What is the effect on steam temperature when the steam flow is increased through a Heat exchanger with all other variables remaining constant?

For me this is thermo 101 increase flow = increase temperature.

I am in a control room with 5 operators that insist I am totally off my rocker. They insist that increasing steam flow through a HX will decrease the steam temp. When they stop MFing long enough to try and explain that the Gas Temperature will go down but the steam temperature will go up they just go back to their point. They also add that "That theory may be true on a conventional boiler but not an HRSG"

So I am looking for a paper or other reference that I can download (I have them in my office but that is a long way from here).

If any one has something with picture, large text, and small words I would appreciate it.

Thanks,
Ron

 

You don't say whether the steam is being heated or the steam is the heat source.

If the heat exchanger is heating the steam, then the outlet steam temperature will drop. By your definition, the only parameter that has changed is the steam flow, so the heat (energy) input to the heat exchanger has not changed, the steam outlet temperature must drop to preserve the energy balance. You have constant energy in, you need constant energy out. The steam mass flow has increased, so the steam enthalpy must decrease to maintain the same energy out. For superheated steam at constant pressure, a decrease in enthalpy means a decrease in temperature (look at the steam tables or a Moliere Chart).
If, on the other hand, the steam is the heat source, similar reasoning on the energy balance will show the outlet steam temperature increasing.

 

It depends on whether the steam is the heating fluid or the heated fluid. But think conservation of energy. If steam is used for heating, you have fixed steam inlet and outlet conditions, and increase the flow, the energy into the exchanger will increase and so the energy taken out will increase. So the temperature of the heated fluid at the outlet will increase.

If steam is the heated fluid and the energy input remains the same, then an increase in steam flow will mean that the temperature rise through the exchanger will not be as great.

Cheers,
Bruce.

 

To clarify, this is an HRSG superheater. The flue gas temperature, pressure, and flow remain constant. The steam flow varies based on valve position. Increasing the mass flow of steam through the heater extracts more heat from the flue gas. The flue gas outlet temperature will drop. The steam outlet temperature will increase.

 

You just changed the rules. Initial question stated that the only parameter that changed was the steam flow. Now you say the gas outlet temperature dropped. So now we need all the numbers to determine what will happen to the steam temperature. Depending on how much the steam flow has changed and how much the exhaust temperature changed, and what the mass flows of steam and exhaust gas are, the steam temperature could rise, fall or be unchanged.

 

Think thermal resistance. To a first approximation, the Log Mean Temperature difference across the exchanger divided by the power transferred will be constant (there will be some small increases in heat transfer coefficient with steam flow rate that can be ignored). If steam inlet temperature is not affected by flow (it could well fall slightly). The air side flow rate will not change.

The air flow is probably transverse to the steam tubes so the LMTD won't change a lot. If the steam inlet end and air inlet temps are the same and your argument holds (steam temperature rises), the LMTD from air to steam will fall. But the thermal resistance will stay the same. So the power transferred from air to steam has to fall.

For an increase in steam flow with no change in exit temperature, the power transfer from air to steam will rise. This can only come about if the LMTD across the exchanger elements increases - hence if inlet temps remain the same and air outlet falls, the steam outlet must fall even further.

My money's on a drop in steam temperature if steam flow increases. But why not do some tests to settle it one way or another? And let us know the results.

Cheers,
Bruce.

 

Okay. I still did not explain myself very well. The HRSG has a superheater with a gas inlet temperature fixed at 1000°F. It has a steam outlet temperature of 800°F with a flow rate of 200Kpph.
In general, without knowing the surface areas, mass flow rate of the flue gas, the moisture content, etc., if the steam flow rate is increased from 200Kpph to 250Kpph what will the Steam temperature do?
I hope this clarifies the question.
Ron

 

Bruce,

Actually we did an experiment to prove that the theory is accurate. Even in the face of the facts the operators will not believe this and have a 101 reasons why this is wrong. That is why I am looking for a fundamental text explaining HX theory. Not for me, for them.

Strange that the majority of times the operators are the biggest asset to help the visiting engineer, me, understand how their plant works. But every once in a while you get a group that has a strange idea that is difficult to over come.

I will be back in my office next week and will be able to send them something from my texts. I am just looking for something while I am here on site to give them.

Thanks,
Ron

 

I still have a problem with the steam temperature increasing as the flow increases as a general principle.

If the steam flow is very small the air outlet temperature will be 1000 F and the steam temp will be somewhere just below 1000 F. So if the steam flow increases and steam temperature rises as a result, eventually you are saying that the steam outlet could reach a higher temp could get above the hot air inlet??

Convective heat transfer inside the tubes is dependent on the flowing velocity and it is possible for instance that there is a flow range where the improvement in overall heat transfer coefficient compensates for the drop in steam temperature that would otherwise have occurred. How are you measuring the steam temperature, and have you got some other readings that confirm the temperature results?

Cheers,
Bruce.

 

Ron,

Can you be more specific on how you are increasing steam flow rate when no other variables are changing? Are you lowering the steam pressure so the saturated steam temperature drops and delta H drops so the waste heat can generate more steam? If you are not changing anything on the steam side of the HRSG then something must be changing on the waste heat side of the HRSG for the steam rate to increase.

Here is one possible scenario assuming the steam production rate and super heat temperature are not controlled but the steam header pressure is held constant and the surface area on the steam latent heat side and super heat side are constant (the water level on the steam side does not change): If the input disturbance is that the gas turbine waste heat flow rate increases, then the exit temperature on the waste heat side (turbine exhaust side) will now be hotter exiting the latent heat section of the HRSG (required for Q = UA Detla T) this will generate more steam.

The waste heat turbine exhaust entering the superheat section is hotter but the saturated steam entering the super heat section is the same temperature since the header pressure is controlled. Since both the steam flow and the turbine exhaust flow have increased, whether the superheated temperature of the steam exiting the superheat section goes up or down depends on whether the Cp of steam is higher or lower than the Cp of tubine exhaust and in the steady state how both the heat balance equation and heat exchanger equations below are satisfied. A simulation would help answer this. Also, to fully answer the question we have to know exactly what variables are controlled, what variables are manipulated and what variables are disturbances. There are other possible outcomes depending on the controlled, manipulated and disturbance variables.

You have two equations that have to be satisfied in the steady state:

Sensible heat balance equation -superheat section:
1. Q = Fs Cps ΔTs = Fwh Cpwh ΔTwh

Q = Sensible heat transferred from turbine exhaust to the steam
F = Steam flow
Cps = Specific heat of the steam
ΔTs = Change in temperature of the steam
Fwh = waste heat flow
Cpwh = Cp of waste heat
ΔTwh = change in temperature of the waste heat.

Heat Exchanger equation:

2. Q=U A ΔThx

Q = Sensible heat transferred from the waste heat to the steam (Same Q as eauation 1)
U = Heat exchanger coefficient
A = Heat exchanger area

ΔThx = logarithmic mean temperature difference between waste heat side and the steam side of the HRSG.

If I run across any papers on this I'll let you know.

Hope this helps,
Jay

 

Bruce,

As the steam flow through the heater increases more heat is extracted from the flue gas side of the heater. This will continue until the pinch point of the heater is reached.

As the heat is extracted from the flue gas the gas temperature at the out let of the heater drops.

As the steam temperature goes up the gas temperature goes down until the pinch point of the heater. After this point further increases in steam flow will not change the temperatures.

No that I am in the office I have access to my references and will send something back to the site explaining this for them.

Ron