Need For Varying Expansion
The pressure of the expanded steam is less than that of steam supplied directly from the boiler. An engine working with an expansion valve set to an early cut-off is thus less powerful than with the valve fully open. Accordingly the engine must now be driven, so that the valve is manually adjusted as the load on the engine changes. An engine running under light load may be operated efficiently with an early cut-off, an engine under heavy load may require a longer cut-off and the cost of more steam consumption.
When Trevithick supplied his 1801 engine for a rolling mill at Tredegar Iron Works, the engine was more powerful when worked without expansion and Samuel Homfray, the ironmaster, preferred to use the extra power despite the potential saving in coal costs
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