Trigger sprayer bottles are an extremely useful type of machine and an excellent demonstration of basic plumbing principles. A spray-bottle head is made up of only a few parts. It has a trigger lever, which activates a small pump. This pump is attached to a plastic tube that draws cleaning fluid from the bottom of the reservoir. The pump forces this liquid down a narrow barrel and out a small hole at the gun’s muzzle. The hole, or nozzle, serves to focus the flowing liquid so that it forms a concentrated stream.
The only complex element in this design is the fluid pump, and it’s about as simple as they come. The main moving element is a piston, housed inside a cylinder. Inside the cylinder, there is a small spring. To operate the pump, you pull the trigger back, pushing the piston into the cylinder. The moving piston compresses the spring, so when you release the trigger, the piston is pushed back out of the cylinder. These two strokes of the piston, into the cylinder and out again, constitute the entire pump cycle.
The downstroke, the piston pushing in, shrinks the area of the cylinder, forcing fluid out of the pump. The upstroke, the spring pushing the piston back out, expands the cylinder area, sucking fluid into the pump. In a spray bottle, you need to suck cleaning fluid in from the reservoir below and force it out through the barrel above. In order to get all of the fluid moving through the barrel, the pump must only force the fluid up — it cannot force the fluid back into the reservoir. In other words, the fluid must move through the pump in only one direction.
The device that makes this possible is called a one-way valve. A trigger sprayer bottle has two one-way valves in the pumping system: one between the pump and the reservoir and one between the pump and the nozzle. Typically, the valve between the pump and the reservoir consists of a tiny rubber ball that rests neatly inside a small seal. The sides of the seal are angled so that the ball won’t fall through. Depending on the design, either gravity or a small spring holds this ball against the seal so that the water passageway is blocked off when you are not pumping. When the piston moves out (when you release the trigger), the expanding area of the cylinder sucks on the fluid below, pulling the ball up out of the seal. Since the ball is lifted up, fluid is free to flow from the reservoir. But when you squeeze the trigger, the outward force of the moving fluid pushes the ball into the seal, blocking off the passageway to the reservoir. Consequently, the pressurized fluid is pushed only into the barrel.
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