Expert advice: Selection and control of condensate pump in condensate recovery systems
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As shown in the figure, it is a set of multiple steam drying systems; after being heated by steam, the condensed water enters the flash tanks respectively. Of these 2 flash tanks, one is in a vacuum state (e.g., -80 kPa); a variable-frequency pump is used, and the liquid level is controlled by adjusting the pump speed ； The other option is positive pressure (such as 25 kPa), in which a fixed-frequency pump is used, and the liquid level is controlled by adjusting the level control valve. Several similar projects have been consulted; in cases where the flash tank operates under vacuum (negative pressure), either fixed-frequency pumps combined with control valves or variable-frequency pumps are used. However, in situations where the flash tank operates under positive pressure, all the reference projects have opted for fixed-frequency pumps combined with control valves. Question: For a flash tank of this type, in a vacuum condition (negative pressure), can a variable-frequency pump be used to control the liquid level by adjusting the pump speed? What will be the consequences if it’s not possible? Could you help analyze the theoretical reasons? Among all the reference projects found so far, no cases of using variable-frequency pumps under positive pressure conditions have been identified. . . If necessary, appropriate red envelope rewards can be set; thank you very much! !
Under negative pressure conditions, it is feasible to use a variable-frequency pump to control the liquid level. Because in this case, the change in liquid level is mainly caused by evaporation, and the amount of evaporation is related to the temperature and pressure inside the flash tank, rather than to the flow rate and pressure of the pump. Therefore, adjusting the pump speed can achieve the effect of controlling the liquid level. However, under positive pressure conditions, flow rate and pressure control the changes in liquid level. The flow rate and pressure output of variable-frequency pumps are affected by various factors, so it is not possible to accurately control the liquid level. Therefore, a fixed-frequency pump combined with a level control valve is generally used to control the liquid level. It should be noted that whether in a negative pressure or positive pressure condition, it is necessary to select the appropriate pump and control method based on the specific process conditions and equipment parameters. At the same time, field testing and optimization are also required to ensure that the system operates stably and reliably and achieves the desired results. .
The actual flash tank is under pressure control; if the pressure in the flash tank remains relatively stable, can the liquid level be well controlled even when there is a positive pressure? In other words, can a variable-frequency pump be used as well?
How do the different specialties within a design institute set conditions for one another?
For negative-pressure flash tanks, it is best to use a fixed-frequency pump combined with a control valve to regulate the liquid level. When variable-frequency pump control is used, if the flow rate is unstable, the pump’s speed will decrease, which leads to backflow – that is, the material flows back into the flash tank against the flow of the pump, making it difficult to control the liquid level in the flash tank.
In the flash evaporation process, in fact, it makes little sense to use variable-frequency pumps regardless of whether the flash tank is under positive or negative pressure. This is true for pumps that rely on centrifugal force to transport fluids. The reason why variable-frequency pumps are not useful lies in their operational characteristics: once the rotation speed drops below the designed value, the head generated by the pump will decrease rapidly. In practical applications, variable-frequency pumps are used in processes where a decrease in head has no significant impact (this is because a high head value is selected during the initial design phase). Especially when the flash tank is under vacuum, such a decrease in head will quickly affect the NPSH.r value, which in turn prevents the pump from fulfilling its fluid transportation requirements. According to your diagram, this can only be avoided if there is enough height between the liquid level in the flash tank and the fluid pump……
For negative-pressure flash tanks, it would be better to use a fixed-frequency pump together with a control valve to regulate the liquid level, which is the exact opposite of what I’ve seen in actual cases. I’ve examined several project examples, and in most of them it is: – Positive-pressure flash tanks: fixed-frequency pump + level control valve; – Negative-pressure flash tanks: variable-frequency pump or fixed-frequency pump + level control valve. It seems that your conclusion is open to discussion.