Page 301 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10G 5/30/2001 4:41 PM Page 263

Heat Transfer 263

the process engineer to evaluate the manufacturer’s bids for Q
air cooled units with the following points in mind: 88 A 1U21MTD2 (10-285)

2
2
1. The dollars/ft of finned surface or dollars/ft of bare or, 251
tube surface in a finned unit do not necessarily give the
Q 1U21A21T t2 mean
only important factor.
2. Determine whether parallel or counter flow exists 1 1 1
r f,t r f,a r w (10-286)
inside tubes. U htc a htc t
3. For condensing problems, determine whether appar-
ent weighted mean temperature difference is used, and where A total bare tubeheat transfer area, ft 2
2
which is applicable. htc a airside heat transfer coefficient, Btu/(ft ) (hr)(°F)
2
4. Determine fouling factors. htc t tube-side heat transfer coefficient, Btu/(ft ) (hr)(°F)
5. Determine tube metal resistance. MTD mean temperature difference, °F
6. Determine net free flow area for air across bundle, and Q heat transfer duty, Btu/hr
2
r f,t tube-side fouling resistance, (hr.-ft -°F)/Btu
determine air linear velocity. Compare air side coeffi-
2
r f,a air-side fouling resistance, (hr-ft -°F)/Btu
cients for same linear velocities.
2
r w wall resistance, (hr.-ft -°F)/Btu
7. Determine required fan horsepower (bhp) per million
t air temperature, °F
Btu transferred. T hot fluid temperature, °F
2
8. Determine total dollars per ft of finned surface U overall heat transfer coefficient, Btu/(hr.-ft -°F)
2
including standard (or specified) support structure, CMTD corrected mean temperature difference, °F
ladders, etc. LMTD log mean temperature difference, °F
And,
From such items and others pertinent to a specific situa- (T t) mean CMTD (LMTD) (F) (10-287)
tion will emerge the conclusions:
31T 1 t 2 2 1T 2 t 1 24 3F4

31T 1 t 2 24
1. The lowest dollar value based on complete structure, ln
31T 2 t 1 2
including the important finned surface.
2. The best dollar value considering amount of basic
F MTD correction factor, dimensionless, corrects log
surface, type of fans, etc.
mean temperature difference for any deviation from
true counter-current flow.
These two may not be the same. In some instances, high-
finned surface area but low bare tube surface means that a In air-cooled heat exchangers, the air flows upward
lot of tall (sometimes less efficient) fins are crowded onto umixed across the finned tubes/bundle, and the tube-side
the tube. In this case, horsepower might be expected to be process fluid can flow back and forth and downward as
higher. established by the pass arrangements. At 4 or more passes,
Bid evaluations must include a study of the peculiar costs the flow is considered counter-current, and the “F” factor
expected to be associated with a given unit, and these 1.0. 215 The other fewer-passes correction factors are given in
include first cost of equipment, power (or driver) operating Figures 10-187A, 10-187B, 10-187C.
costs, maintenance for entire unit, foundations, special Referring to Hudson Products Corporation, 251 used by
structural limitations, pipe layout, and perhaps others. permission:
To simplify the evaluation, it is to the advantage of the
purchaser to advise the manufacturer of the dollar cost per 1. Hot fluid heat capacity rate C h C tube
installed horsepower in his plant and the operating costs for (Mc p ) tube Q/(T 1 T 2 ) (10-288)
power. The manufacturer can select, from a wide combina- 2. Cold fluid heat capacity rate C c C air
tion of units, the size and number that are the most eco- (Mc p ) air Q/(t 2 t 1 ) (10-289)
nomical. Otherwise, the bids should be requested as based 3. Number of heat transfer units Ntu
on “lowest operating cost” or “lowest capital cost,” neither (A)(U)/C min (10-290)
being the best in itself except for certain purposes. 4. Heat capacity rate ratio R C min /C max (10-291)
5. Heat transfer effectiveness E
Design Considerations (Continuous Service)
C h 1T 1 T 2 2 C c 1t 2 t 1 2
E (10-292)
The air-cooled heat transfer exchanger is like other C min 1T 1 t 1 2 C min 1T 1 t 1 2
exchangers in that the basic heat transfer equation must be E 1 e NTU11 R2 (10-293)
statisfied: 265 1 Re NTU11 R2