Need to heat 400l/min water delta 40 Celcius

[roberto100]

Hi Guys,

Please help, I am attempting to solve the problem encountered by a dairy farmer in cleaning his milking machines.

The Cleaning system circulates water (containing detergents) starting at 80 deg C through the system with a flow rate of 400l/min for approximately 5 minutes. The problem is that the return water temp is often as low as 40 deg C. We need to heat the water back to 80 degrees before it goes back for the second, third, etc passes through the system.
The Dairy has an Anaerobic Digester producing excess gas which we intend using as our energy source for this heating.
Please advise.

Many thanks
Rob

 

[zdas04]

That is almost 86,000 kg/hr and 100F temp rise. The definition of a BTU is the amount of heat required to raise one lbm of water from 60F to 61F. If you assume that the BTU input is constant across your range of temperatures (which is reasonably close) then you need 24 MMBTU/hr at 80% efficiency. This means that you'll need around 600 MSCF/day which would be a really big Anaerobic Digester. Without the digester this amount of gas would cost around $7k US each day ($2.6 million/year).

For a heater of this size I would look at an oil pipeline "heater/treater".

You also might be able to mitigate some of your costs by preheating the return water by running the return pipes through the digester--those processes often generate excess heat.

David

 

[roberto100]

Hi David,

Thanks for your reply.

I only need to heat the 400l/min for roughly 10 minutes each day... do your figures take that into account?

Thanks again

 

[MintJulep]

I think you're off somewhere David.

I figure about 1.1 megawatt heater (3.8 million BTU/hr)

Needs about 320 cu. ft for 5 minutes of operation.

Those are "into the water stream" numbers.

 

[zdas04]

Ok, I was rushing to do the math before I left for a job this morning. I just loaded it into MathCad and got:

m = 23989 kg/hr
Q = 1.55 MW or 5.29 MMBTU/hr

if natural gas is 1.100 MMBTU/MCF then at 80% effeciency you would use 112 MCF/day or 780 SCF in 10 minutes.

Sorry about the messed up numbers before. 5 Million BTU's is a pretty big heater, but you can get tank heaters with that kind of capacity and somthing like a 100 bbl tank with a tank heater might be the way to go.

David

 

[Bribyk]

How much water is in the system? Could you not just have a large enough tank heated to 80 degC that you could run the system for ten-minutes?

That might cut down on capital cost of the heating equipment and would cut down the peak load on the digester as you would have 23 5/6 hrs to heat the system back up.

 

[DRWeig]

If it's once-thru, that's only 10 minutes at 400 L/min or 4000 liters...

Seems to me that Bribyk's idea is best. Make a tank of 80C fluid, use it, and take your time heating it back up.

My two cents anyway.

Goober Dave

 

[IRstuff]

Perhaps one can contemplate a tankless approach. The numbers appear to be against it, but that's just web surfing.

The largest commercially available tankless heater puts out abuot 10 gpm at a decent deltaT, so 10 of these in parallel would ostensibly meet the flow requirement.

TTFN

FAQ731-376

 

[DRWeig]

IRstuff,

That's a great idea, but still looking at either a really big gas or electric service to drive it (high demand). There's probably a happy compromise in the middle between all-storage with minimum power and all-power with minimum storage, but I'm not up to the math today...

Maybe someone out there has faced this one?

Good on y'all,

Goober Dave

 

[moltenmetal]

Far better and easier to store 4000 L of hot water than to store enough gas to fire this thing in 10 minutes-

It's a no-brainer- store the hot water.

 

[chicopee]

Roberto100, what is the size of the storage tank containing the water at 80 dC?

 

[Unotec]

Are your pipes insulated? Could they be insulated?

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[gruntguru]

Your process is using a lot of heat. Are you sure that 40*C return temperature exists simultaneously with the 80*C supply temperature, or are both temps cooling down gradually during the 10 minute cleaning cycle?

 

[roberto100]

Hi grunt,

The water is in a continous cycle;
We have a container/water heater containing water with detergent at 80deg C starting temp - the detergent water is then pumped through the milking machines and milk pipes and returns to the container where it mixes with what 80deg C water remains on its way through again. The return water cools to about 40 deg C at its coldest = end of wash cycle (i.e after +-6 minutes of circulating).
We cannot increase the quantity of water as this would increase the amount of detergent needed.
I envisaged a plate heater/heat exchanger situation where we could heat a large quantity of liquid to boiling point and store it to provide the heat for the heat exchanger. As I mentioned the wash cycle is +- 6 minutes in the am and the same in the pm.
Any suggestions would be appreciated. many thanks, Rob

 

[chicopee]

what you are not telling us are the size of the container(or storage tank)forming the water heater and the max and min outlet temperatures for the heater to kick in and shutdown during the washing operation.

 

[gruntguru]

Hi Roberto
OK - the first point is that the heat require is much less than originally calculated because your delta T is not 40 deg as assumed. To calculate the heat requirement we need to know either:
The true delta T. ie what is the difference between supply and return temps - measured at the same time. (Measure supply then return if you only have one thermometer.)
or
The volume of your storage container, initial temp, final temp and process time.

Either of these data sets can be used to calculate the average heat requirement of your process. It may be low enough to utilise the digester surplus.

 

[MintJulep]

What happens to this bacteria soup after the wash cycle?

How are the pipes rinsed after the wash?

As someone who drinks milk, I'm really interested.

 

[roberto100]

Hi Guys,
Thanks for all the replies,
The Water heater contains +-2000-2500l at 80+ deg Celcius at the start of the wash cycle. The returning water is at +- 40 deg C when it comes back, mixing with the hot water in the tank thereby cooling our wash water.
The water temperature seems to stabilise at about 55deg C by the end of the wash (after 6 minutes of circulating).

MintJulep, The wash water is flushed into the lagoon digester along with all the fats etc. Because the wash takes place immediately after milking stops there isn't sufficient time for it to develop into a bacteria soup!
However if we don't solve our returning water temp issue and leave milk fats coating the system pipes... then the bacteria soup starts...

Our milk is cooled from body temp down to 6 degrees immediately it comes out of the cow. from there it goes into a bulk cooler tank and gets cooled to below 4deg C.
i.e. No chance for bacteria to grow!! and really clean fresh milk!!!

 

[gruntguru]

Thanks for the info. The solution to your problem is dead easy. You have 6 minutes worth of hot water in the storage tank. All you need is another tank to receive the cooled water till the washing is finished, then transfer the water back to the existing tank to be re-heated.