Technical Point[3]

 Heat Exchanger Rating: Routine feature of Process Industries 

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n daily chemical engineering practice, Heat exchanger rating is a general feature. Actual temperature data comparison with specified temperature data speaks about major evaluation. In fact, four temperature data can be converted into values of approach, range & finally well calculated log mean temperature difference(LMTD) or Mean temperature difference(MTD) after deriving correction factor,F T ( i.e. MTD = F T*LMTD) from its graphs available in TEMA, DQ Kern & other standard reference books of process heat transfer. Correction factor (F T) should be preferably at unity. F T below unity imposes severe penalty of additional surface & more erratic temperature behavior as approach gets crossed most probably for F T at or below 0.75. Case of suction preheater for one of our main ammonia compressor (centrifugal & motor driven) is a good example to study of primary rating of heat exchangers. As specified in case-I (please, refer Table : I : Case-I) , Suction preheater is provided with ~38% excess surface & LMTD of 79.36 oC to get suction outlet at 50 oC. Extra surface of ~38% is substantial but indicative of Ammonia contamination. Thus, there is a chance of choking & fouling. Finally, it may not give reliable maintenance period for which additional surface is arranged. For Case- I (Base case)

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Range of Utility = 139.5 – 111 = 28.5 oC at given flow of 3345 kgph. % Excess area = 37.5% (Over design).

Later on, it is realized that suction temperature is suitable at 56 oC to decompose any carbamate formation. Case – II depicts its parameters as below:

111°C

CASE II HCV SUCTION PREHEATER (1)

FROM UPSTREAM TOWER 9976 KGPH NH3 VAPOR

40 °C

55 °C 120°C MAIN COMPRESSOR (2)

139.5 °C

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3345 KGPH

Approach = 77 55 = 22 oC LMTD = 48.81 oC (check decrease with respect to base case) o

111°C

CASE III HCV SUCTION PREHEATER (1)

FROM UPSTREAM TOWER 9976 KGPH NH3 VAPOR

55 °C

40 °C 120°C MAIN COMPRESSOR (2)

6500 KGPH

139.5 °C

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Approach = 98 -55 = 43 oC LMTD = 60.92 oC (Check increase with respect respect to Case-II) o Power FKW @ 56 C Suction = 356 kW Range of Utility = 120 98 = 22 oC % Excess area = 2% (Over design)

Case-III represents just right size heat exchanger but it gives no latitude for reasonable maintenance period. This concludes that however pressure drops & Velocities permit to alter flow and thermal profile, there is no safe margin for reasonable

TABLE :1 : COUNTER CURRENT HEAT EXCHANGER UNIT :

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CASE VAPOR NH3 FLOW,KGPH TEMPERATUR TEMP ERATURE E °C IN TEMPERATUR TEMP ERATURE E °C OUT ∆P,BAR VELOCITY , MPS NRE LMTD , °C EXCESS AREA,% UD,KCAL/HR/M2/K HEAT DUTY,MMKCAL/HR

SHELL 3345 139.5 111 0.0016 4.31

II TUBE 9976 40 50.22 0.006 46.61

9021

63242 79.36 37.5%(OVER) 80 0.0507

SHELL 3345 120 77 0.0014 4.02

III TUBE 9976 40 55 0.006 46.95

9690 62743 48.81 -50% (UNDER) 80 0.0745

SHELL 6500 120 98 0.0053 8.04

TUBE 9976 40 55 0.0063 46.95

18300 62743 60.92 2% (OVER) 99 0.0745

TABLE :2 : MAIN COMPRESSOR POWER ESTIMATE : COMPRESSOR SUC PR BAR COMPRESSOR DISCH PR BAR COMPRESSOR SUC TEMP TEM P °C °C COMPRESSOR POWER ,FKw

1.08 2.25 50 349

Cp/Cv=(γ)= 1.29 1.08 2.25 56 356

4.45

0.22 1.08 2.25 56 356