Information Technology Reference
In-Depth Information
Figure 8.1
Pump and turbine input.
sections should be read prior to using the block. The basic idea is given by
p
2
η
h
=
Vdp
(8.1)
p
1
where
is either a polytropic or isentropic efficiency factor which is assumed constant
for a process, where
h
is the change in enthalpy between the two states,
V
the
volume, and
p
the pressure. The various methods available in Aspen Plus are based
on assumptions dealing with how the integral is evaluated.
The polytropic efficiency is given by
η
n
−
(k
−
1
1
)/k
η
P
=
(8.2)
n
where
n
is the polytropic coefficient, with each state described by
pV
n
,
k
is the heat
capacity ratio
c
p
/c
v
,and
η
p
is the polytropic efficiency.
For an isentropic process
h
out
is obtained from the outlet pressure when the entropy
at the outlet equals the entropy at the inlet. The isentropic efficiency is used to modify
an isentropic process to approach a real condition. There are two forms of isentropic
efficiency:
h
out
−
h
in
h
out
−
h
in
η
s
=
(8.3)
for compression and
h
out
−
h
in
h
out
−
h
in
η
s
=
(8.4)
for expansion.
