Page 133 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Compressor Best Practices Be st Practice 3.2

Table 3.2.2 Useful relationships
3
3
Actual flow e m /hr (ft /min) where:
3
m /hr (acfm) ¼ mass flow kg/hr (lbs/min) C ¼ 3;600 ¼ m kgf ft-lbs
3
3
density kg/m (lbs/ft ) hr kW Min-H:P:
HEADm kgf ft lbs

3 3 ðPÞ
Density kg=m ðlbs=t Þ¼ HD ¼
ZRT kgm lb
acfm ¼ m3/hr Nm3/hr x (101) (T) P 289 kg lb
Massflow ¼
(scfm x (14.7) (T)) P 520 hr min
m kgf Eff’ y ¼ corresponding efficiency (polytropic, isentropic, etc)
Energy ðidealÞ ðft lb=lb massÞ
kgm
Use head equation, polytropic is usually used P ¼ pressure e kPaa (psia)
)
Efficiency e % T ¼ temperature e K(R )
)
Derived from impeller test results e does not include mechanical losses K ¼ C þ 273.1 ( R ¼ F þ 460)

Z ¼ compressibility
Work e kW (horsepower) R ¼ 1545/mol. wgt
3
3
Brake power ¼ gas power þ mech. losses Nm /hr ¼ Normal m /hr referenced to 17 C and 101 kPA

3
(scfm ¼ standard FT /min referenced to 60 F and 14.7 psia)

ðHDÞðmass flowÞ
Gas power ¼
ðCÞðeff’yÞ
gas compressors and refrigeration applications with side
IDEAL GAS EQUATIONS loads).
Isothermal Head
HD Enclosed impellers
M − Kgf/kgm = 847.4 1545 (T 1 ) ZAVG LN P 2
(FT − Lbf/Lbm) MW MW P 1 Enclosed impellers are shown in Figure 3.2.5.
Note that the first stage impeller in any multistage configu-
Isentropic (Adiabatic) Head
ration is always the widest. That is, it has the largest flow pas-
HD K − 1 sage. As a result, the first stage impeller will usually be the
847.4 1545 K P 2 K
M − Kgf/kgm = (T 1 ) K − 1 ZAVG − 1 highest stressed impeller. The exception is a refrigeration
(FT − Lbf/Lbm) MW MW P 1
compressor with side loads (economizers).
Polytropic Head Dynamic compressor vendors use a specific speed to select
HD n n − 1 impellers, based on the data given by the contractors and end
n
M - Kgf/kgm = 847.4 1545 (T 1 ) ZAVG P 2 − 1 user. The vendor is given the total head required by the process
MW MW n −1 P 1
(FT - Lbf/Lbm) and the inlet volume flow. As previously discussed, at the stated
Where: inlet flow (rated flow) the head required by the process is in
equilibrium with the head produced by compressor. Vendor
847.4 = Metric Gas Constant “R”
MW calculation methods then determine how many compressor
impellers are required, on the basis of the mechanical limita-
1545 = Customory Gas Constant “R”
MW tions (stresses) and performance requirements (quoted overall
MW = Molecular weight efficiency). Once the head required per stage is determined, the
= Inlet Temperature °K or °R compressor speed is optimized for highest possible overall
T 1
efficiency using the concept of specificspeed asshownin
°K = 273.1 + °C
Figure 3.2.6.
°R = 460.0 + °F
It is a proven fact that the larger the specific speed, the higher
Z 1 + Z 2
ZAVG = Average Compressibility
2 the attainable efficiency. As shown, specific speed is a direct
K = Ratio of Specific Heats C p / C v function of shaft speed and volume flow and an inverse function
n - 1 = Polytropic Exponent = K - 1 1 of produced head. Since the vendor knows, at this point in the
n K h Poly
design, the volume flow and head produced for each impeller,
Poly h = Polytropic Efficiency
increasing the shaft speed will increase the specific speed and
Ln = Log to base A
the compressor efficiency.
= Suction Pressure KPaa (PSIA)
However, the reader is cautioned that all mechanical design
P 1
= Dischrage Pressure KPaa (PSIA) aspects (impeller stress, critical speeds, rotor stability, design
P 2
of bearings and seals) must be confirmed prior to acceptance
Fig 3.2.3 Ideal gas head equations of impeller selection. Often, too great an emphasis on
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