Thermal Dynamics - Where To Start

[kme]

Here`s the scenario:

A 2nd hand cask washing machine has been delivered today. It has an immersion element, to heat a tank of caustic, which is jetted around the inside of a beer keg, at a specified temperature, in order to kill and bacteria within.

The uses it previously had called for the detergent bath to be heated to 47 degrees C maximum ( a typical 2 wire stat & controller are used to fire or hold off the immersion element. There is no internal thermostat fitted to the immersion, which is normal for these machines.)

The use to which it will be put, calls for the caustic to be heated to 85-90 degrees C - allowing for the temperature drop when it is pumped around the inside of a metallic beer keg.

I know the physical dimensions of the caustic tank. I know the material from which the tank is made ( stainless), and I know the size of the element installed ( 6KW element, wired in single phase config).

How do I begin to calculate the immersion size needed to heat the volume of liquid to the required temp? Anyone care to offer any formulae?

Cheers

 

[kerching]

The last time i did any thermodynamics was whilst at Uni in the late seventies!

Now I have trouble remembering if i have had any breakfast

However

http://www.processtechnology.com/electricheatersizing.html

Maybe of little or no use to you

 

[kme]

Both helpful - thanks........( I think)

I`ve now got a 29,600 Kj figure, and a headache. 

I think the losses from the open surface, and unlagged sides of the tank, plus the steel pipework & pump, will be quite substantiative.

I think that this means that, if I allow a 30 min heatup period, from 20 degrees, to 88 degrees, I`d need 16.45KW of power.

If that`s the case - the woeful 6KW element currently fitted is going to have issues 

It doesn`t have to be precise - but I don`t want to size & install a radial for this thing, and find I`m waaaaaaaaaaay off in my loadings. 

I`m thinking of filling it with water, and seeing what it`ll do in an hour or two....

The first thing is getting it to temp.....

Then it must be able to keep it there, as well...................

 

[Evans Electric]

I could only offer the following KME ,   you say previous was heating to 47 deg  and now  90 is req'd   so I'm thinking double at least from the original 6KW  .     The Paddler is your only hope I'd say ,   all that floating about in canals helps. 

Personally I would contact one of the immersion htr technical reps , looking at 12KW or more , & get them to specify .    Protected from brown stuff meeting revolving air mover.

  

 

[Sidewinder]

So you need 29.6MJ of energy to rise the temperature of the liquid, yes?

How quickly does this need to be done?

From the Energy, kJ & the time in seconds you can calculate the energy in kWh (but you need to do some time conversions, obviously!).

1kW = 1kJ/s

So to heat it in an hour you need

29600*3600=106MW of power.

I think you need to check your calcs!

Also, you need to look at the process losses.

However, I think that 6kW is too small to be realistic, this depends however, on how much liquid you are heating.

BTW, I've found you a dozen reliable ind/comm sparks if you need them, all come recommended, and there are deals to be done, and, they are free next month.

I think we need to chat tomorrow.

 

[binky]

how many barrels an hour?

 

[Andy™]

if it doesnt heat up quick enough, light a fire under it.

If your stuck with starting a fire, just ask those at Didcot power station for some help...

 

[kme]

Binky - up to 40 barrels an hour - cycle takes approx 3 mins, and does 2 barrels at once.

Obviously, there is some temp. transference differences between 9 gal and 18 gal units; without getting involved in the ambient temp. of the barrels themselves...........

I think the decision has been made, to fit an 18KW TP unit - the existing cask washer holds 80 L of caustic, has 2 x 3KW elements, only does 1 barrel at a time; has lagging around the tank, and still occasionally drops below 80 degrees.  ( Snakey) The new unit holds 120 L of caustic - so I`m of the opinion that twice the power would possibly still be borderline. Because we have thermostatic control over the unit - the only downside to over-estimating the immersion is a larger supply cable - and given the other circuits going out, It won`t be an issue.

Thanks for all the help. Snake hips - I`ll call you tomorrow mate  

 

[Apache]

Is it possible to pre-warm the barrel with warm water? That's what we do with milking parlours before hot water circulating.

 

[kme]

The barrels are pre-rinsed with fresh water; held in a separate bath - but that doesn`t ( currently) have the ability to be heated...........Though we could drill the side of the bath, and stuff another immersion in - but I`ll bet that the Chiefs wouldn`t like that idea......... :slap

Actually - it wouldn`t be effective. The wash cycle is in 3 parts:

1. Rinse from unheated tank "A". Drained to waste

2. Wash from heated tank "B" ( Caustic). - recycled to caustic tank.

3. Final rinse from cold main. Drained to tank "A".