Heat Exchanger Basics 1

[Josh2008]

Hi,

Can anyone please provide a basic rundown on what information is typically provided to vendors when spec'ing a heat exchanger?

-Type: Tube or Plate
-Fluid Mediums: Fluid or Gas and what type
-Heat Transfer Load
-Temperatures incoming and outgoing for both mediums
-Mass Flow Rates
-Volumetric Flow Rates

Anything else?

Any tips on the selection between heat exchanger type as well as the secondary fluid medium.

I have to cool water, from T1(fluid1) to T2(f1). I have calculated the cooling capacity required based on Cp of water at Tavg(f1), and the mass flow rate.

I am looking at using water if available on site (open loop, which is a waste) as the secondary cooling medium, so I know the T1(f2). I can calculate T2(f2), but I'm not sure on how to choose an appropriate mass flow rate?

Any information is appreciated, I'm just learning at this point.

Thanks,
J

 

[pmover]

Josh2008,

something to consider is how much time you have to conduct the analysis and whether or not a HT specialist is needed.

a data sheet of typical information on a heat xchr data sheet is at:

http://www.freecalc.com/hxds2.htm

please read through the wolverine databook (

http://www.wlv.com/products/databook/db3/DataBookIII.pdf).

there is much to be learned and hope this is a start.
-pmover

 

[DYV1973]

Water / water heat exchanger at temperatures below 150°C would be a good application for a plate heat exchanger. Please note that pressure drop for a PHE is higher than for a shell & Tube. A PHE gives approx. the same pressure drop on both hot and cold side, so if a too small LMTD (temperature difference) is not getting a problem, it is recommended to balance the flows.

Also use lower values for fouling, as the PHE will be oversized (which is not always an advantage).

If your process conditions are known, you do't need a specialist, but you can send the inquiry to a manufacturer.

DYV

 

[davefitz]

other req'd info is :
a) design code to use
b) design pressure for both shell and tubeside
c) fluid properties, including corrosive properties
d)QC specifications aove and beyond code min required
e) allowable pressure drop, both tube + shell side
f) design cleanliness factor

 

[Josh2008]

Physical Explaination

Counterflow vs Parallel Flow Heat Exchangers

Counterflow is more efficient than Parallel Flow HE.

What is deceiving about this is that I would have thought parallel flow would be more efficient because when viewing the diagram below I would predict that the average temperature difference between the two fluids would be larger over the fluids path through the HE with the parallel flow versus the counterflow. A larger average temperature difference, the more heat is transferred (if the area and specific heat is the same between both scenarios). What gives? What am I missing here?

PARALLEL FLOW
Hot Fluid 150F ----->----->----->----->----- 70F
Cold Fluid 60F ----->----->----->----->----- 103F

COUNTERFLOW
Hot Fluid 150F ----->----->----->----->----- 70F
Cold Fluid 103F -----<-----<-----<-----<----- 60F

I know this can be explained with a difference in LMTD, but that is difficult to put in perspective. Please shed some light on this.

Thanks

 

[vpl]

Is this homework?


Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[IRstuff]

Your parallel flow example cannot exist, since it implies that heat is flowing BACK to the hot side from the coolant at the end of the loop. All cooling physics tells you that heat flows because of temperature differences, so the logical extrapolation of a parallel flow system is where the outlets have equal temperature, and therfore, the latter portion of the parallel flow system actually has little in the way of heat transfer.

A counterflow system, however, will almost always still have a huge temperature difference at both inlet AND outlet, thereby spreading the heat transfer across the entire length of the exchanger.

TTFN

FAQ731-376