Direct Steam Injection or Plate Heat Exchanger

[StoneCold]

I am in a quandry about wether or not to buy a direct steam injection unit (Pick heater)or go with a Plate and Frame heat exchanger package. The service is for a hot water loop on a reactor. The water in the jacket is maintained near 65C as a gentle heat source for a vacuum distillation that is going on in the reactor.
I have estimated the load to be about 125,000 Btu/hr.
The 125,000 Btu/hr is the design load for the exhanger and the Pick heater is an off the shelf size and it will do about 700,000 Btu/hr.
Neither unit is very expensive. The Pick heater is about $8500 and the Plate and Frame unit is about $17000. The units are very similar except one is direct injection and one is a heat exchanger using steam. Both unit have a control valve and controller and a small centrifical pump that delivers about 30 GPM.
I have never seen or owned a direct injection unit. I like the concept, cost, and extra capacity, but I am worried about turn down. The load on both units will drop significantly as the material in the reactor aproaches 65C. The Plate and frame unit is has a condensate pump/trap to help with the turn down the pick heater does not.
I also would have to send my loop expansion water back to my cooling tower which messes up my cooling water chemistry some.

Opinions?
Experience?

Thanks
StoneCold

 

[EdStainless]

Direct injection gets around a number of issues.
(disclamer, we sell material to Pick)
I like the clean simple DI method.
I had thought that they made a smaller unit, but turn-down on them is excellent.



= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[rmw]

There are other steam/water heaters out there that are not DI, and have good turndown rates. Try Leslie, and Graham.

17,000 seems like a whale of a lot of P&F to me, without running the numbers on your heat exchange rate.

rmw

 

[quark]

Direct injection is economic when compared to HX if you don't have condensate recovery. Based upon your heatload and flowrates, I estimate heating up of circulating fluid by 5⁰C. By direct injection, you can extract further 35kCal/kg of steam when compared to HX. You have to bleedoff around 52.0991 l/s of water with direct injection where as condensate removal rate will be around 57.27 l/s, assuming 100% efficiency(not proper to use this term) of HX.

Words of caution

1. You have to check for conditions of stall when using a HX. Pumping trap downstream HX can minimize this problem.

2. You have to operate the off the shelf DI heater at 17% of its capacity. So I foresee some problems with the steam valve control.

3. Use mixing nozzle in case of DI to minimize hammering.

Good luck,

 

[MJCronin]

Stone,

Talk with the PICK Heater guys...

As I recall, the have a technical article, that was publish in Chemical Engineering Progress, that discusses the use/advantages of this type of heater in reactor circulation/heating systems. They have many units installed in exactly this type of service

See:

http://www.pickheaters.com/application_bulletins/Chemical/ap_c1.pdf

The addition of a modest amount of pure condensate to your cooling water should,n't cause problems.

Please complete this thread and let us know what you decide...and your rationalle

MJC

 

[StoneCold]

EdStainless, rmw, quark, MJCronin
Thankyou for your input.
I have decided to go with the Direct Steam Injection unit.
I feel that both the heat exchanger and the direct steam injection unit will do the job. However I think that the added capacity of the direct steam unit can be utilized to shorten the process warm up times. I also think that maintenance on the direct steam injection unit will be less than the plate and frame unit, based on our tower water quality but only time will tell.

Thanks again for you input.

StoneCold