Compressor Selection
Nov.13 2006
Thermodynamic calculation
Engineering Co. utilize in house data from
accumulated vendor information
Customer need vendor’s basic data to perform
z Basic thermodynamic formula with unit conversion
z Table-1. Parameter for Each Frame
z Excel spread sheet (Hand over separately)
Sample compressor selection
2
Basic thermodynamic formula to be used
3
Fundamental Formulas generally used
k -1 / k
ηp = (1)
n-1/n
Polytropic efficiency
n-1 k-1
Td = (Pd /Ps ) n x Ts = (Pd /Ps ) k ηp x Ts (2)
n-1 k-1
Zav R Ts n Zav R Ts k ηp
H= [ (Pd /Ps) - 1 ] = [ (Pd /Ps) - 1 ]
n -1 k -1
n k ηp
m
= Zav R Ts (r - 1) / m (3)
4
k-1
where, r = Pd / Ps, m= (4)
k ηp
From equation ( 2 ), ηp and Td can be back calculated from operation data
k - 1 Log( Pd / Ps )
ηp = (5)
k Log( Td / Ts)
m
Td = Ts x r (6
m
Qd = Qs x r / r (7)
Head coefficient µ can be expressed
H
µ= (8)
2
U/g
πD N
where U = Ft/ Sec Impeller peripheral velocity ( 9 )
60 x 12
D : Impeller diameter in Inch,
N : Operating speed , rpm
5
Gas power GHP
WF x H
GHP = (10 )
33000 x ηp
where WF is weight flow ( lb / min), From equation ( 1 ),
3
v = Z R T / 12 x 12 P ft /lb ( 11)
2
where R= 1545.32 /MW, T = ° R= ° F + 460 , P = psia = ( lb/ in )
3
Volume Q (CFM = ft / min) can be expressed:
Q = WF x v ( 12 )
For multistage centrifugal compressor, operating speed N(rpm) can be
approximately expressed:
1300 Total H
N= ( 13 )
Av. D √ s x µ
where s : Number of impellers , Av. D : Average impeller diameter in inch
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Table 1: DATUM Data
Table-1: Parameter for each Frame( Value for MU & NH are by writer's experience) Rev. 1 Nov 2 2
Frame 2 4 6 8 10 12 14 16 18 20 22 24 26
Nom.Imp dia, inch 9.516 11.045 12.819 14.879 17.269 20.043 23.263 27 31.337 36.372 42.215 48.996 56.867
Max. MCS, rpm 26470 22800 19650 16930 14580 12570 10830 9337 8040 6925 5965 5140 4430
Max.100% spd 25209 21714 18714 16123 13885 11971 10314 8892 7657 6595 5680 4895 4219
Nominal NH, % 82 82 82 83 83 83 83.5 83.5 84 84 84.5 84.5 85
Nominal MU 0.47 0.48 0.5 0.51 0.53 0.53 0.53 0.54 0.55 0.55 0.55 0.55 0.55
Nom. Flow, ACFM 5400 7200 9800 13000 18000 24000 32000 43000 59000 79000 106000 143000 193000
,AM3/H 9175 12233 16650 22087 30582 40776 54368 73057 100241 134221 180094 242957 327907
Case ID, inch 20.617 20.617 23.93 27.774 32.235 37.414 43.424 50.4 58.5 67.894 78.8 91.459 106.152
, cm 51.8 51.8 60.2 69.8 81.0 94.1 109.2 126.7 147.1 170.7 198.1 229.9 266.9
350 ( 24.6)
Case Rating, 400 (28.1) X
Radial 600 (42.1) X X X X
psig (kg/cm2G) 800 ( 56.2) X
1200 (84.3) X X X X X X X X X X X X
1795 (126.2) X X X X X X X X X X
2995 (210.5) X X X X X X X X X
4995 (351.1) X X X X X X X X
7200 (506.2) X X X X X X
8500 (597.6) X
10500 (738.2) X X X X X
170 (11.9) X X X X
Case Rating, 300 (21.1) X X X
Axial 350 (24.6) X X
psig (kg/cm2G) 400 (28.1) X X X X X X
600 (42.1) X X X X X
800 (56.2) X X X X
900 (63.2) X X X
Note: 1.Nominal value of MU is purely by writer's experience.
2. Nominal flow for double flow casing is double of each frame.
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Manual Calculation
Manual Overall Selection( DATUM), Ft/Lbs unit
Project number : Date: Type of Driver:
Customer : By:
Gas : Barometric pressure: psia Disch. pressure: psia
Sour(NACE ) or Sweet gas : Relative Humidity : % Case rating req'd per API(1.25 x Pd-psig)
Operating condition,Sect No 1 2 3 Operating condition,Sect No 1 2 3
Ps-psia Leak Loss, HP ( See table * 2)
o o
Ts- F( or F+460 R) Heat loss(LO&SO), HP(See table *3)
MW( Sum of Mol % x Mol Wt of gas) BHP=GHP+Heat Loss+Leak Loss
R = 1545/MW D -inch, Imp. diameter from table(*1)
Zs MU-pressure coeff. from table(*1)
Zav= Zs+Zd / 2 s = Number of Impeller.
Kav N=(1300/D) x (H/ s x MU)1/2 , rpm
WF -Lb/Min or Max. allowable N from table(*1) for the casing
3
WF=( Nm /H )x MW/610 Selected compressor model
Svc=Zs RTs/144Ps U = π D N/12x60 , Ft/S: Pai=3.1416
Qs= Svc x WF, ACFM Qs/N
r = Pd/Ps Flow Coeff. = 700 Qs/ ND3
m =(Kav-1/Kav)/NH MU from CIX curve (MUc)
NH assumed from table( * 1) NH from CIX curve(NHc)
m
H req'd=ZavRTs(r -1)/m H = MUc x U2 /g where g =32.14
GHP= WF x H /33000 NH Total Head = H x s, Compare with H req'd
m:
Td = Ts x r o
(Td below 380 F) Qd = Qs rm / r
Remark: If no data of table-1,2 & 3 is available, Use NH= 0.8 for below Frame12, 0.83 below Frame 16, and 0.85 for others.
Reasonable MU is in the range of 0.48 through 0.54 except for low head impeller.If low head, MU=0.43 is adequate
Head per impeller is 8000 through 12000 Ft-Lbs/Lbs depend on MW. Never be more than 18000 Ft
Critical speed(NC1 & NC2) has to be checked for adequate separation margin. Min. separation margin per API
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Sequence of Calculation
From given Inlet (Ps, Ts, MW & WF) and Discharge
condition( Pd), Obtain Head, GHP Speed N and
Compressor model using Table 1 attached
No inclusion of mechanical loss by bearing for Brake
HP
No inclusion of leak loss effect nor rotor dynamic
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Ps, Ts, MW, Pd, WF Head, GHP
Calculate Weight Flow (WF) if not given
Calculate Inlet volume Qs = Svc x WF
where Svc= ZsRTs / 1444 Ps ……ACFM, WF……LB/Min
Calculate H= Zav RTs(r m-1)/m …..Ft
Where R= 1545.4/MW, r= Pd / Ps, Zav = Zs +Zd / 2
m= (Kav -1 /Kav) / NH,
GHP = WF H/ 33000 NH ….HP
where 33000= 550 x 60 as 1 HP= 550Ft-lb/Sec
Assumed Value: Zs, Zav, Kav, and NH
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Continued
Assume number of impeller s: Divide total Head by
8000 to 12000 as standard H ≈ 8000 Ft/ Impeller
Calculate N (rpm)
1300 x H
N= √ s µ
D av
• Where μ≈ 0.48 to 0.53 corresponding to 8000-
12000Ft/imp. Use 0.5 as standard. D av :Imp Dia by inch
Calculate Td = Ts x r m … ° R (460+ ° F)
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Excel Spread Sheet
Simple program is separately hand over to audience.
Let’s use this with sample selection
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Ps, Ts, MW, Pd, WF -----------> Head & GHP
2 section or 2 casing
1st 2nd
Ps M Pa G 0.511 1.301
Ps psia 88.8 203.4
Ts degC 45 40
Ts deg F 113 104
Ts deg R Ts+460 573 564
Pd M Pa G 1.377 2.845
Pd psia 214.4 427.3
MW 7.61 6.25
WF kg/h 46663 38013
WF lb/min 1715 1397
Q ACFM 15600 6650
Zs 1 1
k 1.35 1.35
efficiency: E 0.82 0.8
(k-1)/k 0.259 0.259
m (k-1) / k / E 0.3162 0.3241
r Pd /Ps 2.414 2.101
r^m Td / Ts 1.321 1.272
Td deg F Ts * (r^m)-460 297.1 257.4
Qd ACFM assuming Zd = 1 8535 4025
Total
H ft Z*(1545.4/MW)*(460+Ts)*(-1+r 118,274 117,052 235,326
GHP hp WF * H / 33000 / E 7,494 6,193 13,687
kw 5,588 4,618 10,206
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K-value (approx)
H2, N2, Air : 1.4 CH4, NH3, CO2 : 1.3
C2H4:1.24, C2H2: 1.23, C2H6 : 1.19, C3H6:1.15, C3H8: 1.13
“A” Compressor Operating Condition
Weight Flow, kg/s ( Lb/Min) 3.025 (400)
Inlet Pressure, kPa ( Psia) 118 (17.1)
Inlet Temperature, 0C ( 0R) 34.2 ( 553.5)
Relative Molecular Weight 38.8
Discharge Pressure, kPa (Psia) 377(54.7)
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“A” Compressor selection result
Qs= 3580ACFM, Select D4R Frame, D=11.045”
H= 27815Ft , GHP=411 HP , Assume 420BHP with 2% loss
s = 27815/8000=3.5 Either 3 or 4 Impeller
Operating speed N(rpm)
z When s=3, N =1300/11.045 (27815/ 0.5x 3)0.5= 16027 rpm
z When s=4, N =1300/11.045 (27815/ 0.5x 4)0.5= 13880rpm
Conclusion: Either D4R3S at 16000rpm with 420BHP or
D4R4S at 13900rpm
DR’s program for precise selection is D4R4S with 436BHP at
13581rpm. Our Nazri will present later.
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“B” Compressor operating condition
Weight Flow, kg/s, (Lb/Min) 6.898 (911.4)
Inlet Pressure, kPa A (Psia) 2,912(422.3)
Inlet Temperature, 0C (0 R) 34.6 ( (554.3)
Relative Molecular Weight 19.91
Discharge Pressure, kPa (Psia) 9,513 (1379.8)
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“B” Compressor selection result
Qs= 645ACFM Smallest compressor D2R is
good for Max. flow of 5400ACFM, Min.1000ACFM
resulting no suitable quote by centrifugal unit
H= 60441Ft, GHP= 2038HP with centrifugal.
Better go to Recip solution!
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“C” Compressor-2 Section with inter cooler
Description Stage 1 Stage 2
Weight Flow, kg/s (Lb/Min) 21.873(2890) 21.873(2890)
Inlet Pressure, kPa( Psia) 273(39.6) 1170(169.7)
Inlet Temperature, 0C (0R) 33.5 60
Relative Molecular Weight 24.8 24.8
Dis. Pressure, kPa ( Psia) 1239(179.7) 5294(767.8)
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Sample calculation
Qs=17464ACFM ≥ D12R Qs=4427ACFM
H=61631FT H=67083Ft
S= 7.7 at 8000Ft/imp or 5 at S= 8.4 at 8000Ft/imp or 6 at
12000Ft/imp 12000Ft/imp
GHP=6510 GHP=7261
WF= 2893Lb/Min, Total required impeller is 16 at 8000Ft/imp or 11at
12000Ft/imp. Try to squize to 10 imp/case or 2 casing
Total GHP= 13771HP, Total BHP ≈ 14,050 BHP with 2% loss
D12R10B, D=20.043 , N= (1300/20.043)(127814/10x 0.52)0.5= 10245rpm
D14R10B, D=23.263 , N= (1300/23.263)(127814/10x 0.52)0.5= 8792rpm
D16R10B D=27.0 , N= (1300/27)(127814/10x 0.52)0.5= 7575rpm
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Continued
Compressor selection either D12R10B,D14R10B or
D16R10B is depend on rotor dynamic.
DR’s precise program resulted to D16R10B at
14628BHP at 8044rpm. Refer Nazri presentation
Note: More than 10 impeller/case is not recommended
due to Rotor dynamic problem.
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Side stream Side stream
Description Stage 1
inlet 1 inlet 2
Weight Flow, kg/s (wet) 41.13 12.25 31.55
Inlet Pressure, kPa 131 537 1219
Inlet Temperature, 0C -36 4.2 35
Relative Molecular Weight 44.1 44.1 44.1
Discharge Pressure, kPa 537 1219 2197
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Selection by DR’s Program
Ghazali Nazri presentation
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