Using combination of Axial Flow Pump and Airlift pump

Various types of pump have different advantages and disadvantages attached to their use. Best solution is one where we combine two or more different kinds to get the best and the most efficient solution.

In order for the pumpset to give enough flow, consume lesser energy, be easy for maintenance and portable, we propose to use a pump that is a combination of an Axial Flow Pump and an Airlift Pump.

Axial Flow Pump

An axial flow pump has a propeller running in a casing. The pressure in an Axial Flow Pump (AFP) is developed by the flow of liquid over the blades of propeller. The fluid is pushed in a direction parallel to the shaft of the impeller. Fluid particles, while flowing through the pump, do not change their radial positions. It allows the fluid to enter the impeller axially and discharge the fluid nearly axially.

In an AFP, higher the flow, lower is the energy demand. It has relatively high discharge rate. It has lesser energy consumption for similar flow rates compared to conventional centrifugal pump. This is dues to design of propellers, which have lower aerodynamic losses and higher efficiencies. The axial flow pumps have the smallest dimensions among conventional pumps, making it portable and easy to handle.

Airlift Pump

In an airlift pump, air is injected at the base of a submerged riser tube. As a result of the air bubbles suspended in the fluid, the average density of the two-phase mixture in the tube is less than that of the surrounding fluid. The resulting buoyant force causes a pumping action. In other words, the compressed air produces a froth of air and water, which has lower density than water and consequently rises to the surface. The main advantage of an airlift pump is that it has no moving parts and is very easy to use, handle and maintain.

Our Setup

Our setup has both an airlift pump and an axial flow pump installed inside a pipe. This pipe is immersed inside a well. Airlift pump is at a lower level, as is shown in images above. Airlift pump takes air from the compressed air storage tank and pumps the water to 4-5 metres. The axial flow pump is at this level and it pumps this water further up to bring it to the tank on the ground.

The compressor is run using solar photovoltaic system. The compressor fills a compressed air storage tank at 10 bar pressure. This pressurised air is then used to run the airlift pump when pumping. The axial flow pump too runs from the same PV panels. The system is such that it fills the compressed air storage completely before pumping starts and then when it starts, the power from PV system goes to run axial flow pump. This way, we run only one of the two loads (Axial Flow Pump and Air Compressor) at a time on PV. This means that we needs relatively lower instantaneous power from PV and therefore smaller PV system.

Details of equipments

We need a compressor for running airlift pump. In our system, it compresses 450 litres per minute of air and the output pressure is 10 bars. This is typically a single stage compressor of over 1 HP. A compressed air storage tank is provided after this compressor. The storage that we plan to use is 2 metres in length and 1.5 metres in diameter.

Airlift pump that runs on this compressor has a couple of pipes, one for air and one for water. This pump can pump 80,000 litre water in 4-5 hours, this much water is enough to irrigate rice for a day. For other crops, water requirement is lower (35,000 litres per day for wheat, 20,000-40,000 litres per day for various vegetables, for examples). And our pump has to run for 2 hours for these applications. This pump requires air pressure of 4 bars at its startup and then continuously runs at 2 bars. Lightweight PVC pipes are used in these pumps.

Axial Flow Pump that we plan to use is run using 1 HP motor that is running on PV system. Flow from both the pumps will be the same by continuity. This pump too can give 80,000 litres of water in 4 hours and at this flow, it gives 4 metres head. At lower water flow rates, like in case of crops other than rice, the corresponding head is higher.

Thus our both pumps together can give flow of over 300 LPM and total head of 8 metres or more.

The PV system required for this is 1 kW as both compressor and axial flow pump run on 1 HP motor and we run only one of them at a time using PV directly. We also have a set of batteries along with this.

Portability

Axial flow pumps are among the lightest of the pumps available. The model of axial flow pump we intend to use weighs less than 25kgs, making it portable and easy to carry. Airlift pumps are designed with the use of light weight PVC pipes, it weighs around 5kgs. Thus the complete pump assembly weighs nearly 30kgs and is portable.

Airlift Pumps require low or no maintenance as the working unit has no moving parts.

Costing

The complete system together costs INR 96,000. The most expensive part is PV system at Rs. 45,000 including electronics and battery bank. An axial flow pump costs Rs. 13,000 and airlift pump Rs. 6,000. The compressor to run airlift pump costs Rs. 32,000 with its storage.