Hot Water Generation

[mikeb]

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

[jstolaruk]

This would of course depend on the physics of your water network, but we would use a modulating diverter valve to control the amount of chilled water introduced into the hot water stream (using several temp sensors). Unused water would be bypassed back to the chilled water holding tank. Our applications are high pressure (5000-10,000 psi) and we've used http://www.valv-trol.com/home.html diverter valves for the last 15+years. The valve is modulated with variable air pressure. Hope this helps.

[comeng]

Hi Mike All the water blending valves I have used are fairly inaccurate but why bother why don't you use thyrister controls on your water heater and use a standard PID control sensing the the water outlet temp. Flow control just pick your valve and go I tend to use ball valves for most applications fitted with positioners they work well either 0-10V or 4-20ma remember to have your control valve after the heater so that you cannot get heater on and no water Phil

[mikeb]

Thanks for the replies. The water heater I was thinking of using is a Steibel Eltron DHE27, which is an instant hotwater heater 27kw, the heaters switch on when there is flow on the outlet. The temperature is set on a small display/keypad on the front. Its the sort of unit you might find in a hotel. I'm not sure that it could be controlled externally. Can anyone suggest a suitable water heater that culd be controlled.

[comeng]

If this firm does not have the answer I am not sure who will http://www.howden-electroheating.com/ Phil