economizer heat transfer 2

[macmet]

I have an economizer preheating boiler supply water. This is a hot water boiler.

The situation I'm seeing is that the economizer is only absorbing a small amount of energy, and that the stack temperature (gas temp downstream of economizer) is dropping below the water temperature leaving the economizer.

More details,

- Gas temp entering econo = ~ 140C
- Gas temp leaving econo = ~ 85C
- Fluid entering econo = ~ 81C
- Fluid leaving econo = ~ 90C

If you graph my data, you see that the temperatures vs. flw through HX actually cross. And I was under the impression that heat transfer curves cannot cross. Looking at a few resources online it seems to confirm that. My understanding was the delta T would never reach 0, but according to my data, the delta T is reaching zero and then actually going negative. Something is not right.

Upon visual inspection the supply/returns are piped for countercurrent flow as designed.

Any thoughts on what could be happening?

 

[ivymike]

if the flows are in opposite directions, then the fluid leaving could be close to the gas inlet temperature, since the last bit of heat exchanger seen by the fluid will have the hottest gas.

The exit gas temperature is very close to the inlet water temperature, so you're extracting just about all of the heat you could hope to extract (you're getting ~93% of the heat you could get if you had infinite length and the same supply temps, since [140-85]/[140-81]=0.93).

 

[ivymike]

see this diagram, and note that Tbo>Tao.

http://www.taftan.com/thermodynamics/COUNTER.HTM

 

[ione]

In counterflow heat exchanger the exit temperature of cold fluid can be higher than the exit temperature of hot fluid. If the heat exchanger were arranged with parallel, the above scenario wouldn’t be possible.
The parameter of interest in the log mean temerpature which defined as:

(deltaThot –deltaTcold)/Ln(deltaThot/deltaTcold)

In your case the log mean temperature is approx 25.4 °C

 

[macmet]

Thanks everyone for your help.

I see what I've done wrong. My graphs here are not showing the position, just position 1 and position 2.

Simple and stupid mistake on my part.

 

[rmw]

It happens.

Your outlet gas temperature is getting to within a 4C approach to your inlet water temperature, so I'd say that this thing is doing about all it can and doing a bang up job.

I hope you don't have any sulphur in the fuel at those temperatures.

rmw