Hot Water Generation

Michael Bell

Member
Join Date
Mar 2004
Location
Washington England
Posts
96
Hi,
Does anyone have any experience of this.
I need to supply hot water at 50degC at a rate of 10l/min. I cannot use stored hot water as the usage in 24hours will be up to 35000 litres. The water will be supplied from three water sources(different hardness etc). The water is to be used as part of a product development programme. We are bidding to supply some automation to assist this programme.
The hot water generation is a part of the whole system.
I have sourced an instant hot water heater that can supply water at the rate I need (24Kw 3phase 415V AC).
I then have to blend the hot water with cold to achieve the correct temperature. The incoming 'cold' water will be between 10 and 20degC I do not have an exact fiure yet.
The water temperature could change to anything between 20degC and 50degC every six to eight minutes.(each 'test' lasts up to 8 minutes and could use 35l of water). So far I cannot find a blending valve that I can control, with either a 4-20mA or 0-10V signal to control the temperature. I think I may have to build my own system with proportional valves and temperature sensors.
There is one further complication, the actual water flow rate is another parameter and that needs to be controlled by a proportional valve. The Whole system must run unattended.

Does anyone have any experience of blending hot and cold water like this or does anyone know of a blending valve that will fit my requirements. The final system will be controlled by a Mitsubishi Q series PLC, so temperature sensors, analogs could all go back to it.

Huge thanks in anticipation.

Mike B
 
You could try a spirax sarco 3 way B series valve which would do the trick (http://www.spiraxsarco.com/us/assets/uploads/pdfs/TIS%20sheets/TI-1-620-US.pdf) I assume that both hot and cold supplies would have a common pressure source , all you need are a couple of thin thermowells to enable you to get the actual liquid temperature quickly , and I would think you can guarantee good stability . I would tend to drive the heater bank through an Eltron burst fire controller (about GBP900 for the size you need ) , and to have two control loops , one looking after trimming the discharge temperature from the heater bank , and the other operating the midposition valve to obtain final water temperature .

I would also tend to use the SP2 positioner , you can program in any correction for valve non linearity if you really want , and even if you don't need this function , it is worth the extra money for repeatability , as during start up conditions you are bound to use fixed valve positions to allow things to stabilize until you release things in to auto .

If factory steam was available , then I would use a plate type heat exchanger - or even direct sparge steam into an injector .
 
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Hot Water

thanks for your suggestions. I don't see how the valve works exactly. I can see 2 inlet ports and one outlet are the inlets controllable separately? Or am I missing something here?

I had hoped to use an 'off the shelf' heater such as the Stiebel Eltron DHE27. It is a large 27kw instant hot water heater, it heats when it detects flow on the outlet port. The output temperature is controllable by a keypad. I was thinking of setting it to say 50degC then blending it's output to the temperature I need. I am not sure if it could be controlled 'externally'as you suggest. Do you have any suggestions for any water heaters that could be controlled.
 
I'd probably let the water heater do it's job, and use a control valve to inject cold water as needed to regulate the temperature.

Pass the sum of hot water and cold water into a manifold, and through a static mixer or something similar. Use an immersion thermocouple well on the exit of the mixer for temperature feedback.
 
The three port valve has cold and hot in , and you got it , blended water out - a simple PID loop will control the valve position and thus blended water temperature - a couple of check valves to make sure that nothing goes back where it shouldn't , and away you go .Set the stat on the instant heater fairly high to give you a bit of a reserve - you could use a feed forward as well , and you should use a very simple fixed position control to allow temperature to stabilize quickly after startup . Since the Eltron has its own controller , leave it to do its own thing . You should install a simple swirl pot after the 3 port , with the thermowell in the swirl pot , talk to TC Ltd in the UK for a quick response device .
 
I am doing this very setup right now at a bread facility. Flour , water and oil are batched into a mixer to create a dough to be used to manufacture Tortillas. The water temperature is a very important component of the process, as this water must be 85-95f.

To accomplish this, an 'on demand' gas fired heater supplies 140f water at a flowrate of 25 Kg/Min. Cold city water is then blended into the water line using a proportional mixing valve. Both the hot water and city water line are 'tee-ed' together, and appropriate check valves are placed on both lines to prevent back flushing.

Th Micromotion massflow meters used to batch the water also provide the Control Variable , Temperature, used in a PID to throttle the cold water mixing valve.

If you are interseted I can supply manufacturer specs on the mixing valve.

Ian
 
This is a link to the mixing valve

The hot water on demand heater in this application is set @ 140F due to local regulations regarding hot water systems. At this temp in a food plant environment, most bacteria residing in the hot water lines will be killed. The cold city water ranges from 50F to 70F due to the fact the main 6" cold water header resides near the roof of the plant above many ovens. Eventhough the water enters the building around 45f - 50F, by the time the water reaches the mixers, latent heat picked up at roof level heats the water up.
 
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Being an "electric" type of guy, the hot water I like best uses electricity, a lot of it.

Start with a Detroit Diesel 318 genset. 480 volts works good, 60 cycles is prefered locally, but you may use 50 cycles in uncivilized or primitive areas and third world countries.(You may subtitute Caterpillar, Cummins, or other).

Add a large tank of water, a large tank of Diesel fuel (Number one, Detroits don't like #2). Some 535mcm cable and three copper bars, too!

Attach one bus bar via a piece of 535 to each output terminal on the gen set. Bus bars should be insulated from each other, and the water tank, spaced about 18" from each other, and protruding into the water a few feet.

Add some 3 phase from the genset, "salt" the water too taste, and 50*C in short order. If water starts to drop in temp, just add more salt (or electricity).

Boy, I miss genset tests. Specialy the government tests when the inspector would say, leave the engine run, and open the drain valve for the engine oil and see if it shuts down.....
 
This is known as a "load test", that use to be done on generators, got new fangled things today.

NOTE: Adding salt will allow more current to flow but regular tap water (has minerals) will do that anyway so for heating purposes the salt may not be needed.
 
Casey , I am sure that it will shut down at some point .I am not sure what sort of person would allow a generator to be shut down using the low oil pressure switch - turbochargers and big-end bearings object to such things . The oil temperature would go up through the roof before the sump pick-up was uncovered - unless of course you use 60Hz oil which doesn't overheat .

Add some 3 phase from the genset, "salt" the water too taste, and 50*C in short order. If water starts to drop in temp, just add more salt (or electricity).
Interestingly , this won't happens - it will always rise in temperature , and I haven't quite worked out how you can "add more electricity" short of disconnecting the governors (though we only use "uncivilized /primitive /third world electricity ")





 
In the old days they did test oil pump shutoffs by draining the oil while running, the whole point of having it is to save the engine if does not have oil.

NOTE: turbochargers do not normally get lubricated from the system, they have them thar new fangled lubricated and sealed bearings.

Technically adding more salt will increase current i.e. raise temp.

Kasey (Casey) acts out but definitely been thar and done it afore.
 
Did similar myself for a tomato based sauce, the mixed stored product was around 20 degrees and prior to filling raised to about 70 degrees.

We used a spirax sarco to mix the steam and the product to raise the temperature at a fast flow rate (cannot remember the rate now), I had to set up 2 Eurotherm PID controllers on 2 control valves as the pressure was also important and the flow was variable from a pump driven via a VSD. Fun to set up if I remember correct.

(edit: the pressure control was on the steam prior to the mixing with the product, the pressure had to increase when the speed increased, if the pressure was too high when the speed was low you would invariably burn the product)

As this was in London, the water was very hard and unless it was treated would have completely screwed us up, so we had a de-ionised water plant which supplied water for the whole plant.
 
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rsdoran said:
In the old days they did test oil pump shutoffs by draining the oil while running, the whole point of having it is to save the engine if does not have oil.

NOTE: turbochargers do not normally get lubricated from the system, they have them thar new fangled lubricated and sealed bearings.

Technically adding more salt will increase current i.e. raise temp.

Kasey (Casey) acts out but definitely been thar and done it afore.

Only on the biggest of diesels will the turbo have it's own lube supply , otherwise up to 2MW it will be sump fed , and then genarally on two strokes . You don't save any engine if you run the bearings during the test - fair enough , check the function of the oil pressure switch (which may well be duplicated) , but to actually drain the oil on a engine proves very little , apart from a lack of knowledge !

Sure , increasing salt will increase current , as will decreasing the electrode clearance and/or size , but short of increasing the voltage , you cannot increase the electricity so to speak - all down to coulombs and moles .

I am sure Kasey has been there and done it , but I wouldn'r necessarily recommend that anyone goes there and does it be running an engine out of oil to prove a pressure switch - there are other ways , including using a hand pump - many oil pressure switches are checked for both states of operation ,so that if it fails , then a no start will occur , this is part of the control system necessary to bypass the switch to allow a startup .
 
Did not say it was something to do now but like I said, "the old days", you know back when they did not have control systems per se. You would, I was taught too, use a small pump to cycle oil through BUT with plug out you do not (should not) obtain pressure therefore system should shutdown quickly and nothing gets harmed.

Casey was just being funny, and I was attempting to also but do not forget that there was a time many of us had to work without all them thar new fangled gadgets like plcs etc.
 
I am sure Ron and countless others have had to deal with the hard-nosed inspectors, that read a spec sheet or a proceedure and take it literal.

The US Government guys were pretty bad, and the boys from DuPont weren't to far behind.

I almost got fired the day I refused to open the oil drain valve. After reality was expained to me in $imple term$, I opened the valve. Hey, it wasn't my money. Jusy my tax dollars at work!

The DuPont guys came out to the cornfiekds, 16 miles north of Peoria to the plant I was working at. There were two phone lines with six phones total. Now put 6-8 inspectors, bored out of their mines, wanting to call home to ask if Buffy got her braces today, or if Mandy went to the club for her tennis lessons. Meanwhile, the purchasing agent, sales manager, owner and customers are wanting to use those two phone lines. The office was in an old 4 room school, the shop was the old gym and a pole biulding addition. The engineering classroom had enough electricity to run the "schoolhouse" lights, clock, electric erasers (board drawings guys), blue print machine, and one box fan. The original pencil drawings went to the shop. Many came back without being burnt by the welders or covered with paint. "Same As" jobs were easy, there was a poloroid picture of everything that left. They had three crews that went out to correct mistakes and omitted equipment. Ah yes, 1986 was a GOOD year!
 

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