BASIC RULE OF THE EFFICIENT USE OF STEAM
NEVER PERMIT STEAM TO EXPAND FROM ONE PRESSURE TO A LOWER PRESSURE WITHOUT GETTING SOME USEFUL RESULT FROM THE EXPANSION.
As far as steam for heating and process is concerned, there are just two fundamental things that govern everything;
YOU LOSS MONEY FOR EVERY PRESSURE DROP
- The boiling point of water decreases with reduced pressure.
- The latent heat (the "heating heat) of steam increases with reduced pressure.
UNSEEN LOSS THAT EVERYONE IS SUFFERING WITHOUT KNOWING IT.
We have so many ways to save energy and most of us seem to look into any method which gives saving clearly visible to the eye of the management such as waste heat recovery, change of fuel to lower the cost per Kcal. All these methods can give a clear figures to the management as to how much money company can save by calculating value of the heat. However, to the author's knowledge, most of the steam users do not know that they are losing so much money every seconds when their boilers operate with fluctuating pressure. People tend to believe that any heat input can produce energy in a form of steam with a certain consideration of heat loss which reflects in the boiler efficiency regardless of fluctuating steam pressure. They say that even pressure changes, we are getting steam equivalent to what we spend in energy input. This is totally wrong concept. There are hidden losses of energy or unseen losses of great amount of money when your boilers are operating with fluctuating pressure. Every time when pressure drops, you are loosing money. Everytime when the boiler must rise its pressure to what you need, again, you are losing money. In this documents, I am going to elaborate why this happens and how we can prevent it.
LET US KNOW THE FUNDAMENTAL OF STEAM
Now, we have to go deeper into the characteristic of Molecules of Water. The molecule in liquid
is constant motion, and their speed of movement is dependent on the temperature. The hotter the liquid the faster do the molecules move. Owing to the congestion and to their erratic movement collision often take place. As a result of multiple collision some molecules go, for a short time, much faster then their fellows who may have been slowed, or even stopped, as a result of the collision. The more heat we add to the liquid the faster go the molecules, the more molecules can escape.
At the water level of the atmospheric pressure, molecules of the air-born water and molecules of the water are having battles all the times. A hail of air or vapor molecules is raining down on because it always carries some moisture, so that, although some water molecules are jumping from the water surface some air-born water molecules are diving back into the liquid.
HEAT IS ENERGY
Heat is a just a form of energy. When heat is added to a substance it is stored in the substance as extra molecular movement. It's a mechanical movement and mechanical energy. The greater the weight to be moved, the greater the energy needed.
WHAT PRESSURE IS
The pressure exerted by a gas or vapor is due to the myriads of impacts of the molecules bombarding the surface enclosing the vapour. If we add heat to a gas or vapour in a vessel we increase the speed of the molecules and therefore the temperature rises. The faster movement of molecules demands more room, so the vapour tends to expand. If we prevent expansion by keeping the vessel closed the faster moving molecules , having the same density as before, must produce a heavier bombardment effect which shows as an increase of pressure.
If the pressure on the surface of a liquid is caused by the rain of air or vapour molecules, it follows that those molecules trying to jump out of the liquid must be exerting a pressure on the air or vapour above the liquid. The pressure at which the escape of molecules from the liquid just balances the overlying pressure is called the 'VAPOUR PRESSURE OF THE LIQUID".
As more heat added to the water, temperature rises, and more molecules try to jump out of the liquid. Most of the adventurous water molecules get knocked back into the liquid, so that practically whole of the added heat energy is retained as increased molecule speed in the liquid. As the addition of heat proceeds we reach a point where the upward bombardment by jumping molecules overcomes the downward bombardment of the overlying air or vapour molecules. That is to say, the liquid vapour pressure overcomes the overlaying pressure. The speeding water molecules, having won the battle, can now leave the water freely provided they receive sufficient and continuous energy to enable them to overcome the overlying pressure. At this point, it is impossible to raise the temperature of the water, because this would increase the vapour pressure which can rise as it has already overcome the overlaying pressure. The particular temperature at which this state occurs is called as "BOILING POINT".
By knowing the characteristic of steam and its molecules, let us now discuss here how to prevent great energy loss due to the pressure fluctuation. There are three cases. Constant pressure, pressure drop and pressure rise.
When the boiler is operating at the constant pressure, that means the vapour pressure of the water is equalized with the overlying pressure of the surface maintaining the Vapour pressure of the water and the boiling point. As long as these two factor do not change, any heat added to the vessel will let the escaping molecules to go and that is the evaporation made under perfect efficiency because all heat input will be transferred to the steam.
When the boiler pressure drops, it means overlying pressure on the water surface will decrease. Then the liquid molecules suffer less interference and can escape more easily. The water can exert the necessary vapour pressure at a lower temperature. So, the boiling point of a liquid falls with reduced pressure. Instantly, water then will be forced to release energy to the vapour area by allowing molecules to escape. This release energy is in equivalent to the difference of sensible heat of the initial pressure and sensible heat of the lowered pressure. This release is done instantly that the whole water surface area will be used for maximum vaporization with the rate of vaporization (Kg/m2 of steam) at the given pressure. Therefore, this energy is from the energy contained in water and NOT FROM THE ADDED HEAT. Immediately after the overlying pressure is equalized with the vapour pressure, no more molecules can escape without the help of the ADDED HEAT in order to overcome the overlying pressure. Then from this stage, as long as the overlying pressure does not change, all ADDED HEAT will be fully used for vaporization.
During the period of equalization of 2 pressure, steam being released from the boiler is from the stored sensible heat of the water. ( we call this "DISCHARGING TIME) And factory will not be receiving any benefit or result from the continuous firing of fuel, which means A GREAT LOSS OF ENERGY for every pressure drop.
When pressure rises, overlying pressure on the water surface will become stronger than vapour pressure. Escaping molecules will meet bombarding pressure above which prevent molecules to fly off the line. During this period, evaporation of steam stop. Then water will receive energy from ADDED HEAT to increase its sensible heat in order to increase its pressure equivalent to the overlying pressure and increase boiling point. As soon as two pressure equalize, evaporation will start again .
During this period of receiving energy without evaporation ( we call it as CHARGING TIME), all energy from ADDED HEAT is used for increasing water sensible heat without evaporation. Remember what you need is HEAT FROM STEAM and not SENSIBLE HEAT. Factory never receive any benefit out of sensible heat since it is inside the water. Only the LATENT HEAT is to be carried by STEAM. You are losing money every time when this happens without knowing it.
If there is anyone who argues that the additional heat during this charging period is used to raise up the pressure even without evaporation so it is not wasted, to whom I shall say that he might be right if he is needing such a raise of pressure at that time. However, if such a pressure rise is not needed, then added heat without evaporation is a total loss to him as the pressure must be reduced to what he needs immediately after the rise causing another loss due to the pressure drop as explained herein.