The screw pumps we are employing today can rightfully claim to be the
oldest pumps for the conveyance of liquids. In the
3rd century B.C., the Greek mathematician and physicist, Archimedes, invented the original screw pump and gave it his name
calling it the "Archimedian Screw". This was a hand operated, spiraled tube set at an incline.
In the 4th century, the Romans employed the Archimedes Screw in their
highly advanced, for that time, water supply systems.
The Romans also used screw pumps for irrigation and drainage work. Screw pumps also found use in ore mines in Spain.
These early units were all driven by man or animal power.
In the 14th century, screw pumps reappeared as a means of conveying
liquid and constituted the first rebirth of Archimedes'
ancient pump. Historical records refer to its use with artificial fountains. This rebirth was short-lived with the development of
reciprocating plunger pumps which had their heyday during the 19th century and were used in the development of public water
supply systems. Reciprocating plunger pumps were later replaced with centrifugal pumps.
Screw pumps with their ability to pump relatively large quantities of
water at low lifts still found one obvious application as a
drainage pump in low-lying areas, such as the reclaimed land areas of the North Sea and Baltic Sea. During this era the screws
were constructed of wood and were powered by use of wind force with windmills.
Present rebirth of the modern day screw pump dates back to the 1920's
where their primary use was drainage pumps. In
1930 there were some 300 screw pump installations in use in the Netherlands for draining low-lying ground. It wasn't until after
World War II that screw pumps were seriously applied for handling wastewaters. Since this time, their use has grown rapidly.
PRESENT DAY USE
Today the screw pump has taken its rightful place with the numerous
types of pumping units that are available. Where you have
to move a large quantity of liquid at a relatively low lift, you should look at a screw pump as one method of getting this job
Screw pumps are now being applied for:
1) Returning activated sludge
2) Raw sewage lift stations
3) Raw and effluent sewage plant lift stations
4) Industrial product and waste handling
5) Storm water pumping
7) Land drainage
8) Constant flow to treatment works with equalization basins
Screw pumps are available from 12-inch to 144-inch diameter with flows
ranging from 100 to 95,000 gal/min and lifts from 6
to 35 feet. Construction of screws is with single, double or triple helix flights.
CENTRIFUGAL PUMPS VS. SCREW PUMPS
The reasons for the rapid gains in use of screw pumps become evident
by comparing various features of centrifugal pumps
versus screw pumps. A discussion of some of the major features is set forth below:
1) Centrifugal pumps require a bar screen ahead of
the pump to remove debris and fibrous material to prevent clogging.
Screw pumps do not require bar screens and can pump grit, rocks, wood, tin cans, plastic materials, rags and other foreign
objects because of the relatively wide clearance between the flights. Less maintenance is required.
2) Centrifugal pumps require a deep wet well which
requires expensive excavations and concrete work and sometimes can
cause odors. The suction and discharge piping, valves and fittings not only add friction losses but also have to be housed.
Screw pumps do not require a wet well, piping or a pump house. The total lift of the screw pump is less and thus requires less
3) Centrifugal pumps with non-clog impellers are only
relatively efficient and the highest efficiency of a given impeller is
a limited operating range. Screw pumps are highly efficient and will operate over 70% efficiency from 100% of design capacity down to 33% of this capacity. This higher efficiency requires less horsepower.
4) Centrifugal pumps are subject to abrasion and wear
because of the solids in the liquid and the high operating speed.
Repairs are required because of these features as is maintenance required to unclog these units. Screw pumps consist of a
sturdily built tube and flights and operate at slow speeds of approximately 110 to 21 rev/min depending on pump diameter and
are not subject to wear and abrasion thus requiring a minimum of maintenance, repair and upkeep.
5) Centrifugal pumps to meet the present increasing
demand for variable delivery require sophisticated, complicated and
expensive drive and control arrangements. Screw pumps are self compensating in that they will automatically pump the quantity
of liquid that comes to them from zero to their maximum rated capacity and this is accomplished with a constant speed motor.
6) Centrifugal pumps because of their high operating
speed will disintegrate activated sludge floc. Screw pumps lift activated
sludge flow gently and cause a minimum of floc breakup.
7) Centrifugal pumps can pump against relatively high
heads and can deliver liquid to a pressurized discharge. These are
two main disadvantages of screw pumps. Lifts are limited and dependent upon the length you can build a given diameter
screw. Discharge from the screw is to atmosphere and normally an open channel.