Water Pressure with DC motors [RE-wrenches]

Mon Sep 24 09:21:45 PDT 2001

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Hi Carl. I have several suggestions to help clarify and solve your problem

Carl Emerson wrote:

>In a remote installation the most common way in NZ to achieve water 
>pressure for a residence is to use an
>AC pump with a small pressure chamber.
>
>This is when the client does not want to have a 'header tank' on a high stand.
>
>Now this 'on demand' water pressure system is very energy hungry. 
>Typically the pumps have a high surge
>start and cycle on and off constantly under low flow demand conditions.

The cycling on and off constantly is due to two possible factors:
1. Undersized pressure tank
In the US, it is conventional for a domestic well pump system to have 
a 40-80 gallon (150-300 liter) pressure tank. For RE systems we 
emphasize the value of this. Most of the tank volume is taken by air, 
so the actual water storage between cycles is about 1/4 of that. If 
your customers' tanks are much smaller, that would account for 
frequent cycling. There is NO DISADVANTAGE to having a much bigger 
tank -- only advantages. To enlarge the capacity, multiple tanks can 
be paralleled. They need not be matched, and can be located anywhere 
along the pressurized line!

2. Improper pre-charge of the pressure tank.
The air bladder in the tank must be set to a pre-charge pressure of 
just slightly below the cut-in setting of the pressure switch. This 
is measured with NO water pressure in the system (or before the tank 
is even installed). To much pressure, and the air bladder acts like 
cement. Too little, and it functions but at less than full storage 
potential. Pressure tanks are precharged at factory, on the high side 
(easy to let air out). If a plumber is called in to install it and he 
isn't a pump guy, he may not be aware of this, although it's always 
explained in the product instructions.

Go to http://www.dankoffsolar.com/reference.html  and you will find 
these articles that relate to your problem:
Adjusting Water Pressure to Reduce Energy Waste
DC Pressurizing Pumps for Domestic Water Supply and Irrigation

>Now one approach is to replace the AC pump motor with a DC 
>equivalent. Over here 48 Volt 1.5 HP motors
>are hard to find. If you can get one then this will help to solve 
>the problem, it will also reduce the
>light flicker every time you turn on a tap.

I agree (it's my business) that DC is a great way to go. However, 
most AC pumps use a motor with a threaded stainless shaft that runs 
into the wet end. Nobody makes DC motors with this type of shaft. 
Also, the jet pumps that are most commonly used for pressurizing are 
not especially efficient. We commonly replace a 1.5 HP jet pump with 
a 1/2 HP piston pump for similar performance. For a family up to 4 
people without need for large lawn sprinklers, we go down to 1/4 HP, 
with the following two products:

http://www.dankoffsolar.com/waterpumps/specsheets/2900_flowlight/2900_flowlight.html

http://www.dankoffsolar.com/waterpumps/specsheets/3000_solarforce/3000_solarforce.html

>
>... as it stands water pressure is one of
the most energy demanding elements of the overall system design.

The energy draw per day is so small, the DC pumps present a very 
minor load on the system. The energy usage of the Flowlight pump at 
40 PSI (2.8 bar) pressure is .67 watt-hours per gallon pumped (.18 WH 
per liter), as stated in the specifications.

If you contact me off-line Carl, I can suggest another dealer in NZ 
who imports our pumps. Perhaps you can pool in with him to bring in a 
stock of pumps at a big savings in freight cost.

Windy

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