Water Flow vs Temp Difference........ 2

[CamLoch]

We have a water/glycol brazed plate heat exchanger. In general terms, should we expect the delta T to increase with increased flow or decrease?

 

[corus]

Increased flow increases the heat transfer coefficient and so increases the delta T

Tata

 

[IRstuff]

That's seems a bit backwards to me.

Q = htc*area*deltaT. If Q is constant and htc increases, then deltaT decreases, doesn't it?

TTFN

FAQ731-376

 

[speco]

CamLoch,

If you increase the flow on the water side, the heat transfer cofficient on that side will increase, of course, increasing the overall heat transfer coefficient. This will increase the delta T on the other side. However, I don't think it will increase the overall heat transfer coefficient enough to result in a higher delta T on the water side.

As IRStuff pointed out, the water side delta T is inversely proportional to the flow rate if the Q is constant. It's a pretty good bet that the effect of the water flow is greater than the effect of the increased heat load.

Speco

 

[CamLoch]

In our case, the load (Q) is constant. We are only changing flow on a fixed system to determine a possible flow issue. We are seeing a higher than expected delta on the water side and wondered if there my be a flow restriction in the system (as indicated by the increased delta T).

Thank you all very much for your input!!

 

[ione]

If the load Q is constant and the measured water deltaT is higher than expected, the water flow rate M must decrease, being for the water side:

Q = M*cp*?T

with cp = water specific heat capacity

 

[corus]

I'm not too clear as to what delta T is being measured. I presumed it was through the plate thickness so an increased htc will produce an increased delta T for an increase in Q through the plate. If you increase the htc then the delta T between the water and plate surface will tend to zero. The delta T measured in the water will also tend to zero for an increasing mass flow rate M, as per ione's equation.

Tata

 

[speco]

corus,

The fact is, we sometime get a little sloppy in our terminology. Delta T can mean three different things in a heat exchanger.

1. The hot fluit change in temperature. Usually T1-T2

2. The cold fluid change in temperature. Usually t2-t1

3. The effective temperature temperature difference between the fluids, or Log Mean Temperature Differenct (LMTD)

In IRStuff's posting, he meant LMTD

In Ione's posting, he meant the temperature change in either fluid.

I believe the original poster was referring to one of the fluid temperature changes.

Speco

 

[CamLoch]

My apologies for the vague reference on delta T. I was in fact looking for temp difference between inlet and outlet of the fluid.

I will endeavour to be more precise with my future postings as it is quite clear that a thread can come unravelled if not properly mended. One could say "a stitch in time saves 9". No more punny references, I promise!!

thank you all again!!

CamLoch

 

[IRstuff]

Given that piece of information, the coolant deltaT would go down with increased flow, since, for a constant heat load, the increased mass transfer requires less of a temperature increase to carry the same heat load.

In the limiting (absurd) case, an infinite coolant flow would result in a zero deltaT across the inlet and outlet, since you would now have an infinite heat sink.

TTFN

FAQ731-376

 

[davefitz]

The fluid DT will decrease as the flow increases if the DT is defined as DT=To-Ti, provided the HX areas remains constant and the opposing fluid flowrate remains fixed. Refer to "compact HX theory" ( Kays + London) and the relationship of (Ti-To)min vs the HX effectiveness "e" and the opposing fluid (Ti-To)max

 

[25362]

Let's not forget that no matter how much we improve one individual heat transfer coefficient, the overalll heat transfer coefficient would stay at a value smaller than the lowest of both individual values.