CAPITULO 8. SEMILLAS PARA LA PAZ: orientaciones pedagógicas, recomendaciones y
8.5 CONCLUSIONES
3 - 2·Yair
3 - 2·Yair
k\ [3 - 2.Yair]
l' 1
T
2T
2.k2 ·Yairphi
= [
]
fe2+ [
] DSI +
[
]
3 -
2'Yair
3 -2.Yair kJ 3 -2·YairT
T
1T
k2 ·Yair
pe2 =
pIJ2 = [
]
fe2
+[
] DSl
+k
[32
]
3 - 2y�
3 - 2y�
! -
y�(6.2)
(6. 3 )
(6.4)
The mass flow of steam to the DSI unit can be determined by making an energy balance around the DSI .
M C
[T
T ]
M
C
[TDSJ -(1 -YaJTfe2
+Yair]
fe2 · pwater DSl - pc!
fe2 · pwater
!
msteam =
-C pwater
=
-C pwater .TDS1
[3- 2Yair ]
(6.5)These equations show that air has two important impacts on a preheat system. Firstly the mass
flow of steam required by the DSI unit increases, as shown by equation (6. 5). Secondly it causes
the temperature of the second flash vessel to increase, as shown by equation (6.4). Both of these
effects are actually caused by the temperature difference between the top and bottom of the first flash vessel. The temperature at the top of the first flash vessel reduces and therefore more steam is required to heat the DSI unit to the same temperature. However, the increased temperature at the bottom of the first vessel causes an increase in enthalpy to the second vessel and so its temperature rises.
The increase in the steam mass flow to the DSI unit means that the energy efficiency of the unit is reduced. The Evaporator A plant has a temperature control loop placed CJ,round the DSI unit and this compensates for variations in the feed milk temperature. As a result, of the air in the flash vessel, the DSI temperature control loop increases the mass flow of steam. However, since the DSI unit temperature is the same, the energy efficiency of the DSI unit is lower. Obviously we should remove this possibility by providing adequate de-aeration of the flash vessels.
Variations of air concentration in the flash vessel cause milk temperature variations to the MVR
evaporator section. Later in this Chapter it will be shown that the temperature of the MVR
evaporator has an impact on its mass flow of evaporation. Therefore it would be preferable if temperature disturbances to the MVR evaporator could be minimised. Once again this requires adequate de-aeration of the flash vessels.
Industrial evaporator plants have de-aeration lines that should remove any air that accumulates in the preheat system. The de-aeration lines are small pipes, containing an orifice plate, attached to the flash vessels and another lower pressure vessel. It is important that the de-aeration orifice
plates are correctly sized. If the plate is too small then air will accumulate in the flash vessels and the above problems will occur. However, if the plate is too large then large amounts of steam will escape from the system and this will cause other problems. We have shown that a probable indication of air problems is the temperature difference between the top and bottom of the flash vessel. Ideally a flash vessel should operate without any temperature difference and this provides a simple test of the de-aeration lines.
6.2.3) Holding Tube Pressures
Earlier we discussed the importance of maintaining an adequate pressure in the preheat holding tubes. If the milk is not to vaporise in the holding tubes then the pressure at the top of the tubes must be larger than the vapour pressure of the milk. Figure 6- 1 shows the configuration that should be used to remove the possibility of milk vaporising in the holding tubes. The pressure after the holding tubes is measured and a combined orifice plate and control valve is used to regulate the pressure.
OSI unit Steam I I
' @
L _ _ _ _ _ _ 1 584 Backpressure ControllerFigure 6-1 : DSI preheat section back-pressure control loop.
The requirement for an adequate holding tube pressure is given by equation (6.6). However, it should also be remembered that this neglects any additional pressure drop due to the back pressure control valve. A control valve should be sized so it is operating in the middle of its range and this can be accommodated by including a safety margin into equation (6.6).
Where, �, pressure at the top of the holding tubes.
�,ead
pressure drop due to the height of the holding tubes.(Pa) (Pa)
Optimum Operating Regime
pressure drop due to the back-pressure orifice plate. pressure of the flash vessel.
vapour pressure of milk.
(pa) (pa) (pa)
The orifice plate pressure drop is given by the standard orifice plate equation and the holding tube pressure drop is given by the hydrostatic head.
Mhead
=p.g.hl
Where,
Cd
orifice plate discharge coefficient. (-)AOriI
cross sectional area of orifice plate hole. (m2)QOSI
volumetric flow of liquid through the holding tubes. (m3/s)f3 ratio of orifice plate diameter to pipe diameter. (-)
p
density of liquid in holding tubes. (kg/m3)hI
height of holding tubes. (m)(6.7)
These equations show that the pressure at the top of the holding tubes depends on the orifice plate size. Smaller orifice plates give greater pressure drops and consequently higher pressures at the top of the holding tubes. The choice of the orifice plate size thereby determines whether the milk will vaporise at the top of the holding tubes.
The pressure in the flash vessel and the required milk vapour pressure can be determined from the simple preheat model and the Antoine saturation equations. If the DSI temperature varies across a wide range then the flash vessel temperatures will also vary. Consequently the pressure in the flash vessels will change and also the pressure at the top of the holding tubes. This means that the orifice plate needs to be sized for the maximum DSI temperature, when the milk vapour pressure is at its highest. If we neglect the milk boiling point elevation then the flash vessel pressure is given by the following, where
Tphl
is given by equation (3 .30).2 1