COMPARISON OF PUMPS AND INJECTORS
_________________________________________________________________________
| | | |
| Method of Supplying | | |
| Feed-water to Boiler | Relative amount of | Saving of fuel over|
| Temperature of feed-water as | coal required per | the amount required|
| delivered to the pump or to | unit of time, the | when the boiler is |
| injector, 60 degrees Fahren- | amount for a direct-| fed by a direct- |
| heit. Rate of evaporation of | acting pump, feeding| acting pump without|
| boiler, to pounds of water | water at 60 degrees | heater |
| per pound of coal from and | without a heater, | Per Cent |
| at 212 degrees Fahrenheit | being taken as unity| |
|______________________________|_____________________|____________________|
| | | |
| Direct-acting Pump feeding | | |
| water at 60 degrees without | | |
| a heater | 1.000 | .0 |
| | | |
| Injector feeding water at | | |
| 150 degrees without a heater | .985 | 1.5 |
| Injector feeding through a | | |
| heater in which the water is | | |
| heated from 150 to 200 | | |
| degrees | .938 | 6.2 |
| | | |
| Direct-acting Pump feeding | | |
| water through a heater in | | |
| which it is heated from 60 | | |
| to 200 degrees | .879 | 12.1 |
| | | |
| Geared Pump run from the | | |
| engine, feeding water | | |
| through a heater in which it | | |
| is heated from 60 to 200 | | |
| degrees | .868 | 13.2 |
|______________________________|_____________________|____________________|
The injector, considered only in the light of a combined heater and
pump, is claimed to have a thermal efficiency of 100 per cent, since all
of the heat in the steam used is returned to the boiler with the water.
This claim leads to an erroneous idea. If a pump is used in feeding the
water to a boiler and the heat in the exhaust from the pump is imparted
to the feed water, the pump has as high a thermal efficiency as the
injector. The pump has the further advantage that it uses so much less
steam for the forcing of a given quantity of water into the boiler that
it makes possible a greater saving through the use of the exhaust from
other auxiliaries for heating the feed, which exhaust, if an injector
were used, would be wasted, as has been pointed out.
In locomotive practice, injectors are used because there is no exhaust
steam available for heating the feed, this being utilized in producing a
forced draft, and because of space requirements. In power plant work,
however, pumps are universally used for regular operation, though
injectors are sometimes installed as an auxiliary method of feeding.
Table 18 shows the relative value of injectors, direct-acting steam
pumps and pumps driven from the engine, the data having been obtained
from actual experiment. It will be noted that when feeding cold water
direct to the boilers, the injector has a slightly greater economy but
when feeding through a heater, the pump is by far the more economical.
Auxiliaries--It is the general impression that auxiliaries will take
less steam if the exhaust is turned into the condensers, in this way
reducing the back pressure. As a matter of fact, vacuum is rarely
registered on an indicator card taken from the cylinders of certain
types of auxiliaries unless the exhaust connection is short and without
bends, as long pipes and many angles offset the effect of the condenser.
On the other hand, if the exhaust steam from the auxiliaries can be used
for heating the feed water, all of the latent heat less only the loss
due to radiation is returned to the boiler and is saved instead of being
lost in the condensing water or wasted with the free exhaust. Taking
into consideration the plant as a whole, it would appear that the
auxiliary machinery, under such conditions, is more efficient than the
main engines.
[Illustration: Portion of 4160 Horse-power Installation of Babcock &
Wilcox Boilers at the Prudential Life Insurance Co. Building, Newark,
N. J.]