Thermal Fluid Distribution Systems - Energy Conversion and Management - page 135

8.4.1 Heating a Mass by Steam

m steam ¼ ð c m Δ t Þ= h v

where m steam ¼ steam mass flow rate, c

¼

specific heat of the material to be heated,

m ¼ mass flow rate to be heated, Δ t ¼ temperature increase of the mass,

h v ¼ evaporation enthalpy or latent heat of the steam.

8.4.2 Exchanging Heat Between a Fluid and Steam
(See Also Chap. 15 )

Δ

t
R th

1
h v ¼

m steam ¼ A A Δ t U = h v

where m steam ¼ the log-mean

difference between the temperatures of the two fluids at the two sides of the exchanger
(sec Chap. 15 ), R th ¼

steam mass flow rate, A ¼ surface of exchange, Δ t ¼

overall thermal resistance ¼ 1/ U , U ¼ overall heat transfer

coefficient (see Chap. 15 ), h v ¼ evaporation enthalpy or latent heat of the steam.

8.4.3 Extraction of Water in Industrial Dryers by Evaporation

The necessary quantity of heat medium (steam, oil) is related to the
quantity of water contained in the product and to the efficiency of the
system. In industrial applications at least 4,000-5,000 kJ/kg of water
(1,710-2,160 Btu/lb of water) is generally required, that is roughly
1.6-2 kg of steam, to evaporate 1 kg of water contained in the product.

Operation of most process equipment can basically be derived from the above
relationships; low-consumption process equipment is designed to ensure heat
recovery inside the equipment itself and the minimum requirement of thermal
energy (hot water, hot air, steam, hot oil).

8.5 Practical Examples

There follows examples of insulation with reference to Figs. 8.2 and 8.3 .

Example 1 Compare losses from a composite wall with different thicknesses of
insulation (see Fig. 8.2 and Table 8.4 )
A flat surface with air on both sides is heated on one side. The difference
between inside and outside fluid temperatures is 100 C (180 F).