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kengineering
25-03-2006, 03:12 AM
I was always under the impresion that temperature follows pressure period. So if a cut out pressure setting produces a 20 degree coil temperature, which produces a 30 degree discharge air temp, which in turn gets you a 40 degree box temp ( all approximates) and nothing could change that. However, pulling down a nite shade closing an open self select merchandiser seems to produce lower case temperatures than when it is open and sometimes even freezes the product.

Could someone explain this. Thanks, Ken

US Iceman
25-03-2006, 03:46 AM
Hi Ken,

The big difference is the amount of ambient air that gets drawn into the boxes when the covers are removed during the daytime.

When the covers are pulled down for the after business hour periods a greatly reduced volume of ambient air is allowed to be entrained into the refrigerated air.

The lower temperatures at night are due to the decreased heat loads the refrigeration system is exposed to. Therefore the system actually has extra capacity.

This is why they can freeze, when it is normally just cold.

Dan
25-03-2006, 04:30 AM
I was always under the impresion that temperature follows pressure period. So if a cut out pressure setting produces a 20 degree coil temperature, which produces a 30 degree discharge air temp, which in turn gets you a 40 degree box temp ( all approximates) and nothing could change that. However, pulling down a nite shade closing an open self select merchandiser seems to produce lower case temperatures than when it is open and sometimes even freezes the product.


Sorry, I had to quote you to easily refer to the question.

I am okay with everything you said until the following:


However, pulling down a nite shade closing an open self select merchandiser seems to produce lower case temperatures than when it is open and sometimes even freezes the product.


Iceman's answers are on target, but maybe I can twist your thinking further.

You need to control temperature with a thermostat. Air temperature has no direct relationship with evaporator temperature. That's the short answer. But permit me to throw the question back at you.

The only constant is the low pressure control cut out setting which produces a 20 deg F coil. What have you changed when you introduce covers? You have turned an open display case into a reach in. Is it unreasonable to expect the air temperature to approach closer to the evaporator temperature?

kengineering
25-03-2006, 06:46 AM
Ok, so reducing the load is the reason the air temperature can approach closer to the evaporator temperature. Which stated another way would mean the TD of the evaporator (entering air temp vs. leaving air temp) will generally be greater with a higher load than with a smaller load.
So the 20 degree coil (I presented as an example) now produces a 25 degree dicharge air temp , instead of 30. Which produces a lower temperature in the product.
Does that sound right?
Ken

US Iceman
25-03-2006, 06:20 PM
Exactly right. The actual temperatures might be different than those you used as an example but the same logic applies.

The night covers are used to reduce the heat load on the display cases. By doing this the system run time is reduced at night.

I like Dan's response about using a thermostat. The air temperature is what you are trying to control, not the suction pressure.

If you control by suction pressure, the setting has to be adjusted for different operating conditions. When the night covers are pulled down you change the operating conditions of the display case.

jamcool
25-03-2006, 07:00 PM
So,is there a way to control the 2 temperatures? night temp with not alot of external factors compared to day temp. with shades up and all? the only way i can think is have someone physically adjust the stat for night or use maybe a timer or computer program as in buildings for comfort cooling,but then again what effect would it have on the product?:confused: Just thinking aloud guys,what you all think:)

US Iceman
25-03-2006, 07:05 PM
When the night covers are down, less ambient air enters the display case. As the case starts to cool down, the discharge air temperature off of the evaporator decreases. The thermostat shuts off the system.

When the covers are up, more ambient air enters the case and the system runs more of the time.

No need for two thermostats. One will work just fine.

Peter_1
25-03-2006, 07:07 PM
Many controllers in supermarket applications ((Radford, Danfoss, Eliwell, Dixell, Carel...) are having a special digital input for that so that a second higher setpoint can be chosen automatically when the curtains are closed.

Sometimes a light sensor detects the closing of the shop - shutting down the lights - closes the curtain and increases the temperature.

Dan
26-03-2006, 03:23 AM
Sometimes a light sensor detects the closing of the shop - shutting down the lights - closes the curtain and increases the temperature.

You have me curious, Peter. What temperature are you increasing with this robotic sounding controller?

Dan
26-03-2006, 03:27 AM
So the 20 degree coil (I presented as an example) now produces a 25 degree dicharge air temp , instead of 30. Which produces a lower temperature in the product.
Does that sound right?
Ken

Precisely, Ken. The return air (entering air) will be significantly cooler, thus the discharge air (leaving air) will be significantly cooler. Even with the same evaporator pressure settings.

Peter_1
26-03-2006, 12:21 PM
Dan,

The light switch: there are/were different systems.

Packo made long time ago a unit that was installed on the top of a counter and this device measured the lux of the store via an LDR.
If that went under a preset value - light out in the store - then some relays were activated to shut down the lights in the cabinets, closed the curtain and switched to a second thermosta with a higher setting.

We made this device ourselve in a small switch-cupboard.

The controllers used these days for supermarkets like Danfoss, RMS/Radford, Eliwell, Dixell, Carel.... all have a second build-in second setpoint, activated via a digital input.
Many of the standard controllers (Carel, Eliwell, Dixell, and CArel) have this feature as a standard.
You can trigger this input with an LDR, or an auxillary contact of the curtain and/or the cabinet lights.

So as soon light swiches out or curtain closes, the second setpoint is activated.

You then can increase or decrease temperature relative to the setpoint (a certain DT of the setpoint) or give in an absolute value.

Some devices (Eliwell and Dixell for sure) even can regulate the temperature according to the mathematical average temperature of the air-on and the air-off temperature.
This to avoid freezing during the night.

For a counter, we increase temperature +/- 4 K higher.
For a freezer we decrease temperature 5K so that we can use the benefits of the lower electricity costs and lower ambient temperatures of the night.

US Iceman
26-03-2006, 06:57 PM
After thinking about the use of two set points I am beginning to think this is a very smart thing to do. The resetting of the night time temperature could reduce the potential for partial freezing of the food at night (on coolers).

Dan
26-03-2006, 09:16 PM
Peter, is it possible, for the counters, that the temperature you are referring to is the evaporator temperature? I don't see why you would choose to warm up the discharge air temperature.

How you handle freezers intrigues me. In this case, are you actually permitting the air and product to get colder, sort of like a banking or fly-wheel system?

Peter_1
26-03-2006, 09:17 PM
Eliwell parameter H11 and E00 till E03

An IR33 from Carel has a very intelligent self learning defrost algorithm. It can learn from the previous defrost to optimize the next one.
This device is one with the most sophisticated software available and +/- same price of an Eliwell, so cheap.

Andy T
01-04-2006, 11:16 PM
Electrolux used a temp controler with an air on and air off the evap probes. Which calculated the product temp which you could set the air on/air off %. for use with night blinds.Worked well apart from the probes failing after time causing drift.

Peter_1
02-04-2006, 01:17 PM
Peter, is it possible, for the counters, that the temperature you are referring to is the evaporator temperature? I don't see why you would choose to warm up the discharge air temperature.

How you handle freezers intrigues me. In this case, are you actually permitting the air and product to get colder, sort of like a banking or fly-wheel system?

Noticed now you asked me a question.
For the counters, it's done for Carrefour even with controllers with 1 probe. The settemperature is raised some degrees. Evaporator temperature remains the same because they're mainly connected to a pack which keeps a stable suction temperature all the time.
We have a pack running on cold rooms (-2C) and meat cutting rooms (10C) The setpoint of the pack is increased when no cold rooms are asking for cold so that we can increase COP. There's also a 3th higher setpoint for night operation.

The freezers..well we store latent energy in the products and in the larger freezer rooms with a concrete floor, a lot of energy can be stored also in the floor.
A client of us with a big storage freezer room (+/- 15 x 15 x 10 m) runs his freezer (full of vegetables) from Friday evening till Monday morning at a strongly reduced electricty rate. He pulls it to -30C and it slowly rises to -20C till the Friday.

Test this once with your domestic freezer at home..run it the whole night in the 'Super' position and then switch it off in the morning. It will take hours before the thermostat will swich on the compressor.

Andy
02-04-2006, 07:58 PM
Noticed now you asked me a question.
For the counters, it's done for Carrefour even with controllers with 1 probe. The settemperature is raised some degrees. Evaporator temperature remains the same because they're mainly connected to a pack which keeps a stable suction temperature all the time.
We have a pack running on cold rooms (-2C) and meat cutting rooms (10C) The setpoint of the pack is increased when no cold rooms are asking for cold so that we can increase COP. There's also a 3th higher setpoint for night operation.

The freezers..well we store latent energy in the products and in the larger freezer rooms with a concrete floor, a lot of energy can be stored also in the floor.
A client of us with a big storage freezer room (+/- 15 x 15 x 10 m) runs his freezer (full of vegetables) from Friday evening till Monday morning at a strongly reduced electricty rate. He pulls it to -30C and it slowly rises to -20C till the Friday.

Test this once with your domestic freezer at home..run it the whole night in the 'Super' position and then switch it off in the morning. It will take hours before the thermostat will swich on the compressor.

Not strictly latent energy, as the product will have been frozen below the latent phase before entering the freezer.

But I know what you mean. Thermal storage using mass of the product and the floors.

Kind Regards. Andy:)

Peter_1
02-04-2006, 09:56 PM
You're completely right Andy, we passed of course the latent phase of teh floro and the products.
But If you count what the mass is of the concrete floor, then you have a huge storage medium.

Dan
02-04-2006, 11:55 PM
In a dormant or hidden stage

One of the definitions of latent, Peter and Andy. So not a bad usage of the word. But I prefer to think of latent heat being the change of state sort of heat. But, back to the subject:

We use floating suction setpoints with a target temperatuer of the most demanding refrigerator... ice cream and meat, for example. Nothing warms up, but we take advantage of letting the suction pressure rise. Using refrigeration inertia is an intriguing concept. It is almost the opposite to floating the suction pressure. Interesting.

US Iceman
03-04-2006, 12:20 AM
Hi guys,

The load shifting ability you are discussing is based on the thermal capacitance of the building and/or product being stored.

The more product in storage you have provides a higher thermal capacitance.

Think of it like this: Mass X Specific Heat

This provides a certain amount of stored energy at a specific temperature. After some heat is absorbed by the product, the temperature will change.

With cold storage facilities the very large volume of stored product allows the temperature rise to be minimized during the daytime with the refrigeration system OFF, or at some reduced capacity.

The added mass of the concrete also helps.

In a true load shifting strategy, the room is over cooled during the night (lower wet and dry bulb temperatures and possibly less expensive electricity).

During the daytime (when the wet and dry bulb temperatures are higher and the electricity is more expensive) the refrigeration system is idled to minimize energy and demand charges.

OCC Foodservice
10-04-2006, 06:39 PM
I would like to install these on my merchandisers/coolers and I am having the worst time locating a manufacturer of these night curtains/blinds. Can anyone refer me?

Peter_1
10-04-2006, 07:10 PM
Did this poster PM'ed you also with same message?

And this for his 1st post. You know what I think about this.

I learn my kids that they first have to introduce themselves. Am I old-fashioned?

US Iceman
10-04-2006, 07:20 PM
I received the same message Peter.

It seems our friend must have sent this to a lot of people posting in this thread.

OCC Foodservice
10-04-2006, 07:31 PM
Apologies for leaving out the introduction. My name is Jessica and I am a marketing manager for Aux Svcs at a community college in Michigan. I am not an engineer- so I didn't really intend to post often enough to be introduced. I was just desperately looking for some help/guideance on something that you folks may know more about so that is why I contacted several people in the same thread seeking the same information. But again, I didn't realize that there was specific protocol for such formal introductions in this forum- I did not mean to offend. Thank you.

coolman
10-04-2006, 07:44 PM
You have me curious, Peter. What temperature are you increasing with this robotic sounding controller?

Say you've a meat display cabinet that during daytime switches between -4 and -1C with curtains closed you would like to switch -1 and +3C so discharge air wil be well above freezing point and still within limits of the stored products.

Regards Victor

Peter_1
10-04-2006, 08:09 PM
Jessica,

No problem, it's just we had some troubles the last weeks and we want to keep some etiquette on this forum.

We're almost with 5000 members worldwide and there need to be some rules.
As you already perhaps have noticed, this is a very helpfull forum with posters from China to the US, Russia, Japan Belgium, the UK....

So your question: I can give you immediately 3 addresses in Belgium but you can't do anything with this.

Have a few days patience and posters from the US will see your question and I'm sure they will give you an answer.

HELLLLOOOOO, someone can help this friendly lady?

US Iceman
10-04-2006, 08:24 PM
Peter, some information has already been sent to her via PM.

Dan
11-04-2006, 12:59 AM
Yes, all I had to do was a search for "night covers for refrigerated display cases" and I found pages of results. I also recommended Economax and Chase doors, specifically, just in case my search results did not work.

I would have responded the same had I read her question here. But all I did was a search for what Jessica was asking and there were pages of results, so I am puzzled why it was so difficult finding manufacturers. Anyway, no offense taken Jessica.