Performance of Steam Engine
In this post, we are discussing the performance of steam engine i.e., the efficiency of the steam engine and heat balance sheet.
The Efficiency of Steam Engine
It is the general term used for the ratio of work done to the energy supplied to an engine.
The following are the important efficiency of a steam engine:
- Mechanical efficiency
- Overall efficiency
- Indicated thermal efficiency
- Brake thermal efficiency
- relative efficiency
Read also: Steam Engines: Parts, Working and 14 Different Types of Steam Engines
1. Mechanical Efficiency
It is the ratio of the brake horsepower (B.H.P.) to the indicated horsepower (I.H.P.) Mathematically, mechanical efficiency,
It may be observed that the mechanical efficiency is always less than unity (i.e., 100%) because some power is lost in overcoming the engine friction.
In other words, the indicated horsepower is always greater than brake horsepower. This power which is lost in overcoming the engine friction is known as frictional horsepower. Therefore, frictional horsepower,
F.H.P. = I.H.P. – B.H.P
2. Overall Efficiency
It is the ratio of the work obtained at the crankshaft at a given time to the energy supplied by fuel during the same time.
Let
- Wf = Weight of fuel burnt per hour.
- C = Calorific value of fuel in kcal/kg of fuel.
and work obtained at the crankshaft/min
3. Indicated Thermal Efficiency
It is the ratio of heat equivalent of indicated horsepower to the energy in the steam supplied per minute.
Let
- Ws = Weight of steam used in kg/min.
- H = Total heat of steam supplied in kcal/kg.
- h = Sensible heat of feed water in kcal/kg.
and heat equivalent to I.H.P.
4. Brake Thermal Efficiency
It is the ratio of the heat equivalent of brake horsepower to the energy in the steam supplied per minute.
Mathematically, brake thermal efficiency
Note:
Whenever thermal efficiency is mentioned without qualifying the name, i.e., “indicated” or “brake”, the indicated thermal efficiency should be calculated.
5. Relative Efficiency
The relative efficiency is known as efficiency ratio. It is the ratio of thermal efficiency to Rankine efficiency. Mathematically, relative efficiency,
Heat Balance Sheet
The complete record of heat supplied and rejected during a certain time (say one minute) by a steam engine is entered in a tabulation form known as a heat balance sheet. The following values are generally required to complete the heat balance sheet of a steam engine.
1. Heal supplied to cylinder per minute
Let,
- Ws = Weight of steam supplied to the cylinder in kg/min,
- Wj = Weight of steam supplied to jackets in kg/min, and
- H = Total heat of steam supplied in kcal/kg.
Note:
If some water is received from receiver drain (Wrj, heat supplied to the cylinder/min.
2. Heat absorbed in I.H.P.
We know that heat in I.H.P
3. Heat rejected to the cooling water
Let,
- Wc = Weight of the cooling water/min,
- to = Outlet temperature of cooling water
- ti = Inlet temperature of cooling water.
4. Heat rejected in condensate
Let,
- tc = Temperature of the condensate.
Note:
The sum of heat rejected to the cooling water and condensate is known as heat rejected to exhaust steam.
5. Heat rejected in jacket drain
Let
- Wj = Weight of water drained for jackets/min
- tj = Temperature of jacket water.
6. Unaccounted heat
There is always some loss of heat due to friction leakage, radiation etc., which can not be determined experimentally. In order to complete the heat balance sheet, this loss is obtained by the difference of heat supplied to cylinder per min and heat rejected in I.H.P., exhaust steam and jacket drain.
Finally, the heat balance sheet is prepared as given below:
That’s it. Thanks for reading. If you have any questions about “The efficiency of a steam engine or heat balance sheet” you can ask in the comments. If you found this article helpful please share with your friends.
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