GPM vs. PSI

[Sergei]

Hi guys. Great board, helps a lot, thank you. I'm a beginner, need an advice.

I wash roofs with a cold 3500psi/4gpm unit. Thinking about bying a better machine.

Should I go for 5000/5 gal OR 3000/8 gal ? They both have same amount of cleaning units, but should I be shopping for more pressure or more flush?

Thank You!

 

[Mark]

That is an "Old Argument"

There are many different schools of thought on that subject.

Here is mine:

Go for the higher volume and lower pressure, we used to custom
build skids for the roof washing trade, our best sellers were usually
8-10 GPM @ 1500 - 2500 PSI. The last one we did used an 11 GPM
@ 2300 PSi pump we ran the pump a little slower to deliver 9 GPM
@ 2000 PSI. It kicks butt!

5000 PSI on roofs? Scarey thought.

My vote is for higher volume..

Here is a picture of our new 5 GPM @ 5000 PSI portable unit.

 

[Larry L.]

I would go with the 3000/8,more flush.I've never been able to clean with high psi anyway without useing chemicals unless flatwork whats not every dirty,oily.3000 psi is enough psi,need more flush.

 

[Mike Hughes]

I agree with the above........go with the higher GPM's and forget about anything over 3000 psi.

 

[oguard]

While the physics is complex the short answer is always go for flow over pressure.

"Cleaning units" is a term coined by the marketing department of a supplier to the mass merchandisers. They DO NOT represent actual cleaning ability.

Without going into the whole physics of it I can best illustrate the differences this way:
Start with a 2 gpm at 1000 psi unit. You now wish to halve your cleaning time; you can either.... double flow or quadruple pressure. Flow is directly related to horsepower

Therefore 4 gpm @ 1000 psi unit cleans in the same time as a 2 gpm @ 4000 psi unit: in almost every instance.

As pressure rises so too does the possibility of injury or damage. Keeping in mind that water weighs 1 kg / litre or 8.4 pounds / US gallon that is why flow is worth more than pressure- impact force!

Plus it helps you apply and rinse any chemicals faster. HP is KING, go with the FLOW.

 

[Sergei]

Thank you so much for thoughtful replies!

The last question is - will double volume really halve the cleaning time?

 

[oguard]

Probably not if you timed it but it IS a fairly accurate 'Rule of Thumb' performance measure. Cheers.

 

[ron]

one other thing where are you getting that water from[got a big tank?] a house cant supply that volume. It can assist in refilling.

 

[Walt Graner]

Flow is the way to go, however like Ron said better look into your supply. Go to get a balance most houses have poor water flow to say the least, too much flow is no go.