Is Pump Flow Rate in a Closed Loop Constant?

[KingNothing]

Is the overall flow rate through a closed loop with a pump constant? I've always though that if a pump can do a specific amount of work, it doesn't matter if the 'loop' of tubing goes up and down, or what types of turns it takes. I've thought this because if the tubing goes down then up (or vice versa) the force needed to push it upwards would be the same amount of force exerted when it goes downwards.

Can anyone confirm this? If I'm wrong, why?

 

[Q_Goest]

Not exactly. If a closed loop goes up and down the pressure head cancels as you point out, but that is just one consideration. Here are two others:

- There are 'frictional' losses in a piping system. These frictional losses don't cancel out like pressure head. The longer the pipe, and the more restrictions in it, the more pressure loss there will be for any given flow rate.
- There is also a problem with elevation changes in a pipe system. If the pressure in the piping system drops below the 'saturation pressure' of the fluid, the liquid will begin to flash or boil.

So given any piping circuit, there will be a pressure loss which increases with flow rate, and when designed the piping system must also not change phase without care being taken to accommodate that phase change.

One other point to note is that there are fundamentally different types of pumps which can be broken down into two catagories, positive displacement and what is sometimes called a rotodynamic pump (ie: a centrifugal or similar). A positive displacement pump puts out a given quantity of liquid regardless of pressure, as long as it has the power of course. A rotodynamic pump or centrifugal pump puts out a flow rate which is dependant on the difference in pressure between the inlet and outlet. So for any given closed loop system, you can change a restriction in the system to give you more or less pressure restriction, and the constant displacement pump won't change flow, but the centrifugal pump will.

 

[charng]

The answer to this question depends on the specific system and conditions involved. In a closed loop system, the overall flow rate can be affected by various factors such as the pump's capacity, the size and length of the tubing, and the resistance or friction within the system. Therefore, the overall flow rate through a closed loop may not necessarily be constant.

However, the pump flow rate, which refers to the volume of fluid passing through the pump per unit time, can be considered relatively constant in a closed loop. This is because the pump is designed to maintain a consistent flow rate based on its specific capabilities and the pressure it is operating under.

It is important to note that while the pump flow rate may remain constant, the overall flow rate through the closed loop can vary depending on the factors mentioned above. For instance, if the tubing has a smaller diameter or is longer, it may create more resistance and decrease the overall flow rate, even though the pump flow rate remains constant.

In summary, while the pump flow rate can be considered constant in a closed loop, the overall flow rate through the system can be affected by other factors. It is essential to consider all these factors when designing and operating a closed loop system to ensure optimal performance.