(3) Energy conservation by reducing steam pressure

The steam table introduced in the previous volume exhibited the sensible/latent heat quantity per weight at varying steam pressures. In this volume, we will look at an example of steam conservation using [Table 1].

Generally, small boilers have the combustion control settings to generate steam pressured at 0.9 Mpa・G or up to 1.0 Mpa・G. This is because the pressure control settings in this range are economical.

In many cases, rare metals with superior corrosion resistance, such as titanium or niobium, are adopted for heating tubes and heat exchangers used for heating plating bath. In order to reduce the production cost, manufacturers are trying to minimize the metal usage by adopting thinner materials. Using the thinner heat transfer materials for heat exchangers is also beneficial in terms of improving their heat-transfer effect.
However, using thinner materials may pose problems concerning the pressure capacity. A heater made of thin materials will not withstand the inflow of high-pressure steam. In order to transfer the desired amount of heat using such heater, it is necessary to reduce the steam pressure. Commonly, the maximum pressure of heaters used for surface treatment is set to 0.2 Mpa・G.

This has produced an unexpected energy-saving effect. Steam has sensible heat and latent heat. The sensible heat per unit weight decreases as the steam pressure drops. On the contrary, the latent heat increases as the steam pressure decreases. Let's look at a specific example in [Table 1].

[Table 1] Energy-saving effect by a decrease of steam pressure in heating systems

Let's assume this factory uses 20 tons of steam per day. If the factory reduces the gauge pressure from 0.9 Mpa・G to 0.2 Mpa・G, they can save the following quantity of heat.

20 ton × 1000 Kg × 149.6 KJ/Kg = 2,992,000 KJ

This will save the steam pressured at 0.2 Mpa・G for the following weight:

2,992,000 KJ/2163.2 KJ/Kg = 1,383 Kg

This is a daily value, but if they continue every day, this will be a huge saving.