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Peter_1
27-09-2005, 08:23 AM
I have a judicial case with a nitrogen tunnel for cooling (not freezing) French Frites from 20°C to 0.5°C.

I would like some feedback from you guys because, something is very wrong with following case, at least taht's my opinion.

They're cooling now 5.000 kg French Frites with nitrogen in a special constructed tunnel - same they're using to freeze products.

The estimate is a production of 5.000 kg/hour, 8 hours/day, 5 days/week and 50 weeks/year.

The price for 1 l Liquid Nitrogen LIN is 0,0756 € and they need now 0.27 to 0.30 l LIN/kg product.

In the tunnel are 10 fans of 0.75 kW and two exhaust fans of 1.5 kw.

They also have to pay a rent for the isolated LIN storage tank of 900 €/month.

The tunnel cost was +/- 125,000 €

I will not say more for the moment but those capable of doing some rough load calculations and the appropriate energy price for this, do once the calculation and compare it with how we, the classical refrigeration engineers should solve this problem.

What's your conclusion?

US Iceman
28-09-2005, 03:17 AM
Hi Peter,

If my numbers are correct I calculated about $240.000 € per year for only the LN2 system. Adding in the fan energy will increase this amount.

The last time I worked on one of these provided some interesting facts.

These cost less to install, but are extremely expensive to operate.

This is even more interesting since the cooling temperature for the product is only 0.5C. A fairly high temperature for a LN2 system.

A single stage refrigeration system operating at approximately 6C would be much less expensive to operate. However, the mechanical refrigeration system will cost more to install.

Using the LN2 system will make the french fries more expensive per kg, so the owners cost and selling price will be higher.

Did the installation people use vacuum jacketed piping to distribute the LN2 from the tank to the tunnel? This piping is expensive also.

Is this similar to your conclusion?

Peter_1
28-09-2005, 06:51 AM
That was exactly my conclusion: it consumes in 1 year more then it has cost.
But did you made once the calculation for the electricity bill when you should do this cooling with conventional compressors?
With the kWh price of Belgium, I came to a cost divided by 15!!
You only need +/- 60 HP compressor for this and will cost more, that's true but the higher price will be payed off in less then 2 years if you take only in account the LIN price. You have almost 500.000 $ you can spend and you never will need this amount.

The judge asked me in his conclusions that I also had to investigate if the seller has given the buyer the correct advice and if the buyer has told all the pro and contra's of his systeem so that the seller made the right decission.

hiitsme
28-09-2005, 07:03 AM
Hi Peter,

just thinking. Aren't there any restrictions??? (noise, vibration, use of refrigerants, energy consumption...etc)

Peter_1
28-09-2005, 07:18 AM
US Iceman, indeed vacuum Liquid lines which are indeed extremely expensive.

Hiitsme, there were no specs prescribed, only the fact that client would like to cool 5.000 kg/hour potatoes from 20°C to 1°C.

The problem now is that they can't go any faster then 2.000 kg/hour + all teh water is freezing around the nozzles to ice in a ball shape of +/- 15 to 20 cm.

I'm convinced that the gas company together with the tunnel manufacturer gave the wrong advice to this client.

botrous
28-09-2005, 12:08 PM
I'm convinced that the gas company together with the tunnel manufacturer gave the wrong advice to this client.

After reading you posts and US Iceman posts I bet that the gas company and the tunel manufacturer are a cartel.

US Iceman
28-09-2005, 02:52 PM
Peter,

I did not calculate the operating cost for the conventional refrigeration system. However, I am not surprised that the operating costs for the LN2 system are 15 times greater.

I believe the LN2 systems are sold on low first costs and quick installations. I doubt the LN2/tunnel salesman discussed the differences between mechanical and cryogenic freezing with the owner.

If the salesman did discuss the differences, I suspect the owner made a bad decision to save initial costs.

All of the above is only one of the issues based on your past posting.

The more important issue is the tunnel system does not work.:eek:

The tunnel is operating at only 40% capacity. This is probably costing the owner more money by loosing production, rather than the higher operating costs.

Some of these tunnels can be provided with pre-cooling chambers to absorb the entering moisture (product and air). This could reduce the nozzle problem.

How is the LN2 sprayed into the tunnel? I believe LN2 tunnels are counter-flow, and the CO2 tunnels are parallel flow. (fluid flow versus product flow)

This has to do with the CO2 generating dry ice crystals during expansion. The dry ice lands on the product and absorbs heat as the product flows down the tunnel.

Since LN2 does not generate frost during the expansion, the very cold gas is used to cool the product in a counter-flow arrangement.

The lack of production capability is a major concern. The operating costs are too, but at this rate the owner is loosing income very quickly.

US Iceman
03-10-2005, 09:44 PM
Peter,

Do you have any new information you can share on the liquid nitrogen cooling tunnel?

I am interested in the nozzle freezing and the loss of capacity the owner has been experiencing.

Thank you,
US Iceman

Peter_1
04-10-2005, 09:30 PM
Well, we did a test run last Thursday.

The maximum we achieved was a batch of 2950 kg/u, entering 19°C and leaving 7.8°C.

So, pure thermodynamically seen, we're missing a lot of capacity right now.

Around noon, the tunnel sudden cooled anymore. One relay output on the Siemens S7 PLC went broken. The LIN SV-safety valve wasn't energised anymore.

We bridged it manually so that we could continue.

Wednesday before the test, the manufacturer replaced in 2 years the 6th VFD and while changing this one in the switch-cupboard, they saw that another VFD went failure (number 7)

So I insisted in a second run next Thursday (this week)but the manufacturer postponed it till next week because they're not ready with the replacement of the VFD and the programming of the PLC.

We measured with an 8 channel PICO thermocouple recorder, connected to a portable
I took pictures of the nozzles, the tunnel, the product, the washing machines, the cutting machines but the camera is now not with me here.

I will make some pictures next week when I'm busy with measuring and also of the testcase we made with the PICO unit.

The manufacturer states that the French frites are entered to humid in the tunnel and that this increases the needed capacity. But this was a known parameter when making the offer for the tunnel.

US Iceman
04-10-2005, 10:54 PM
the manufacturer replaced in 2 years the 6th VFD and while changing this one in the switch-cupboard, they saw that another VFD went failure (number 7)

This is a lot of VFD's to replace in two years. Are the frequent replacements due to electrical problems or under sized VFD's?

Were the VFD's rated for the low temperature air? The amp draw on motors is much greater in very cold air. The VFD's may have been sized on nameplate motor amps, instead of the actual running amps with cold air.


The manufacturer states that the French fries are entered to humid in the tunnel and that this increases the needed capacity. But this was a known parameter when making the offer for the tunnel.

I think you will find this is one of the major problems you are facing. High moisture loads will deduct from the actual tunnel capacity.

I think this will be verified by the nozzle freezing.

I hope you have better luck with the next test.

Peter_1
05-10-2005, 06:56 AM
In that factory are +/- 30 VFD's, even outside the building some in switch cupboards.
The only one that fails are those in the tunnel.
I think they're not undersized for several reasons but I haven't checked it actually. Will do it next Thursday.
Reasons why I don't think: this company is making no more then 25 years N2 freezers Cooling applications are very rare.
The VFD's were rated for a freezing condition but we're in a cooling situation.

They just replace them without repairing and that's what bothers me.
Danfoss said that when they receive one for repair, they always give a repair report what was wrong , what could be the cause and what had to be done in the future to prevent such a failure.

Replacing the VFD's without solving the cause is carrying water to the see.
The VFD is internally (software) current protected.

And it's not only the VFD: also the Operating Panel the PLC, I/O's of the PLC, ...

What I have seen is on the stainless steel of all the equipment in the room (tunnel, stairs, containers..) are small rust particles. I will take some pictures of it.
I asked the owner what this corrosion was and he said that there is always some iron in a potato. This 'iron dust' is flying in the air and sticks to the tunnel, stair...The humid environment starts riusting this particle.
In the switch cupboard is a fan which pulls air from the room in the closet to cool the VFD's. Perhaps this particles are entering th PCB's and makes a short circuit after some time.

I insisted now that they give in the broken VFD's and that they ask for a very detailed repair report.

US Iceman
05-10-2005, 05:04 PM
Replacing the VFD's without solving the cause is carrying water to the sea.

A very good comparison. From the information you provided it seems the VFD's are experiencing repeat failures from some outside source; "iron dust", high humidity, insufficient cupboard cooling, etc.

Or, a strange electrical fault isolated to this system.

I think your idea of getting the inspection report is very good. This should help to isolate one of the problems.

For the low cooling capacity issue: Have you verified the liquid nitrogen use per hour? If you calculate the nitrogen flow rate for the 5.000 kg per hour cooling load for the product, and compare it to the flow rate used, this may help.

Are the exhaust fans pulling air through the tunnel? You could also calculate the nitrogen flow to cool the exhaust air pulled into the tunnel by the exhaust fans.

Comparing the total nitrogen flow rate used to the calculated flow rate required may show some interesting facts.

This is an interesting problem and I will be interested to see how it develops.