506 Farm Mechanics. 



If the area of the water surface at the water level in the 

 boiler is small in proportion to the pounds of steam deliv- 

 ered the water carried over will be large and for this rea- 

 son the horizontal boilers of a given H. P. tend to supply 

 dryer steam than the vertical boilers do because there is 

 more surface from which the steam may escape and the 

 agitation is less. (3) If the volume of steam space above 

 the water is large there is more opportunity for the sus- 

 pended water to fall back and leave the steam dryer, 

 hence one means of preventing "priming," as carrying 

 over water is called, is to work with the water level low 

 in the gage glass. (4) If the size of the boiler compared 

 with the amount of steam required for each stroke of the 

 piston is small the tendency will be to cause the pressure 

 in the boiler to vary and this variation will agitate the 

 water and cause "priming." When this is the case prim- 

 ing may be lessened by throttling down the steam supply 

 at the stop valve. 



632. Wetness Due to Condensation in Steam Pipes 

 and Valve Chest. When the steam pipes are long, lead- 

 ing from the boiler to the steam chest, and when they are 

 not jacketed and are exposed to the cold, priming is pro- 

 duced. Jacketing the steam pipes and the steam chest 

 reduces the priming from this cause very materially be- 

 cause the loss of steam from uncovered iron steam pipes, 

 per degree of difference of temperature between steam and 

 outside air, is about 2.4 heat units per square foot of out- 

 side surface of the pipe per hour, while covering the pipe 

 with wool felt one-half an inch thick reduces the loss to .7 

 heat units in the same time. 



633. Initial Condensation. The temperature of the walls 

 of the cylinder is always, in practice, colder than the 

 entering steam as it comes from the boiler and so there 

 must be a greater or less condensation as it enters until 

 both are brought to the same temperature. So great is 

 the condensation of stoam when the engine is first started 



