![]() This slug of water is dense and incompressible, and when travelling at high velocity, has a considerable amount of kinetic energy. This will allow condensate to drain from the system, but will not allow the passage of any steam. The moisture droplets run down the baffles and drain through the bottom connection of the separator to a steam trap. The baffles create an obstacle for the heavier water droplets, while the lighter dry steam is allowed to flow freely through the separator. In the separator shown in Figure 2.4.3 the steam is forced to change direction several times as it flows through the body. The droplets of water entrained within the steam can also add to the resistant film of water produced as the steam condenses, creating yet another barrier to the heat transfer process.Ī separator in the steam line will remove moisture droplets entrained in the steam flow, and also any condensate that has gravitated to the bottom of the pipe. It has already been shown that the presence of water droplets in steam reduces the actual enthalpy of evaporation, and also leads to the formation of scale on the pipe walls and heat transfer surface. The overall result is that steam arriving at the plant is relatively wet, and the droplets of moisture carried along with the steam can erode pipes, fittings and valves especially if velocities are high. Although these pipes may be well insulated, this process cannot be completely eliminated. In addition to this, as the steam leaves the boiler, some of it must condense due to heat loss through the pipe walls. These deposits will accumulate over time, gradually reducing the efficiency of the plant. Incorrect chemical feedwater treatment and periods of peak load can cause priming and carryover of boiler feedwater into the steam mains, leading to chemical and other material being deposited on to heat transfer surfaces. This is only true however if the temperature is constant, and there is no chemical reaction between the liquid and the gas. ![]() This states that the mass of gas that can be dissolved by a given volume of liquid is directly proportional to the partial pressure of the gas. The concentration of dissolved gas in the water can be determined using Henry’s Law. The concentration of dissolved carbon dioxide is also kept to a minimum by demineralising and degassing the make-up water at the external water treatment stage. The temperature of the feedtank is maintained at a temperature typically no less than 80 ☌ so that oxygen and carbon dioxide can be liberated back to the atmosphere, as the solubility of these dissolved gases decreases with increasing temperature. This means that ‘air’ dissolved in the boiler feedwater will contain much larger proportions of carbon dioxide and oxygen: both of which cause corrosion in the boiler and the pipework. However, the solubility of oxygen is roughly twice that of nitrogen, whilst carbon dioxide has a solubility roughly 30 times greater than oxygen! When the water is heated in the boiler, these gases are released with the steam and carried into the distribution system.Ītmospheric air consists of 78% nitrogen, 21% oxygen and 0.03% carbon dioxide, by volume analysis. Make-up water and condensate, exposed to the atmosphere, will readily absorb nitrogen, oxygen and carbon dioxide: the main components of atmospheric air. Then the maximum possible heat transfer rate (q_max) is calculated by the shown formula.Other sources of air in the steam and condensate loopĪir can also enter the system in solution with the boiler feedwater. mass flow rate multiplied by specific heat) C_h and C_c for the hot and cold fluids respectively, and denoting the smaller one as C_min. The method proceeds by calculating the heat capacity rates (i.e. Therefore one fluid will experience the maximum possible temperature difference, which is the difference of T_h,i – T_c,i (The temperature difference between the inlet temperature of the hot stream and the inlet temperature of the cold stream). To define the effectiveness of a heat exchanger we need to find the maximum possible heat transfer that can be hypothetically achieved in a counter-flow heat exchanger of infinite length. In heat exchanger analysis, if the fluid inlet and outlet temperatures are specified or can be determined by simple energy balance, the LMTD method can be used but when these temperatures are not available The NTU or The Effectiveness method is used. The Number of Transfer Units ( NTU) Method is used to calculate the rate of heat transfer in heat exchangers (especially counter current exchangers) when there is insufficient information to calculate the Log-Mean Temperature Difference ( LMTD).
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