Pressure washer pressure

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richardabeattie

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Pressure washing my hull made me ask myself how is it that the water jet is so much stronger than the jet I would get by simply squeezing the end of the hose that feeds it. Don't just tell me it's because there is an electric motor inside - I worked that out all by myself - but the pump can only pump the water coming to it from the hose pipe. Baffled.
 
Pressure is force divided by area (e.g. lbs/sq in) i.e. if you decrease the area then the pressure must be increased to maintain the same rate of flow.

Given that the rate of flow (litres per minute) remains essentially constant in a pressure washer, the variables that are linked are the pressure and the velocity of the water, As the pump increases the pressure, the exit nozzle can be decreased in size and so the velocity of the water increases to maintain a constant flow rate. The same effect occurs when you put your finger over the end of the hoze but to a much smaller extent as the pressure remain constant.
 
Yes, Playtime. I suspect that because of the very high pressure of a pressure washer, people get the impression the volume of water passed is also high. It isn't. Try filling a bucket and then doing the same from the hosepipe feeding the pressure washer. You might be surprised how slowly the pressure washer fills it.
 
Typically a pressure washer outputs at 6 to 10l/min, but at around 150bar. Your tap can do the first part, but not the second. If you put your hand in front of a pressure washer jet you can strip the flesh off , right down to the bones.
 
sarabande said:
Typically a pressure washer outputs at 6 to 10l/min, but at around 150bar. Your tap can do the first part, but not the second. If you put your hand in front of a pressure washer jet you can strip the flesh off , right down to the bones.
Click to expand...

I am always surprised at how little water a pressure-washer uses. As for the hose, the pressure starts off very low, with the main gradient being at the tap. If you put your finger across or squeeze the hose, the pressure rises within the hose until ultimately it would reach the mains pressure if you completely blocked the hose, when there would be no gradient at the tap. This is way below the pressure produced by a pressure-washer.
 
richardabeattie said:
But as I keep saying I can squeeze the end of the hose but not get anywhere near the pressure the pump provides.
Click to expand...

The pressure from the mains is roughly 10 bar (pumped and regulated at the local pumping station); the pressure from the pressure washer is roughly 120 bar (one bar is approx 10 metres pressure head) pumped and regulated by the pressure washer pump. Volume is a different thing.
 
That does not explain anything. I'm well aware that the pressure washer raises the pressure. The question is how does it do it. I can squeeze the hose to produce the same small volume as comes out of the washer. But can't match the pressure.
 
richardabeattie said:
That does not explain anything. I'm well aware that the pressure washer raises the pressure. The question is how does it do it. I can squeeze the hose to produce the same small volume as comes out of the washer. But can't match the pressure.
Click to expand...

The washer is in principle a high pressure pump - if it answers your question. The maximum pressure you can achieve by totally closing the output is the pressure the pump is capable to develop and it is given by its design and power of the motor that drives it.
The maximum pressure the pump supplying the mains can develop is much lower than the the pressure generated by washer.
And just to be sure: aren't you trolling? ;)
 
richardabeattie said:
That does not explain anything. I'm well aware that the pressure washer raises the pressure. The question is how does it do it. I can squeeze the hose to produce the same small volume as comes out of the washer. But can't match the pressure.
Click to expand...
Because it puts out less water at a higher pressure. That's what the pump does but someone has already tried to explain that. The pump pushes water down a very narrow nozzle at great pressure. All that electricity has to be doing something.
 
Being as how pumps are my day job, maybe i can help.

The water from the mains is supplied from either centrifugal pumps or head tanks. These pumps will generate a maximum pressure down the line when the flow is stalled or close to it. Importantly the pumps can continue to run with no flow going through them - they just churn away and the absorbed energy warms the water. If the mains is from a header tank /water tower you can see that you can stop the flow from both of these sources by just closing the valve/tap.
When you partially close the hose you don't actually increase the line pressure, just the pressure drop across the nozzle which increases the velocity, but you are also decreasing the rate of flow and if you continue to restrict it you will stop it altogether.

(slight simplification there but near enough-apologies to Bernoulli!)

Pressure washers normally use a type of pump called positive displacement. A packet or quanta of water is transported from one side of the pump to the other via a chamber or chambers that are closed during the transition. PD pumps don't generate pressure at all, they just transfer packets at a constant rate.
Say 10 litres/min for example.
If you unscrewed the hose on your pressure washer and ran it you would see the truth of this. The water would flow gently out at say 10 litres/minute.
Where it gets interesting is what happens when you restrict the outlet. The same 10 litres /minute has to get through a small hole and the only way this can happen is if the flow velocity goes up and the only way this can happen is if the pressure difference from one side of the nozzle to the other is increased.
When you release the trigger and stop the flow through the nozzle, that 10 litres/min has to go somewhere or else something will burst or the pump will break. So pressure washers have a relief valve that is set just above the working pressure and that pops open to recirculate the water.

Hope this helps
 
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