View Full Version : Help with capacity calculations

23-09-2016, 02:04 PM
First, I am sorry if this is a redundant post, I've searched through all the posts and online but I want to make sure I haven't made a horrible miscalculation. Secondly, I'm sorry if this is in the wrong topic.

So Now the part that's got my brain overworking.

I am in the process of redesigning a freeze tunnel for individual hash brown patties to get rid of our current spiral freezer. More headaches than its worth, belt is constantly breaking, limiting us to about a max of 5.7 ton/hr production, having to defrost a coil every hour (4 coils). I would like to suggest replacing it with a linear tunnel freezer, depending on cost is the biggest factor if it gets approved or not. I would like tol take the current time of freezing from 1 hr to about 10-15 minutes depending on production and how long the tunnel has run. We are a continuous 24/7 two week run facility, so we only shut down for a day every two weeks for major maintenance and heavy cleaning. I will be using a water defrost for the coil defrost method as that is what we currently have available and plumbed in.

I am basing the design off our other main line freeze tunnel that we run 54 ton/hr through without any problem until a defrost is needed about every 48-60 hours depending on what product we are running.

Design parameters:

Max design tonnage 15 ton
max running tonnage 12 ton (if you tell production they can only run 12 ton / hr they will try and push 14-15 ton / hr)

The freeze tunnel will be in two sections, a precool section to take the initial heat of the patties from ~190 degF to ~32 degF. This ideally will be accomplished in about 60% of the total time the patties are in the tunnel. The second section will be the main freeze section of the tunnel that takes the patties down to a core temperature of ~14 degF +/- 5 degF.

I used the basic calculation of
Q = BTU / hr
M = lbs / hr
Cp = Specific heat, above and below freezing
dT = temp change
L = latent heat

M = 30000 lbs / hr
Cp1 = 0.81 above freezing
Cp2 = 0.42 below freezing
T1 = 190 degF
T2 = 40 degF
T3 = 14 degF
L = 113.5 BTU/lbF

Q1 = 2460000 btu above freezing
Q2 = 934800 btu below freezing
L = 3405000 btu latent
Btu precool = Q1 + L = 5865000 btu / hr
Tons precool = 488.75 tr
Btu freeze = Q2 + L = 4339800 btu / hr
Tons freeze = 361.65 tr
tons total = tons precool + tons freeze = 850.4 tr

I know I am probably headed in the right direction but those numbers don't seem too accurate when compared to our current need of ~200 TR when running 5 ton/hr currently in a spiral freezer with the full capacity in it with no precool section.

Thank you for your help!

23-09-2016, 09:34 PM
I am coming up with different figures.
Unless I misunderstand you are including latent heat in your precool calculations. they do not belong there as there is no change of state in the precool, although I may have missed something. If so I apologize.
I presumed a freezing point of 32F, although it is likely a bit lower.

My figures:
Precool: 30,000 lbs. X sensible heat above freezing (.81) X temperature drop (150F) =3,645,000 BTU
Freeze section 30,000 lbs. X sensible heat above freezing (.81) X temperature drop (8F) =194,400 BTU
Freeze section 30,000 lbs. X Latent heat (113.5) = 3,390,000 BTU
Freeze section 30,000 lbs. X sensible heat below freezing (.42) X temperature drop (18F) =226,800 BTU
Total = 7,456,200 BTU/Hour 12,000 = 621.35 tons

23-09-2016, 11:39 PM
Hi Brian,
add base load to calculations to account for fan horse power and heat ingress through structure.

Interesting read about current spiral, the belting has an average life expectancy of 10 years max if ashworth type. With the need to defrost every four hours could be due to poor internal air balance drawing in massive amounts of moist factory air. If you are getting clouds of cold air coming out of infeed or out feed, then moist is getting sucked. Generally hash browns are low in moisture content after the flash fryer.

With proposed line tunnel, I would suggest a three section design, first section being a thermo syphon and standard evaporator coils to take advantage of ambient wet bulb condition
using and evaporative condenser to knock a large chunk of sensible heat out of product with out mechanical refrigeration. second section the pre chill and cyclic air defrost on multi coils, this section will do max moisture removal, without having to stop for defrosting third section the freezing area, and extremely low snowing of coils
The design can run 24/7 continuously only requires a deep clean every 2>3 weeks.
I worked on a french fry line of similar capacity. The air flow was vertical up for fluidizing product , your product would vertical down.

Have fun, magoo

24-09-2016, 12:24 AM
Thank you NH3LVR, I thought I was a little off. And yes the freezing point is a little lower as I found out today, its closer to 26 degF. So I'll fix my spreadsheet for the new data.

Magoo, thank you for your idea, I will see if I can fit that third section in the space we have to work with. On our main fry line we use a water coil pre-precool before the refrigerated precool section. Our main line is also vertical up air flow to keep the fries from freezing together too much, which isnt too bad until you have to crawl under the coil to replace the motor (about every week we loose one of 20 of our 20hp motors in the tunnel). As far as the spiral belt, its an itralox spiralox 2900 2.2 belt, 30 tiers go up configuration. It's about 5 years old from what i'm told and since I've started working here in march it has snapped in half at least a half dozen time, once a month, twice a week, depending on the week and conditions. Part of the problem is usually high tensions caused from a poor cleanup and excess fat left on the support rails, along with what I believe to be the wrong kind of material for the inner drive cage railing, UHMW instead of a higher coefficient of friction SS vertical bars. As far as the coils frosting over, yes it is from high humidity air getting into the freezer. For whatever reason the upper management decided to add new AMU's to the plant over the raw processing area instead of the packaging area and since our freezer is right in the middle of the two it basically acts as huge dehumidifier for the packaging area as the air moves from one area of the plant to the other.

As far as adding in more capacity for fans and other heat infiltration I was going to add about 30%, all fans will be 10hp. Now just to find coils with the capacity that I need that will fit in a 50' long main tunnel and 25' long precool tunnel, unless I use the three section idea and make each one 25' long... More to ponder. But I definitely like the idea of not having to defrost as often and make longer runs before a deep cleaning is required, if we can add another week before cleaning is required it would make a better case to install the new tunnel, as most of our down time on the line is from something breaking at startups or being out of grade for a few hours or shifts until things settle into their routine.

Thank you for your advice and ideas,

24-09-2016, 12:47 AM
Another quick question, does dwell time make a difference in capacity required?


24-09-2016, 03:12 AM
Yes the longer the retention time the less refrigeration load. Becomes a balance between design product flow rates and out put temperatures and how fast you can realistically achieve that for long periods, evaporator surface areas, fin coil design and progressive fin spacing for snow building on coils. the coils have to be purpose built, not your off the shelf units. Specify ammonia over feed. for efficiency.
Fan motors, should be spec'd as tropicalized with sealed bearings and basically sealed every where with low temp silicon to stop water ingress, totally enclosed.
There are in situ automatic snow removers available, basically a huge air compressor supplying air to sonic nozzles travelling up and down air on face and basically shocks snow off coils. you end up with snow drifts inside enclosure and snow on the product. Haha adds product weight though.
If you have a space problem go for a wider belt selection.
The spiral belt sounds like Atrax plastic, in a word do not even go there, totally hopeless with loads and temperatures , and drive drum sounds like a friction drive, the drum wear strips should be vertical up, friction drive s suffer from cooking oil and slippage so becomes jerky to look at which in turn sresses belt and loads so the they break. Cleaning solutions should be selected accordingly, cold foam and water rinse, be carefull the cleaning solution does react with all metal components.
Have fun magoo PM me if you want to.

24-09-2016, 08:05 AM
Thank you for the information magoo, I will definitely be picking your brain a little more here in a short bit, but now it's time to go to work.


27-09-2016, 11:00 PM
Hi magoo, I tried to PM you and the site said I didn't have enough permissions to send a pm so I think the message posted somewhere on your profile i think... I'm sorry for that.