Trying to establish if my cooling tunnel is performing as expected

[JoeReade]

I have a cooling tunnel that is being used to chill chocolate covered biscuits, but it does not seem to be performing as well as I would expect it to. I'm hoping that you can calculate either a)what performance I should expect from the installed cooling units and or b) what capacity the units should have to achieve the performance I'm looking for. Two sides of the same coin!
The tunnel is 17 metres long, 900mm wide and 260mm high. Including the enclosures for the evaporators, the internal volume is 4.5m3. The ambient temperature in the room it's being used in is 25c. It's insulated with 25mm thick polystyrene, and all internal and external surfaces are stainless steel. It needs to cool 28kg of chocolate covered biscuits from 30c to 12c in 8 minutes. ie that's how much product fills the tunnel, and the belt speed is such that they take 8 minutes to travel the full 17 metres of the tunnel.
The tunnel is fitted with 3 cooling units equally spaced down the length of the tunnel. Each one has a stated capacity of 2.75kw.

Is this combined capacity (8.25kw) sufficient?

We run the tunnel with all three units set at 6c. The first unit will not go lower than 13c. The second reaches 6c, and the third 8c. This is without biscuits passing down the tunnel.
In order to test the tunnel, I can close both ends so that escaping air is minimised. For the purposes of testing in these circumstances (ie closed ends and no passing product), how long would you calculate it should take for the empty tunnel to chill from ambient to 6c? Should it be capable of reaching 6c in these conditions and with this cooling capacity?

I hope I've covered all the necessary points for someone with the necessary knowledge to calculate this. If not, let me know!

I WILL SEND A CASE OF OUR DELICIOUS BISCUITS TO WHOEVER CAN GIVE ME THE HELP I NEED!!

Thanks

Joe

[hookster]

Lovely choccy biscuits for bribery, you should be inundated with posts!!

Chocolate setting is notorious in its complexity to avoid the dreaded fat blooms (not sure what your production specialists call it?)

Some further bit of information required though.
What is your product conveyor mass? (some description or photo of this would be useful. always tends to be forgotten and is it fed direct from automated baker (oven) to cooler or rested at ambient prior to entering setter?

Is the air recirculated at any stage or is it single pass?

What is your issue with product in the tunnel? Is it chocolate not setting or biscuit base becoming soggy or too much weight loss?

Fan speeds and volume of air pass is required along with evaporator surface area etc.
Rudimentary calculation could be done but I would advise that you rather consult a cooling specialist if it is for retail. ( I travel )

[JoeReade]

Thanks Hookster, you are first in line for the biscuits.... to answer your questions -
Conveyor mass - I don't think this will matter too much, because the conveyor returns inside the cooling tunnel, and only a short stretch is outside. Perhaps a small amount of required cooling should be added for this, but I think it will be minor.

Product temperature - the biscuits are at ambient (25c) and they get coated in chocolate at 30c immediately prior to entering the tunnel. Hence my figure of 30c for the product prior to entering the tunnel (in fact the temp will be a little less than this, since only 25% of the mass is chocolate at 30%, the other 75% which is biscuit will be at 25c)

The air is recirculated. Each end of the tunnel is open of course, but it's only a very narrow slit about 20mm high.

The issue is that in hotter weather, the chocolate is not setting within the required 8 minutes. Our refirgeration engineer has done all he can to find a problem (the obvious things like gas, valves etc etc), but still the performance of the three units is so different that I suspect at least one of them is not working as efficiently as it should. One can can easily reach 6c; another will only get to 8 and the other only gets to 13c. The manufacturer maintains that the cooling is sufficient, but in order to verify this, I need to know how the tunnel SHOULD perform in theory.
Afraid I don't know the volume of air passing through the evaporators or the evaporator surface area. Both are quite large relative to the compressor however I think, since it is designed to work constantly with minimal freezing, with the evap surface area at a relatively low Delta t (if that's the correct term - I mean a relatively low difference between set temp and evaporator temp). Can you make some assumptions about this for your calcs? Or perhaps i can find this out.

Thanks. biscuits waiting!!! Joe

[install monkey]

try more insulation on your tunnel- cheap test- get a roll of duct wrap- but u may need to seal it first as i dont like rockwool in my choccy bic's!!
also are the condensers located in direct sunlight?

[chemi-cool]

Did the tunnel performed well before?
If two of the cooling units don't reach the desired temp. have you checked why?
Are all the evaporators clean? Have you checked air velocity?

[hookster]

Hi I will try some rudimentary design spec but I would guess you rather have a configuration issue rather than refrigeration problem. Final temperature of chocolate needs to be a delta T of 10 degC (ambient to finished product)on tunnel exit to reduce condensation problems on chocolate coating.

Majority of tunnels that we build operate on superheat control for evaporators rather than set point on delivery air. Do your evaporators work on this principle or expansion valves?

You may find that it is dwell time on the design that is the issue not the refrigeration process as this may be at design limits with higher ambient and start temperatures

Initial load on first stage evaporator would be expected as 30 enter and 13 delivery with product 15-18
Second stage you will see as observed lower delivery
Final stage is adjusted to give final product ambient delta T of 10

As Chemi says check evaporators are clear and have your technician check superheats of all stages, a too low superheat at stage 2 will reduce efficiency if moisture on coil is freezing.

On a hot day measure your ambient, chocolate tempering temperature and check manufacturer data for bands you may find with the higher ambient your dwell time will require adjustment via belt speed. Do you rest product prior to packaging post tunnel?

[JoeReade]

Thanks Mr Hookster.
It's definitely a refrigeration problem with one or more of the three units. I ran a test yesterday by blocking both the inlet and outlet of the tunnel, no passing product and a benign ambient temperature of 20c. I set all three units to 6c, just to see what they were capable of. On unit got down to the set point in 7 minutes. Another took 16 minutes, and the third could get no lower than 9.5c. Each unit has the same volume of air to cool, none was more heavily loaded than the other. But very different performance.
It seems to me that one unit is performing fine, one not so well and the third very badly. Would you agree?

[r.bartlett]

In which case your refrigeration specialist should be able to work out what is causing the difference in performance.

[JoeReade]

Yes, absolutely. The problem is that he can't find a fault and the supplier maintains that it is working correctly. What I'm after is a calculation or an expectation based on the size of the units and the load that they have, of what the cooling curve ought to be - so that I can with some authority tell the supplier what we should expect. The difference in performance between the three units ought to be enough, but it's not.

To make it simpler, lets forget that its a cooling tunnel. If I block both ends, have no product passing through it and the conveyor not running - can someone tell me how quickly units of this size (3 x 2.75kw) should bring the internal temperature (volume 4.5m3) down from say 20c to 6c?

[r.bartlett]

download coolpack and put in the numbers

http://coolpack.software.informer.com/1.5/

I know you are remote but is there anyone else you can call on for an independant opinion.