Injectors

[NW Mailing List]

Jimmy, thanks for the info. Can you explain how a feedwater pump works?  Is it a pump, like the airpumps? If so, is there a piston?

Mike Rector



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From: NW Mailing List <nw-mailing-list at nwhs.org>
To: NW Mailing List <nw-mailing-list at nwhs.org>
Sent: Friday, March 11, 2011 12:07 PM
Subject: Injectors


 
"An 
injector puts cold water (ignoring the steam used to operate it) into the 
boiler, so an engineer could use his injector in such a way as to make life 
tough for the fireman by killing steam pressure with cold water while keeping 
the throttle way open."
 
Gordon,
    I guess we have all heard this same story and it has 
been ingrained in our brain as "that's the way it is". Here is another one you 
may have heard: "as the safety pop valves were going off, the engineer 
turned on the injector to put cold water in the boiler and knock the pops down." 
This all sounds plausable, eh?
    The end results in each example are true. 
However, the way those results are achieved are different from what we have all 
been told. 
    "Cold"  water is a 
relative term. Water in the tender can be considered as being at the current 
ambient temperature or shall we say "Cold". Water in the working boiler is "Hot" 
and under steam pressure. Saturated steam from the boiler is routed from the turret to and through the injector in order to 
supply feedwater to the boiler. Inside the injector this steam directly comes in 
contact with the cold feedwater from the tender and actually is a type of 
"feedwater heater". 
    From this contact the steam thus heats the feedwater 
quite a bit. Naturally it can only heat the water a relative amount. However, 
this amount is quite a lot and thus the water entering the boiler at the check 
valve is not "cold" by any means. It is not as hot as the water in the 
boiler, but, it is not cold either. It can be said that feedwater from the 
injector absorbs, for all intents and purposes, 100% of the heat used to 
force the water into the boiler. By natuee, the injector can never supply "cold" 
water to the boiler. Now hold that thought.
    Switching to the other side of the engine is what we all 
know as the "Feedwater Heater System". This includes a steam driven "Cold Water 
Pump", the "Feedwater heater" and a steam driven "Hot Water Pump". 
    A very important note here is that the 
"Feedwater Heater" only supplys heated water when the locomotive is being worked 
and there is exhaust steam to mingle with the cold water. 
    When the locomotive is "working", (read under load), the Cold Water Pump takes "cold" water from the 
tender and pumps it to the Feedwater Heater section. Here the "cold " water is 
heated, directly or indirectly (depending on the type of system), by exhaust 
steam used by the cylinders. Again this "cold" water can only be heated a 
relative amount by this exhaust steam. Once heated the "Hot Water Pump" is used 
to force the water into the boiler. This water is also not as hot as boiler 
temperature. 
    The amount of heat transmitted to the water by the "FWH" 
depends on how much exhaust steam moves through the heater which is 
determined by how hard the locomotive is being worked. Since the pumps of 
the "FWH" system are steam driven, it would be possible to actually force "cold" 
water  into the boiler.Thus the "FWH" is not used while the loco is sitting 
still.     
    But, for the most part, "cold" water never 
enters the boiler. Neither the injector or the feedwater reason can supply the 
boiler with water as hot as what is already there.
    Back to the injector. As explained, when the injector is 
turned on, it uses live steam from the boiler to operate. The use of this live 
steam naturally reduces the pressure in the boiler in order to supply the 
boiler. It is this use of live steam "from the boiler" to supply itself with 
water is the root cause of the reduction of boiler pressure and not so much from 
the relative lack of heat of the water being fed to the boiler.
    The savings in using the "FWH" is that it utilizes heat 
from the exhaust steam that has already been charged to the boiler. The effect 
on any boiler pressure reduction caused by the use of live steam to supply the 
FWH pumps is minimal.
 
Jimmy Lisle
        
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