b. Proper air ejector operation
(1) Clean nozzle and strainer
(2) Correct quality and quantity of steam
c. Ample flow of circulating water
(1) Clean strainer, pipeline, and tubes
(2) Proper valve settings
(3) Proper operation of the circulating
d. Effective surface in the distilling condenser
(1) No undue deposits inside the tubes
(2) Proper venting of the condenser
(3) Proper operation of the condensate
A distilling plant cannot maintain its full output
unless it is supplied with dry steam at the designed
pressure. The orifices were constructed to pass the
proper amount of steam plus about 5 psig pressure to
safely produce the designed plant output. Orifices
should be inspected annually. An orifice should be
measured and the reading compared with the figure
stamped on the plate. If necessary, the orifice should be
If the steam pressure above the orifice varies, the
exact source of trouble should be located and corrected.
First the weight-loaded regulating valve and then the
pressure-reducing valve (if installed) should be checked
to determine whether or not each valve is operating
properly. If they are functioning properly and the
pressure cannot be maintained above the orifice, you
may assume that an insufficient amount of steam is
being supplied to the plant.
The auxiliary exhaust steam supply for the distilling
plants, after passing through the regulating valve, is
usually slightly superheated because of the pressure
drop through the reducing valve and the orifice plate. A
small amount of superheat has little or no effect on the
plant operation or the prevention of scale formation.
However, when live steam must be used, the installed
desuperheater spray connection should be used to
control the superheat. The water for desuperheating
must be taken from the boiler feed system, preferably
from the first-effect tube nest drain pump. Water for
desuperheating must NEVER be taken directly from the
fresh water distilled by the distilling plant.
Fluctuations in the first-effect generating steam
pressure and temperature cause fluctuations of pressure
and temperature throughout the entire plant. With
increased salinity of the distillate, the fluctuations may
cause priming, as well as erratic water levels in the
shells. These fluctuations may he eliminated by proper
operation of the automatic pressure regulators in the
steam supply line.
First-Effect Tube Nest Vacuum
The range of the pressure maintained in the
first-effect tube must be between 16 inches of mercury
(in.Hg), with clean tubes, to 1 to 2 in.Hg as scale forms.
The output of a submerged-tube type of distilling plant
is not greatly reduced until the deposits on the tubes have
caused the vacuum to drop to about atmospheric
pressure. When the first-effect tube nest vacuum is lost
entirely, the reduction in output becomes very great.
Assuming the reduction in vacuum is due to scale and
not to improper operating conditions, the tubes must be
Keeping the vacuum in the first-effect tube nest as
high as possible reduces scale formation to a minimum,
enabling the plant tooperate at full capacity.
A vacuum reduction that results from any factor
other than deposits on tube surfaces should be corrected
to reduce deposits and greatly extend the intervals
between cleanings. The primary factors affecting the
first-effect tube nest vacuum are air leakage, low water
levels in the evaporator shells, improper venting of the
evaporator shells, scale or other deposits on the tubes,
and improper draining of the evaporator tube nests.
Loss of vacuum resulting from deposits on
evaporator tubes should be gradual. Under normal
conditions, there will be no major loss of vacuum for
any one days operation. Any sudden drop in vacuum
can be traced to causes other than scale deposits.
The generating steam circuit operates under
vacuum and is subject to air leaks. Leaks from the steam
side of the first-effect tube nest to the first-effect shell
space cause losses of capacity and economy. Loss of
vacuum and loss of capacity may be due to air leaks. The
air leaks may he from the atmosphere into the generating
steam line (downstream from the orifice plate); from the
first-effect tube nest front header; or from the first-effect
tube nest drain piping. Air leaks in this part of the
distilling plant may be less noticeable than air or water
leaks elsewhere, because the effect on the plant is similar
to the scaling of the tube surfaces.