Travaini Pumps USA SA Series, Operating & Maintenance Manual

Page: 22 / 27

17.3 - OPERATION WITH PARTIAL RECOVERY OF SERVICE LIQUID
Where the working conditions will allow it, the ser vice liquid temperature can be
increased utilizing a smaller quantity of fresh liquid from an outside source. A
similar flow as the make-up is discharged to the drain while the balance of liquid
required by the pump is recirculated. In these cases the ser vice liquid working
temperature rises and the pump capacity will require correction per cur ves of fig.
29 and 30. The system installation will be similar to the schematic of fig. 31.
Depending upon the affordable loss of capacity the ser vice liquid temperature T2
may be set and the make-up flow of fresh liquid QF can then be calculated:
Q
F
(m 3 /h) =
Q
A
•
∆
T
T
2
- T
1
+
∆
T
Where:
Q
F
= Fresh make-up flow from outside source in m 3 /h
Q
A
= Total ser vice liquid flow required for the operating
conditions in m
3 /h
∆
T = Ser vice liquid temperature rise (see section 17.2)
T
2
= Ser vice liquid temperature to pump
T
1
= Temperature of make-up liquid
The fig. 31 indicates a generic schematic of a liquid ring vacuum pump in a par-
tial recover y system. By closing the recirculation line the system would become a
“once through” installation where all the ser vice liquid is drained, therefore:
Q
A
= Q
F
and T
2
= T
1
– 38 –
17.2 - SERVICE LIQUID TEMPERATURE CHANGE ACROSS THE PUMP
The ser vice liquid of a liquid ring pump absorbs total heat QT as follows:
Q
T
(BTU) = Q
c
+ Q
K
+ Q
R
Where:
Q
c
= 0.9 x BHP x 2545 = Isothermal compression heat
Q
K
= m
V
x r = Condensation heat
Q
R
= m
g
x c
p
x
∆
T
a
= Cooling heat (Generally negligible,
ignored in calculation of Q
T
)
m
V
= mass condensed incoming vapor in PPH
m
g
= mass incoming gas in PPH
P = absorbed power at operating point in kW
c
p
= gas specific heat in BTU/1B/°F
r = heat of vaporization in BTU’s
∆
T
a
= differential temperature in R, between incoming gas TG and service
liquid discharge temperature (T
2
+
∆
T)
K = Kelvin temperature
Once the Q
T
is known it is possible to calculate the differential temperature
∆
T of
the pump ser vice liquid:
∆
T =
Q
T
Q
A
•
ρ
•
c
P
Where:
Q
T
= total heat load before calculated in BTU/hour
Q
A
= pump ser vice liquid flow in GPM
ρ
= ser vice liquid density in kg/m 3 (water = 1.0)
c
P
= ser vice liquid specific heat
NOTE: It can be assumed that the discharge gas and ser vice liquid have same
temperature.
– 37 –