.

thanks again, Ken