HEAT BALANCE SHEET

Procedure: For preparing a heat balance sheet for a steam engine cylinder, the engine should be tested over a period of time under conditions of constant load and steam supply. An indicator diagram should be taken and the steam pressure noted at regular intervals of time, an account should also he kept of the steam supply to the jackets. The mass of the steam supplied to the cylinder supplied can be obtained from the steam condensed by the condenser.

Analysis: The heat balance for the steam in engine is more easily drawn up than that of the internal combustion engine. The working fluid in the steam engine does not undergo any chemical change, and consequently changes in properties may be ascertained with reference to an arbitrary datum.

Figure 5.19 shows, diagrammatically, a steam engine and various quantities entering and leaving have been indicated there on. Treating the engine as a flow system as shown in Fig. 5.20, an energy equation may be written as follows:

where m = mass, W = work, h = specific enthalpy, Q = heat transferred, and suffices have the following meanings, s₁ = steam supplied, s₂ = condensate discharge, c = cooling water, R = radiation.

Figure 5.19 A steam engine

Figure 5.20

The Eq. (5.19) in tabular form is expressed as follows:

Heat in		Heat out
In steam	ms1 hs1	Heat as shift work	W
		Heat to condensate	ms2 hs2
		Heat to coolant	mc (hc2 − hc1)
		Heat to radiation	QR

The value of enthalpy may be obtained from the steam tables. The heat to coding water is given by m_c × c_pw (t_out − t_in) and the heat to radiation obtained by the difference. In the absence of leakage m_s1 = m_s2.

The heat balance may therefore be written as follows:

Heat in	Heat out
Heat in steam	Heat as B.P.
	Heat to condensate.
	Heat to cooling water.
	Heat to radiation, by difference.

It is to be noted that friction terms are absent, since any work done against friction is included in heat to the condensate, cooling water and radiation loss.