500 MODERN GASWORKS PRACTICE
(6) Small ground space required.
(7) Access easily and quickly gained to working parts.
Fig. 311.— Rateau High-speed Exhauster driven by Steam Turbine.
EXHAUSTERS PRIOR TO CONDENSERS
The position of the exhauster is entirely arbitrary. In almost all cases, how-ever, it will be found in sequen.ee between the condensing and washing plant. Some engineers prefer to place the machine either before the condensers (so that con-densing takes place under pressure), or after the washers (so that both condensers and washers are under a vacuum). The practice of condensing under pressure has assumed some importance, and finds a stout advocate in Dr. Carpenter. The problem of the most effective means of lubricating an. exhauster is always a vexed one, and is said to be more troublesome when the light oil tars have been removed from the gas in the process of condensation. When, however, the exhauster is placed prior to the condensers, so that it is operated at a temperature of about 60° C., no lubricant is necessary, the condition of the gas being sufficiently oily to ensure smooth running of the machine. By placing the exhauster at a point nearer to the liydraulic main a shorter length of main under vacuum is assured, so that there is less probability of the intake of air through, defective joints. The disad-vantage attached to exhausting hot gas is that owing to its increased temperature the bulk of the gas is augmented, hence additional exhauster capacity is necessary. If the average temperature of the gas is about 60° C., the additional capacity required (compared with the ordinary practice of exhausting at 20° C.) will amount to about 15 per cent.