The relationship between flow rate, flow rate and pressure

If the outlet pipe diameters of the two pumps are the same, then the flow rate will be the same, but the outlet pressure of the 90m lift pump is higher, which is not certain.

Agree with the point on the 2nd floor. Water can be regarded as an incompressible fluid, and the volume flow rate of water (m^3/s) can be regarded as not changing with pressure.

I think to answer this question we must first clarify two basic concepts. First of all, the flow rate mentioned by the poster is actually the volume flow rate/cross-sectional area. When the pipe size is determined, the flow rate reflects the mass relationship, which can be easily determined using mass or material balance calculations. When using a pump to increase the lift, this part involves doing work through the pump and increasing the mechanical energy of the material. This reflects the energy relationship and can be explained by energy balance. For fluid transport, there is Bernoulli's equation to explain the energy relationship. Let’s explain it intuitively with an example. A pump is used to transport 3m^3/s of water from 0 meters to a storage tank 30 meters high. This 3m^3/s is the flow rate, divided by the cross-sectional area of ​​the pipe is the flow rate. For the same material, the flow rate must be the same. The function of the pump lies in the pressure head or lift. A large pump can transport to 30 meters without any problem, but a small pump may only transport to 10 meters. This is related to the total work done by the pump, which is the total mechanical energy obtained by the material. Some people equate the kinetic energy of fluid with mechanical energy, which is very wrong. From the above small example, it can be made clear that what the big pump and the small pump change is the potential energy of the fluid, that is, the pressure. This also explains why the pressure of a fluid is so important in chemical engineering calculations.

From the perspective of the pump alone, a 30m3/h pump with a lift of 30m has different characteristic curves from a 30m3/h pump with a lift of 90m, so the specific structure of the pump is different. The size of the outlet depends on the design and usage conditions. For the same pipe cross-section, there is no temperature change, and the flow rate is the same when the flow rate is the same. ； Pressure is a physical quantity associated with flow, which is determined by the performance of the pump itself, that is, the characteristic curve.

Flow rate = flow rate/cross-sectional area. The flow rate and cross-sectional area are equal, the flow rate is the same, but their pressure heads are different. The lift of 90m is large. In fact, in actual production, what is considered is the installation height, which not only satisfies the production process but also saves costs.

This problem is a problem between the pump characteristic curve and the pipeline characteristic curve.

Agree with the 7th floor, this is related to the characteristic line of the pump and the characteristics of the pipeline.

Thank you, 4th floor. Maybe my two concepts are a bit confused. If calculated using the wave-effort equation, is part of the work done by the pump converted into kinetic energy, potential energy and losses along the way of the fluid? So is the flow rate in this formula related to the flow rate in the material balance calculation you mentioned? Thank you on the 5th floor. The basic parameters of the pump above are an ideal state that I assumed. It is assumed that the pump works at that operating point.

The flow rate of a forced (such as pump-driven) flow depends on the pipe diameter and hourly flow rate. The size of the flow rate determines the pressure at the delivery end.； The flow rate is the cause and the pressure is the effect because there is pipeline loss. The outlet flow rate of a 30m3/h pump with a head of 30m and a pump of 30m3/h with a head of 90m are the same as long as the pipe diameter is the same. ； The difference is their outlet pressure. In layman's terms, the lift of a pump "gives" a higher pressure to low-pressure or normal-pressure fluid, so that it can move forward against back pressure (operating pressure of the target equipment) and frictional resistance.

The flow rate is proportional to the square root of the pressure difference.

Thank you all! Now I understand

I think the lift of a pump is a concept of rated operating conditions. When selecting a pump, we will select a pump that meets the system requirements. For example, if your system requires a lift of 85m, but you choose a pump with a lift of 90m, then the outlet pressure of the pump you face is actually related to the lift of your system, not the rated operating conditions of the pump. You can imagine that the above pump does not exert its maximum capacity but is operating under load.

Not the same. It is true that the flow rate does not change with the change of pressure, that is, the flow rate within a certain period of time is certain, but the flow rate under different pressure conditions must be different if converted into the flow rate under equal pressure conditions.~~~

I think the previous statement of the poster is correct, but the latter statement is incorrect. The flow rate of the two pumps is equal, but the head is different. If used in the same system, if the inlet and outlet resistance of the system are the same, Q will change.

Most people confuse the relationship between pressure and flow. Just like the example on the fourth floor, it is very simple to explain it by mass balance and energy balance respectively. If the lift is large, mechanical work is added. If you study Bernoulli's equation and the characteristic curve of the pump, it will be clear.

According to Bernoulli's equation: p1+1/2ρv1^2+ρgh1=p2+1/2ρv2^2+ρgh2=C It can be seen that the relationship between pressure, head and flow rate does not require any further explanation, right?

For the same pump, there is a relationship between pressure, flow and flow rate. For different pumps, it has nothing to do with the pressure of the pump, that is, the head. If the flow rate of the pump is the same, it depends on the thickness of the outlet pipe of the pump. The thicker the flow rate, the smaller the flow rate.

You can find an engineering Bernoulli equation in any chemical engineering principles textbook, and you will understand it after looking at it!