The objective should be to achieve the process temperature with minimum steam costs. For instance, heating water to 85 deg C can be achieved by using Steam at any pressure above atmospheric.
From the steam table one can note that the Latent Heat of Steam(hfg) decreases as the steam pressure increases. For instance, for the above example if we have to choose between Steam at 2 Barg or at 7 Barg – following factors will govern the right choice –
Latent Heat of Steam at 2 Barg = 517.6 Kcal/Kg
Latent Heat of Steam at 7 Barg = 489.9 Kcal/Kg
If someone chooses 7 Barg Steam for this application, Steam consumption (and costs) will increase by 5.4 % for a given heat load. Hence choosing 2 Barg steam is right choice for this application.
It is economical to go as low as possible. However since temperature difference drives the rate of heat transfer, there is a limit to how low can we go. To make it easy to remember, the thumb rule below can be used to select the right steam pressure for any process.
If T is Process Temp, select the steam pressure that corresponds to T + 35 deg C