Multipurpose Evaporator

[nambiandy]

Hi

One of our client here has a specific requirement.
He wants a Refrigeration system ( ***** based)which can maintain -20 degC room temp and also the same time when desired can be operated at 5 degC room temp.

as far as the compressor is concerened we can check its performace envelop but i am not sure of the evaporator which can match capacity difference at various condition.

Is it possible to have multipurpose evaporator?
Can the evaporator be designed for above condition?

Regards

Nambiandy

[US Iceman]

You have several problems that need to be addressed with this application.

The evaporators need to have a face velocity under 600 feet per minute (3 meters per second) to prevent water from being blown off of the coil when it is operating above freezing temperatures.

The coil will need some type of positive defrost method (electric or hot gas) to defrost when the evaporating temperature is below 32F (0C).

The TXV (if direct expansion is used) will also have to be checked for performance at the two operating conditions.

The drain pan piping will have to be heat traced to prevent freezing also.

The last comment relates to the fin spacing on the coil. This should be wide enough to prevent frost bridging across the fins when the evaporating temperature is below freezing.

What you want to do is not impossible, but it will require some thought to properly work.

[Peter_1]

We've done it several times.

We have a market truck where all the cabinets (4) can be used as a cooler or as a freezer, all connected to a VFD controlled compressor with an additional unloader.

The customer can use between 2 different evaporating temperatures.

You can do it with a 2 speed compressor, a VFD controlled one or one with un unloader system or a small pack with 2 compressors (one running for cooling and both for freezing)

Just some quick ideas.

An evaporator at -10°C with a DT of 8K has +/- the same capacity as evaporating at -30°C with a DT of 10K.
But you will need a 'freezer compressor' which has the same cooling capacity as the 'cooler compressor' but at different evaporating temperatures.
A compressor at -8°C / -10°C has +/- triple the capacity as it has at -30°C.
So, a rate of +/- 1/3

[lana]

Hi everybody,

If a system is designed for operating at -20°C evaporating then the TEV would be selected according. Now if we change the room thermostat to cut at +2°C then we have a room with higher temperature, but the evaporating temperature is still low.
I can not imagine such an operation. Maybe you can explain this more clearly.
Thanks:)

[Peter_1]

Lana,
Or you must accept that the room works with a DT of 25K to 30 K while cooling and take the necessary precautions to avoid overheating or you must install a capacity control system (controlled hot gas injection, VFD, unloader, 2 or more compressors...)
The TEV is not the problem, the unbalanced/uncontrolled compressor capacity is the major concern in my opinion.

[lana]

Thanks Peter.



you must accept that the room works with a DT of 25K to 30 K while cooling and take the necessary precautions to avoid overheating or you must install a capacity control system (controlled hot gas injection, VFD, unloader, 2 or more compressors...)


That is exactly my concern.
It is much better to have a room for specific conditions. There will be much less problems to worry about.:)
Cheers

[US Iceman]

you must install a capacity Control (http://www.refrigeration-engineer.com/forums/glossary.php?do=viewglossary&term=144) system (controlled hot gas injection, VFD, unloader, 2 or more compressors...)


That is a very good point Peter. By allowing the suction pressure to increase due to a higher evaporating temperature you would need to unload the compressor or some other way to change the compressor displacement.

[TXiceman]

At the higher ET, you r compressor will provide substantially more capacity if fully loaded. The TXV sizing will nedd to bechecked and the evaporator cross balanced at both load conditions.

If it is a coil, you will need to check circuiting in both cases. On some various designs, I have used multiple coils and also coils with two TXV's mounted, each sized for the specific application.

Usually on this wide of a temperature spread, you will wind up with seletion at some intermediaqte condtion that is compromise at the two extremes.

Ken

[nambiandy]

That is a very good point Peter. By allowing the suction pressure to increase due to a higher evaporating temperature you would need to unload the compressor or some other way to change the compressor displacement.

Thanks for the input from all memebers


I was just wondering if the suction pressure be controlled using CPRV ?

nambiandy

[Peter_1]

What's a CPRV?
If it's a evaporatin pressure regulator you mean, it will make it even worser, your crankcase pressure will go below atmospheric pressure.
Only installing some sort of capacity control will do it, the more expensive ones (VFD or unloader) will consume less energy .

[lana]

Hi,
He means :
CPRV : Crankcase pressure regulating valve

Cheers

[nambiandy]

yes , i meant Crank case pressure regulating valve (CPRV).

The CPRV will allow a constant suction pressure to compressor . it is widely used in blast freezeing application.

I think it can work for this application

nambi

[Peter_1]

I think you have to look once again closer to the working principle of a CPRV.
Thats now exactly your problem, your flow to the compressor varies widely when using it as a freezer or as a cooler.
You may not have a constant flow to the compressor.
So a CPRV will certainly not work in this case.

You must once plot on scale paper compressor refrigerant capacities verus evaporating temperature and condensing temperature, room temperature, then the evaporator and finally the condensor.
You then praphically ca nsee how everything will behave in a refrigeraton cycle.
I attached a pdf file how I teach it.

[Peter_1]

On the left, lines going from left upper to right under is evaporating capacity.
Different lines are for different room temperatures (kamer = room)
Lines going relative flat from left under to right up are compressor capacity lines. Those a litlle bit more inclined and higher in gray are needed condensor capacities (refrigerant capacity + absorbed power).
At the right, you see the graphs for different ambient conditions strating from 25°C ambient till 40°C, (in gray in teh middel along the lines)

ll these lines were simply drawn by taking 3 values out of the tables of the manufacturer. If you want to do it more accurate, you can take more readings but the lines can be drawn almost flat.

You now can clearly can see where the equilibrium point will float when room temperature or ambient or both changes.

[Peter_1]

Vertical axis is refrigerant capacity, horizontal is left evaporating temperature till 5°C and right condensing temperature starting from 25°C till 55°C.

Putting all this in a program (where you can add your own configuration) should be very nice. Isn't this something our member Autt from China has made already?

[lana]

Hi Peter,

What you explained is exactly what we (my brother and I) did in our thesis.
Mathematical solution for refrigeration balancing is done exactly that way.
The programme uses the manufacturers data to establish the mathematical equations for each component and then it solves the equations to find the balance point.

It's been a long time. I am getting old:D .

Cheers

[Peter_1]

If you draw it this way, you perfectly can predict how the system will behave when conditions change.
Nambiandy, it's perhaps a good way to understand completely what will happen and make once this exercise.

[Peter_1]

LAna, I never saw this graphical method in any handbook. And I have a closet full of books.
I learned it this way somewhere in 1980 and our teacher then had learned it via an article in a Dutch magazine.

[lana]

LAna, I never saw this graphical method in any handbook. And I have a closet full of books.
I learned it this way somewhere in 1980 and our teacher then had learned it via an article in a Dutch magazine.

Hi Peter,

In a book by Stoeker (not sure of the spelling) this graphical method is mentioned.
In the university we were taught this method and in our project we had to convert this method into mathematical one so a computer program could be written.
My project was with screw compressors and my brothers with recipes (one and two stage).
As you mentioned with balancing a lot of info can be obtained specially for part load operation. This method insures that the selected equipment are suitable for the job before buying them.

Cheers

[Peter_1]

I have 2 books of Stoecker, I will earch once in it how he explains it.

[TXiceman]

I believe it is in my early edition of the text by Wilhem Stoecker. I was also taught this method when I was at York GETP (graduate engineers training program) which was in the days of slide rules and no real access to the computer programs that are available today.

I'll try to remember to look up one that I balanced graphically and scan it in to post here fro reference. It is really pretty simple and does not take very long. It also goes a long way in helping a young engineer to understand what is going one when you balance a condenser, chiller and compressor.

Ken

[lana]

Dear Peter,

The book is "Refrigeration & Air Conditioning" By :
Wilbert F. Stoecker / Jerold W. Jones
Second Edition
McGraw-Hill International Editions
ISBN 0-07-066591-5
Chapter 14 : Vapor-Compression-System Analysis

Hope you can find it.
Cheers

[Peter_1]

Lana I have this book and also his book industrial Refrigeration.

Ken, I teach that taking 3 points/line is enough because they're goin +/- straight.
Have you taken more points?

I'm with you that you better understand what's happening in a refrigeration abd how parameters float if you do it this way.