# Pressure required to move a liquid

• KenR
In summary, the conversation discusses a container full of water with a fluid outlet and air inlet, connected by a hose to an empty container. The question is how much air is needed to move all the water from the full container to the empty one. The initial calculation is incorrect and Bernoulli's principle is mentioned as a potential solution. The volume of air required is discussed, with the correct calculation being 1 X 1.36 = 1.36 cubic feet. The conversation also touches on the importance of water levels and the possibility of adiabatic versus isothermal compression.
KenR
I have a container that measures 1 cubic foot and is full of water. I have another empty container that is 4.5 feet higher than the empty container. The full container has a fluid outlet on the bottom, and an air inlet on the top. A hose (1 inch diameter, 4.5 feet long) connects the fluid outlet to the empty container. I want to know how much air I need to add to the full container to move all of the water out of the full container. I tried figuring this out, but my answer is clearly incorrect.

I reasoned that since 1 pound of pressure is required to raise the water 2.31 feet, 2 pounds of pressure should be sufficient to accomplish the job. Next (based on this thread: https://www.physicsforums.com/threads/volume-and-pressure.87911/) I calculated that (assuming no temperature change) it would take 2/14.7*1=0.136 cubic ft. This does not make sense, as it would not even fill the container, let alone pressurize it.

You first have to decide how fast you want the water to flow from the lower tank to the upper tank . Once you have that information you can determine the necessary air pressure .

Speed is not important to the practical application, but if it is required to calculate an answer let's assume 2 minutes.

Bernoulli's principle can be used to find the pressure needed . Are you happy working with simple formulas ?

I think so. Certainly willing to try.

Note to Nidium. I am not so much interested in pressure as I am in volume of air required. I don't want to pressurize the container and then open a valve. I want to add air to the system and as the air is added, the liquid moves. I do recognize that it is necessary to first calculate the pressure required, and then from that the volume.

KenR said:
I calculated that (assuming no temperature change) it would take 2/14.7*1=0.136 cubic ft.
0.136 atmospheres of pressure is required, yes. 0.136 atmospheres more than ambient, to be precise. One assumes that the empty container has a vent, so it is under 1 atmosphere of pressure already.

So you need to displace one cubic foot of water with air that will end up at 1.136 atmospheres of absolute pressure. How many cubic feet of room pressure air would you expect that to take.?

Your equation is just upside down, that's all that is wrong with the second part.

For the first part, remember that it is the top surface of the water that matters, not the bottom surface.

Jbriggs, Would it be as simple as 1 X 1.36 = 1.36 cubic feet?

KenR said:
Jbriggs, Would it be as simple as 1 X 1.36 = 1.36 cubic feet?
Yes. But @russ_watters makes a good point about the water levels.

Thanks everyone. This is a great resource. It looks like my idea for a refueling system is feasible.

## 1. What is the definition of pressure required to move a liquid?

The pressure required to move a liquid is the force per unit area that is needed to push or move a liquid from one point to another. It is typically measured in units of pounds per square inch (psi) or newtons per square meter (N/m^2).

## 2. How is pressure required to move a liquid related to the viscosity of the liquid?

The viscosity of a liquid, which is its resistance to flow, is directly related to the pressure required to move the liquid. The higher the viscosity of a liquid, the more pressure is needed to move it.

## 3. What factors affect the pressure required to move a liquid?

The pressure required to move a liquid is affected by several factors, including the viscosity of the liquid, the density of the liquid, the surface tension of the liquid, and the size and shape of the container in which the liquid is being moved.

## 4. How does the pressure required to move a liquid change with depth?

The pressure required to move a liquid increases with depth due to the weight of the liquid above it. This is known as hydrostatic pressure and is described by Pascal's law, which states that the pressure at any point in a liquid is equal in all directions.

## 5. What is the relationship between pressure required to move a liquid and the rate of flow?

The pressure required to move a liquid is directly proportional to the rate of flow. This means that as the pressure increases, the rate of flow increases as well. However, the relationship between pressure and flow rate may vary depending on the specific properties of the liquid and the system in which it is being moved.

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