Mass and Energy Balance

What goes in comes out, law of conservation of mass. Water in a Tank with one input and one output is easiest system to understand the mass balance. If water input is equal to water output the level in the level will remain unchanged. If input is more then output the level will rise and if the output is more then input the tank will exhaust. It seems simple but if we take example of water level in fire tube boiler then the example becomes very critical. As level control in boiler shell is the most important parameter when dealing with fire tube / smoke tube boilers. The supply should be maintained to cope with demand keeping the acceptable safe level of water in shell. The low water level is safety issue and high water level is steam quality issue (reduction in steam disengagement area). This is not simple as pressure and temperatures of the system is high.

Energy streams also follows the law of conservation. For example water heating in a paper is a good example of energy input and output. The paper will not burn as long as the no accumulation of heat in its and all the heat supplied is transferred to water. Similarly boiler operation will be safe as long as it transfers the heat of fuel combustion to water. If some part of the outside of tubes gets dry, the material of tube will be overheated and material damage can occur. The same thing can observed when you operate the electric heater rods in a low level or empty container. The supply of heater input energy remains the same (kW) but the heat recipient is air in place of water which act as insulator so same exposed area cannot transfer the heat from electric rod to air. The same energy needs a larger area to transfer heat to air and avoiding accumulation of heat in the rod itself. Here a very important parameter "Area of Heat Transfer" comes into play. 

It always starts with a simple but when conditions rise (Temperature, Pressure, Concentration, pH) throughput increases more and more variables come into play. The insignificant things becomes significant and creates impacts. For example if you transfer drinking water (say 250 ml) from a bottle in a glass it looks simple but when you have to transfer the 101 m3 (101 tons) of water to a plant many things come into play. For example lifting a glass of water (250 gms + weight of glass) for drinking is very easy but when we talk of million gallons per day lift requires external sources. Further more when this water has to interact with the systems at higher temperatures and pressures more and more variables come into play and complex models are developed which represents the real systems up to a satisfactory level.

Process conditions and interacting systems decides the stability of systems. For example one drop of bad milk can deteriorate the 10,000 liter milk tank. Minor change in the pH can disturb the whole process through changing the reaction rates.