lect 29 c-d other heat exchange f2013 r0

8/13/2019 Lect 29 C-D Other Heat Exchange F2013 r0

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Other Heat Exchange Equipment &Networks - Extended Surface Heat

Exchangers


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I. Review

Last time, we discussed shell-and-tube heat exchangers. We began with an

introduction to shell-and-tube heat exchange designs available.We readily learned about DTTM , which is used in the overall heat transfer equation,

q = U oAoDTTM .

DTTM for concentric pipe heat exchangers and single pass (1-1) shell-and-tube

heat exchangers is equal to DTLM . This equality does not hold for multiple passshell-and-tube designs. For the multiple pass variety of heat exchange equipment,we require the introduction of the F G correction factor, such that DTTM = F GDTLM .The F G, correction factor is easily determined from correlation charts of

Z = DTh/DTc and h h = DTc/DTc,max .


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With shell & tube heat exchangers, we have encountered the Donohue equation ,

an approximate correlation used to estimate the outside heat transfer coefficient,ho . The inside heat transfer coefficient, h i, is found using the classicSieder-Tate relation. Note that h i and h o are the film coefficientsinside and outside the tubes, respectively.

We have also briefly reviewed technologies for increasing boiling and condensingheat transfer coefficients both inside and outside tubes. This includes use of porouscoatings, grooves, low-fins, and fluting on tubes. For very low coefficients,such as air and other gases, traditional large fins on the outsides of tubesare often used.

Such fins allow air to be used as a cooling medium with banks of tubes and fans.The main disadvantage of such air cooling is fouling and larger P on the processside.


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L fin G fin

Spiral

fin

Extended surfacetube bundle

Radial Fin Types


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Let's consider our overall heat transfer coefficient for a typicalclean heat exchange system:

1U oA o

1h i A i

DxkA LM pipe 1hoA o

How can we modify the above equation to account for fins?

where A b =A f =

hf =

F F bo pipe LM iioo A AhkA x

Ah AU h

D 111

Area of bare tube (not occupied by fins)

Surface area of fins

Fin efficiency


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A Fin -Fan Air Cooled Heat Exchanger


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Induced Draft vs. Forced Draft Fan Coolers& Condensers


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Air Cooler Design Air ambient temperature is based on the dry bulb temperature that is not

exceeded 5% of the time during summer Highest temperature would lead to uneconomic design Sometimes add a few degrees to allow for recirculation

Minimum approach of about 15F at cold end F factor is typically 0.9 for air coolers

Face velocity (based on standard air) is typically about 500 ft/min (2.5m/s) basedon bundle area. Bundle area 2 x tube horizontal cross section area Air side pressure drop is usually very low (0.6 water) Fan power is calculated from air flow and p:

6837pACFM

hp power FanWhere:ACFM = air flow in actual cubic ft/min p = air side pressure drop in inches of water = fan efficiency (typically 70%)

variation of U with Tubeside Coefficient

Tubeside h [Btu/(hr.ft2.F]

2.5

3.0

3.5

4.0

4.5

100 150 200 250 300 350 400

Typical


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Processheating

Condensate return

LP Main

MP Main

HP Main

Boiler &Superheat

BFW

preheat

Processheating

Processheating

Processheating

Make-up

Vent

Degassing

Typical Steam Distribution System


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Make-up andadditives

Process Heat Load Typ. S&T Exchangers

Losses

BlowdownCooling tower sized

To flow, wet bulb approach,and return water temp

Circulation pumps

Evaporation losses

Typical Cooling Water Distribution System


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The Pinch point minimizes costly utilities


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Gas Grille Building Codes, Conduction, Convection,Radiation, and Fire Hazard


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lect 29 c-d other heat exchange f2013 r0

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