Chapter 15. -PUMPS AND FORCED DRAFT BLOWERS 



the turbine. The same throttle valve is usually 

 used to control the admission of steam to all 

 blowers serving any one boiler. K only one 

 blower is to be operated, the root valve of the 

 nonoperating blower must be kept closed. 



When admission of steam is controlled by 

 the four nozzle valve arrangement, no additional 

 nozzle area is required to bring the blower up 

 to maximum speed. In the other type of instal- 

 lation, a special hand-operated nozzle valve is 

 provided for high speed operation. This nozzle 

 valve, which is sometimes called an overload 

 nozzle valve , is used whenever it is necessary 

 to increase the blower speed beyond that ob- 

 tainable with the fixed nozzles. As a rule, the 

 use of the overload nozzle valve is required 

 only when steam pressure is below normal. 



Checking Blower Speed 



Many forced draft blowers are fitted with 

 constant-reading, permanently mounted tachom- 

 eters for checking on blower speed. Sometimes 

 the tachometer is mounted ontopof the governor 

 and is driven by the governor spindle. The gov- 

 ernor spindle is driven by the main shaft through 

 a reduction gear, and therefore does not rotate 

 at the same speed as the main shaft. However, 

 the rpm of the governor spindle is proportional 

 to the rpm of the main shaft. The tachometer 

 is calibrated to give readings that indicate the 

 speed of the main shaft rather than the speed 

 of the governor spindle. 



Some blowers are equipped with a special 

 kind of tachometer called a pressure-gage tach- 

 ometer . This instrument, which may be seen 

 in figures 15-35 and 15-37, is actually a pres- 

 sure gage which is calibrated in both psi and 

 rpm. The pressure-gage tachometer depends 

 for Its operation on the fact that the oil pres- 

 sure built up by the centrifugal lube oil pump 

 has a definite relation to the speed of the pump 

 impeller; and the speed of the impeller, of 

 course, is determined by the speed of the main 

 shaft. Thus the instrument can be calibrated 

 in both psi and rpm. 



Some forced draft blowers of recent design 

 are equipped with electric tachometers which 

 have indicating gages at the blower and at the 

 boiler operating station. The electric tachom- 

 eter (sometimes called a tachometer generator) 

 consists of a stator and a permanent magnet 

 rotor mounted at the bottom of the turbine shaft. 

 The wire from the generator plugs into a con- 

 nector inside the sump. Another connector is 



provided outside the sump for attaching the wire 

 from the generator to the transformer box. 



Forced Draft Blower Operation 



Forced draft blowers supply the air required 

 for combustion of the fuel oil. The amount of 

 air that enters the furnace is determined by the 

 air pressure in the double casings. Although 

 the air pressure is affected by the number of 

 burners in use and by the amount that the air 

 registers are open, it is primarily determined 

 by the speed at which the forced draft blowers 

 are operated. The speed of the forced draft 

 blowers is controlled by manual adjustment of 

 the blower throttle in all installations except 

 those having automatic combustion control 

 systems. 



The forced draft blowers should be operated 

 in such a way as to furnish the required amount 

 of air for the complete combustion of the fuel 

 being burned. In actual practice, it is neces- 

 sary to supply just over 100 percent of the 

 amount of air theoretically required, in order 

 to ensure the complete combustion of the fuel. 

 Higher percentages of excess air are wasteful 

 of fuel, since all air that does not actually enter 

 into a combustion reaction merely absorbs and 

 carries off heat. 



On the other hand, an insufficient quantity of 

 air for combustion is also detrimental to boiler 

 efficiency. If there is not enough air for com- 

 plete combustion, there may be a greater loss 

 in efficiency. Or, if even less air is supplied, 

 some of the carbon will not be burned at all 

 but will pass out the smokestack as black smoke. 

 Insufficient air is also detrimental because it 

 causes the boiler to pant and vibrate; this is one 

 of the major causes of brick work failure in 

 the boilers. 



The air pressure in the double casing must 

 be increased BEFORE the rate of combustion 

 is increased, and must be decreased AFTER 

 the rate of combustion is decreased. There is 

 usually little difficulty in teaching fireroom 

 personnel to increase the air pressure before 

 the rate of combustion is increased, since fail- 

 ure to do so results in panting and vibration 

 of the boiler and in heavy smoke. It is more 

 difficult, however, to teach the men to decrease 

 the air pressure after the rate of combustion 

 has been decreased. However, operating the 

 blower at a faster speed than is required for 

 the rate of combustion is definitely a poor 



419 



