Wash Water Heating System Design

[Pavan Kumar]

Hi All,

I am tasked to propose and design a Heating system for a Wash Water system. The system I am thinking is as shown in the below copied sketch. I want inputs from you all if there is a better or more efficient solution that mine. My task at this time is to propose a solution, get budgetary cost for the system involved namely - Storage Tank, Pumps, Heat Exchanger, Steam Boiler, Deaerator etc. The client after looking at various options and cost will decide on the best solution. If this option gets selected then I will do the detail engineering as well.



A storage tank that holds city water heated to 40 Deg C is passed through heat exchanger to 80 Deg C and supplied to the users. The wash water after passing through the users ( used for cleaning reactor kettles etc ) is sent to the sewer. I have to check if they have to get this water to 60 Deg C before sending it to the sewer. The City water supply is at 4 Deg C and heated to 40 Deg C using a Preheating Exchanger. A main heat exchanger that heats the water from 40 to 80 Deg C will be designed for the required flow rate of 14 Litre/sec (222 US gpm). The storage tank that will supply one hour's worth of water of 50 m3 will be sized. I have not yet done the calculations but with this system I see that the steam requirement will be very high and make it un-feasible. I want to know if there are better ways to do this.

Thanks and Regards,
Pavan Kumar

 

[TBP]

Have you considered using the hot wash water going to drain to preheat the incoming cold make-up water? You could recover a lot of the heat (already paid for) plus cool the hot water before it hits the sewer system.

 

[Pavan Kumar]

Have you considered using the hot wash water going to drain to preheat the incoming cold make-up water? You could recover a lot of the heat (already paid for) plus cool the hot water before it hits the sewer system.

Hi TBP,

Thank you very much I did not consider that. That will save some heat load. I will calculate the required heat duty to heat from 4 to 40 Deg C and with the give flow rate of Wash water cooling from 80 Deg C to say 20 Deg C. That will be the basis for Pre-Heaters HT Area calculation.

Thanks again.

Thanks and Regards,
Pavan Kumar

 

[georgeverghese]

How could exit wash water at consumer HX be at 80degC also, must be much lower ? Is exit flow from users continous and constant - if not, a surge / storage tank for exit water will be required if you want to recover the heat in this exit stream at the cold water feed preheater.

If hot water demand is not continuous and constant, temp control will be difficult. One way to address this is to install a pressurised hot water storage / surge tank upstream of users and let the level in this tank vary between LCH and LCL. Alternatively, use a cascade control scheme - surge tank LIC cascading onto feed flow controller.

 

[pierreick]

Hi,
Can you use direct steam injection or are you worried about cross contamination?
You can recover sensible heat.

Pick Direct Steam Injection Industrial Liquid Heaters

Steam Injection Industrial Liquid Heating Systems offer application solutions for the process industries with unlimited hot water on demand.

www.pickheaters.com

Pierre

 

[Pavan Kumar]

How could exit wash water at consumer HX be at 80degC also, must be much lower ? Is exit flow from users continous and constant - if not, a surge / storage tank for exit water will be required if you want to recover the heat in this exit stream at the cold water feed preheater.

If hot water demand is not continuous and constant, temp control will be difficult. One way to address this is to install a pressurised hot water storage / surge tank upstream of users and let the level in this tank vary between LCH and LCL. Alternatively, use a cascade control scheme - surge tank LIC cascading onto feed flow controller.

The wash water is used for cleaning the vessels. There will be some drop for sure. Lets say by 20 Deg C. This is continuous as far as I know. I will confirm. As you are saying we Exit Wash water tank might also be required to use it to heat the cold feed water.



Thanks and Regards,
Pavan Kumar

 

[georgeverghese]

Yes, else the exit water temp to the sewer may be erratic.

 

[Snickster]

In any case the total heat input to the water is going to be the same. However, if you use plate coil heat exchangers attached to the outside of the tank, with insulation over that, to heat up the water you can slow down the heat transfer rate and required flow rate of steam. That is if you have enough time available to slowly heat up the water before it is needed for the wash water.

 

[udyman]

One option you could consider, assuming your steam pressure allows it, is to use direct steam heating (i.e. a pick heater). This would eliminate your condensate recovery system, and would replace both heat exchangers with simpler and less costly direct steam heaters. You would be able to absorb all the heat from the steam, and not lose any heat to condensate, reducing your steam requirement.

I would also consider heat recovery on the used wash water as others have mentioned.