Total Steam Plant Efficiency

[firefastball]

Hi there,

I'm trying to figure out how can I assess a steam process plant efficiency? not a steam power plant.
I try to scourge through the internet and most of my searches are related to a steam plant which is not my objective.

So consider steam is produced by a 15 ton/hr boiler at saturated 10 bar(a). Feedwater is coming in from the deaerator at 95 degrees C.
The steam is then supplied to a number of autoclaves and no condensate is return back to the system.
Makeup water is coming in at 32 degrees C
The boiler is using Fuel oil.

If I'm going to calculate the boiler efficiency, I would use the below method:
Boiler eff = 15ton/hr X (steam enthalpy @ sat 10 bar - FW enthalpy @ 95 C)/Fuel Oil flow X HHV.

How can I calculate the total steam plant efficiency then?
Does the below equation correct?
Steam Plant Eff = 15ton/hr X (steam enthalpy @ sat 10 bar - Makeup enthalpy @ 32 C)/Fuel oil flow X HHV

How would it change if there is condensate return? Say 50% of steam flow.
Does the total steam plant efficiency is usually lower than boiler efficiency?

The reason is I would like to calculate the increase in efficiency if we do some improvements to the plant.

Thanks. Sorry for the long question

 

[racookpe1978]

Useful "load" is the total heat you need into going into the process heat exchanger - not the "electric power sent to the breaker" as in a power plant.

Other than that, the two are comparable, but I don't see it in your equations.
Net efficiency = "Useful energy supplied" / "Total heat input" (what is burned in the boiler)

 

[firefastball]

Thanks racookpe1978 but can you explain a bit on what do you mean by useful energy supplied?

Lets say Steam is going in the autoclaves at 10 bar and going out as condensate at 120 C.
Would the useful energy supplied be:
15ton/hr X (Steam enthalpy @ 10 bar - condensate enthalpy @ 120 C)?

But the condensate are not returning back to the steam plant.
How can I show by returning condensate the plant efficiency would increase?

 

[racookpe1978]

OK, you see the first half of the problem.

Now, look at the boiler side.
Now.
15 ton/hour Liquid Water "in" at delivery temperature of what ??? = How much energy_0_in. (40 degrees in winter, 65 deg F in summer maybe).
15 ton/hour steam "out" at your delivery enthalpy = ???? How much energy_0_out

So, condense the water, run it back to the boiler.
Condensate temperature = ???? (Assume something reasonable.)
Your efficiency improvement is the delivery of the hotter water back to the boiler
(T_delivery_water is colder than T_condensate_water) and that energy difference is water that does NOT have to be reheated to boiling.

Plus, you now have pure water back to the boiler, not needing to make up (purchase and filter and purify) 15 tons per hour.

 

[firefastball]

So if there is condensate returning to the boiler, say 50% at 80 deg C, and makeup at 30 deg c. my water going in to the boiler would be around 55 deg C.

Which would then decrease the fuel consumption and increase steam generation efficiency, right?
How about net efficiency:
Net efficiency = "Useful energy supplied" / "Total heat input" (what is burned in the boiler)

Will it increase as well?

Thanks

 

[racookpe1978]

Yes. Net efficiency goes up.

Your "purchased" makeup water temperature will not likely be 30 degrees C (86 deg F). More like 15 deg C (60 F) or lower.

Makeup water (after installing the condensate system) should be less than 2% losses, not 50%.

In the powerplants, there is a tradeoff calculation between generating more power (electricity) by running more steam through the turbine; and by passing the turbine at various points as steam pressure lowers to improve thermal efficiency by pre-heating the condensate and feed water.

Do you pre-heat the current feed water with the boiler exhaust gasses?

 

[firefastball]

Yes. We have an economizer installed at the stack.
and since we have touched the point, may I know what is the typical exhaust gas after the economizer if we are running on natural gas.
I have seen the temperature after exhaust reached 90 deg C which I am sceptical because based on the boiler performance data, the lowest is 116 deg C.

 

[lilliput1]

The steam to the autoclave would be contaminated so it should not be returned. The exhaust gas downstream of the economizer would condense causing corrosion if the temperature gets too low.

 

[EdStainless]

In most plants even 'dirty' condensate is cleaner than fresh water, and hence it still costs less to clean and polish for use as feedwater.
You useful work is the heating of the autoclaves. If they were being electrically heated how much power would it take? Hot condensate or other steam is not producing useful work for you, so it does not count.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[lilliput1]

If the autoclave is for a hospital and used to sterilize hospital equipment or render contaminated stuff prior to disposal, condensate should not be returned.

 

[firefastball]

After some research, we don't think we will recover the condensate but we would put a heat exchanger to recover the heat to heat up our makeup water before going to the deaerator.

lilliput1 - you mentioned about temperature limitation on the economizer outlet. How low can the exhaust temperature be before it starts to condense?

 

[lilliput1]

Package economizers are designed for about 300F leaving exhaust gas temperature. See attached. Note recommended economizer piping to always have water flow in the economizer to prevent water from boiling in the economizer tubes.

Consider economics (unless Owner wants to get more LEED points). How often is autoclave used. How much steam. Cost of heat exchanger (double sheet to safeguard against contamination) plus pit mounted condensate pump.