Direct Steam Injection vs Indirect Steam Injection 3

[bctarala]

I have a project for a process equipment which is using direct steam injection (motorized valve use to inject steam to water supply pipe) to produce hot water. I will convert it to indirect type using plate heat exchanger and steam as heating medium to produce hot water. Is my project will reduce energy cost?

Note: Condensate will be returned using steam trap on indirect steam injection

Thanks,
Benedict

 

[Terminus0]

There are so many things we would need to know to answer that simple question.

 

[DRWeig]

It seems to me that your change to indirect will increase energy use for the same amount of heat put into the water:

1. You'll need a pump on the water side.

2. The surface of the heat exchanger, pump, and extra piping will lose heat to the surroundings. If you are putting your exchanger directly into the water flow rather than pumping in a loop, you will add pressure drop. The pump supplying the water flow will have to raise its pressure to maintain the same flow.

3. You'll have to pump the condensate up to boiler pressure to return it, and you aren't doing that now.

Wait and see what the real thermo experts have to say, though.

Best to you,

Goober Dave

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[TD2K]

Have you done a mass and energy balance around the system to see how the two designs compare? I wouldn't expect much in the way of energy savings (gut feel, I haven't done any number crunching on it) but you will recover the hot condensate in your proposed design. What value you place on that condensate (versus having to treat additional raw water to turn it into BFW and bring it up to temperature) might be your major savings.

Currently with direct steam injection all the heat of the steam is used in heating up the cold water to the desired temperature. With an exchanger because you have a hot condensate stream being trapped off, you will use more steam to compensate for the enthalphy exiting with the condensate in heating up the cold water to the final temperature. The savings from recovering condensate in terms of chemical treating costs can be estimated from your current operation. The energy value of that hot condensate depends on your system. If for example you have a surplus of low pressure steam, the energy value of that hot condensate might be little or none. However, if you have to let down steam to heat up the additional cold BFW, there might be a significant energy savings.

Or it might just be the end of a long day and my brain isn't working

 

[EdStainless]

I agree, all that you can save is the cost of the condensate.
You will actually need a little more steam and there will be pumping losses.
In terms of raw efficiency you can't beat direct injection.

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Plymouth Tube

 

[ione]

Good advices from other members.

In a direct steam injection process you exploit not only the latent heat, but also part of the sensible heat of condensed steam, whilst in an indirect process you trap and discharge hot condensate.
Moreover in an indirect process you introduce further thermal resistances which reduces the heat transfer efficiency.
On the other hand the hot condensate pumped back to feed your boiler reduces the need of fresh makeup water and the need of raising its temperature (feeding a boiler with fresh cold water is definitely not a good way to go).
One last point, though this is probably not your case as the direct steam injection had been already adopted for your specific application. There are applications were the direct contact between the heating stream and the fluid to be heated is not allowed and so the only option is an indirect process.

 

[rmw]

Simple answer:

Diret heating is the best use of the heat available. Indirect heating is the best use of the water available.

With direct heat, you have to treat and heat the make-up water and that has a cost too. (You do treat the boiler make-up water don't you?)

rmw

 

[psafety]

When direct consider failure of vakving when the tank is empty. Stean condensing on interior surfaces will be your largest vacuum hazard.