January 2017 Archives

#276 Heat-Saving Measures - High Quality Steam

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(1)Steam with high dryness

In the previous volume, we learned that the steam generated from a boiler is classified into dry saturated steam, wet steam, and superheated steam. Among of all, dry saturated steam at 100% dryness shown in the steam table is the type of steam required in surface treatment processes.
Even if a boiler outlet supplies dry saturated steam, the dryness of steam decreases (the wetness increases) due to the heat loss along the way. When the dryness of steam decreases, the latent heat of steam also drops, which ultimately reduces the heating capacity.
In addition, the steam demand may surpass the amount of steam generation during the peak period, such as the preparation hour for daily operations. During such period, the boiler produces wet steam with less dryness.
Let's compare the latent heat of steam at varying dryness in [Table 1].

[Table 1] Difference of latent heat at varying dryness (steam pressure 0.2 MpaG)
Steam typeDryness of steam (%)Sensible heat (KJ/Kg)Latent heat (KJ/Kg)Latent heat ratio
Dry saturated steam100561.432163.2×1.00=2163.24
Wet saturated steam25561.432163.2×0.25=540.81

This table shows that the latent heat of the steam at 25% dryness is only 25% of the steam at 100% dryness. This means that four times more of this steam is equivalent to high quality steam at 100% dryness. As you can see, the dryness is an important factor for the latent heat.
The recent models of boilers generate superheated steam by heating dry saturated steam using a superheater operated with recycled exhaust heat. If the boiler supplies this steam, heat loss during the transport will only reduce the temperature of superheated steam and will not affect the latent heat of condensation during heating.
In addition, as explained in the previous volume, reducing the steam pressure at the boiler outlet will increase the dryness of steam by taking advantages of superheating performance at a throttle.
The dryness of steam, which is an important factor for boilers, can be measured by using a throttling calorimeter as shown in [Fig.1]. Just like the figure, insert a tube with holes into the steam pipe and measure the temperature and pressure of the steam passing through the throttle. Based on these measured values, you can find out the heat quantity and compute the dryness X using the steam table. As you can see, the heat retention control for steam piping is extremely important during steam transport in terms of the dryness control as well.

fig1

Heat insulation tubes made of glass wool are generally selected for heat retention control of steam piping. Adopting a thicker heat insulation layer can improve heat insulation, but it will be more costly. As a middle ground between the cost and effect, heat-insulation materials of 25 to 100 mm thickness are commonly used. Realistically, you cannot reduce the piping heat loss to zero. It is also important to remove drained water generated in the pipes.

#275 Heat-Saving Measures - Steam Conservation

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(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
Steam pressure
(Mpa・G)
Steam temperature
(°C)
Total heat
(KJ/Kg)
Latent heat
(KJ/Kg)
Difference of latent heat
(KJ/Kg)
0.9179.882776.22013.60
0.2133.542724.72163.2149.6

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.

#274 Heat-Saving Measures - Steam Properties -2

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(2)Heat quantity of steam

Now, let's see how much heat energy the steam has. To find out, "steam tables" are useful.
Pressure-based and temperature-based steam tables are available. Values in both tables are expressed in absolute pressure (Mpa).
The table here is in the pressure-based format. Since a pressure gauge is used for displaying the pressure of ordinary steam boilers, [Table 1] lists the temperature and heat quantity based on the readings of a gauge pressure (MpaG).

[Table 1] Steam table
Gauge pressureTemperatureSpecific enthalpy
(KJ/Kg)
MpaG Sensible heatLatent heatTotal heat
0.0011004192,2572,676
0.11205052,2022,707
0.21345612,1632,725
0.31446052,1332,738
0.41526402,1072,748
0.51596702,0852,756
0.61656972,0652,762
0.71707202,0472,768
0.81757252,0302,772
0.91807632,0142,776
1.11887981,9842,783

Note) 0.1 MpaG = 1.0 Kg/cm2G (former display unit), KJ = 0.2389 KCaL, KCaL = 4.186 KJ

In the above table,
gauge pressure is the steam pressure displayed on a pressure gauge. Specific enthalpy refers to the amount of heat for 1 kg of steam at the pressure displayed on the gauge.
It breaks down to sensible heat and latent heat. In this table, Total heat = sensible heat + latent heat. Sensible heat and total heat increase as the steam pressure becomes higher. However, latent heat decreases at this time.
As described in the previous volume, the heat circulates in the heating system of plating bath. See the figure below.

fig

Indirect heating is a common method for heating plating bath in general, such as using a heater with corrugated tubes or a heat exchanger to circulate plating solution in a pump. Therefore, the latent heat of condensation of water is the only heat effectively used in this heating system.
Now, suppose you have used the steam with the gauge pressure 0.2 MpaG. In this table, the total heat is 2,725 KJ/kg for 1 Kg of steam, but the heat quantity available for heating the plating bath is only 2,163 KJ/kg.

#273 Heat-Saving Measures - Steam Properties -1

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(1) Steam properties

[Fig.1] illustrates the process of room temperature water turning into steam. First, prepare water at 15°C in an airtight container. Then, heat the container. The water temperature gradually rises and starts boiling when it reaches at 100°C. If you keep heating it, water evaporates into steam although the temperature stays the same. Once all the water evaporates into steam, the temperature starts rising again. The heat quantity used to be described in units of kilocalories (KCal), but using kilojoules (KJ) is more common nowadays.

[Fig.1] Transition of water by heating

[Fig.2] Three states of water, sensible heat & latent heat

[Fig.3] Saturated water, wet steam, and superheated steam

[Fig.2] shows the relationship between water and heat when this phenomenon occurs. In this phenomenon, heat transforms water into three states: ice (solid), water (liquid), and steam (gas). There are two types of heat: sensible and latent heat. Sensible heat is in a form of heat energy that can be measured by thermometers whereas latent heat cannot be measured by such instruments. In addition, the steam in the right container shown in [Fig.1] can be in various forms as explained in [Fig.3].

Boiling water and steam coexist in a boiler. This steam is called "wet (moist) steam". The latent heat of this steam changes by the quantity of heat energy applied.
In addition, the moisture content of steam is called "wetness". When the wetness becomes 0%, the steam turns into a form called "superheated steam" and the steam temperature increases by heating.

In surface treatment processes, the outlet of a boiler should have superheated steam while the inlet of a heater or a heat exchanger should have dry saturated steam (dryness of steam is 100%). During these processes, the latent heat of steam vaporization travels through the wall of a heater to be transmitted to liquid substances, such as a plating bath.
After losing the latent heat, the steam turns into hot water at 100°C called condensed water. The water used to be lost down the drain is now collected and recycled by most factories to be used as feedwater in boilers. The process is called "drain recovery (sensible heat recovery)".

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