Green system design - the big picture revealed
Reducing the lifetime carbon impact of the pumping system, will become increasingly important in the future, says Wayne Rose, with a move away from the issue of component-specific efficiency and focus on standards for overall system efficiency
There has been a lot of discussion recently about the motor efficiency of pumps. But with so much focus on just one element of the efficiency of one component in a system, are we in danger of missing the big picture? Pump motors are just one element of overall system efficiency. All aspects of sustainability need to be considered, from avoiding the installation of unnecessary equipment at the outset, to minimising energy consumption throughout the system's lifetime.
Perhaps the most common cause of wastage is 'over-sizing'. In previous decades, most systems had fixed operating speeds, with pumps operating day after day at the top end of their capabilities. So HVAC system designers would over-specify 'to be on the safe side'.
With today's all variable speed equipment and advanced control methodologies however, equipment ramps up and down automatically to match energy consumed with the exact building load. In this scenario, inefficiency designed into the system by over-specification at the outset will significantly reduce carbon reduction potential throughout the lifetime of the plant.
By over-sizing I don't just mean specifying a larger than necessary pump for the required duty. There are other less obvious practices. It is common, for example, to encounter HVAC pump installations incorporating full-duty stand-by, with all of the carbon (and indeed physical) footprint this entails. In many cases this is unnecessary. Parallel pumping (covering the duty with two smaller pumps) can provide an effective alternative in many applications, halving the carbon footprint relating to the manufacture of the equipment itself, and halving the kWs installed.
This approach is frequently overlooked because system designers assume that, in a parallel pumping scenario, if one pump fails, the remaining pump will only cover 50 per cent of the duty. This is not so. Half the installed kW does not equal half the capability.
In addition to reducing carbon footprint, smaller, lighter pumps are easier to install and maintain. Upfront cost is lower and the variable speed drives are smaller, as is the electrical supply cabling. Developments in technology such as split coupling of pumps with external seals also reduce the time needed for routine maintenance to minutes rather than hours, increasing uptime.
Impeller must be right for duty
Another working practice that can lead to inefficiency in fixed speed systems relates to the size of the pump's impeller. When optimising the efficiency of medium and large pumps for fixed speed systems it is crucial to ensure the impeller is perfect for the duty rather than a 'near fit'. Armstrong therefore trims impellers as a matter of course.
To standardise their product ranges, however, some manufacturers only offer the 'nearest fit' rather than machining each impeller. Significant wastage of energy can result throughout the pump's lifetime as the impeller size will always be selected to over-perform, leading to over-sizing of the pump.
This problem is made worse on commissioning when the engineer throttles the pump to get back down to the desired design flow. This, of course, hikes up the head. The pump motor has to work harder, absorbing more energy than planned, to create a head (and maintain an operating point) that nobody actually wanted in the first place!
Waste can also be eradicated during the design phase by more intelligent approaches to installation. The latest generation of 3D computer modelling (see picture above right) has revolutionised this aspect of pumping system design. More effective pipework drawings can be created quickly and easily to reduce the cost and carbon impact of materials. It can also be possible to remove the need for unnecessary civil engineering work. Armstrong 4300 vertical inline pumps for example are now frequently installed in the pipework. This removes the need to build concrete inertia bases, reducing the time and cost of the installation as well as reducing its carbon impact.
The latest generation of pump trims can also offer benefits in this respect, by performing multiple functions with a single accessory. Armstrong's Suction Guide, for example, creates optimum flow conditions at the pump inlet with a single component. This eradicates the need for the conventional Y Strainer, flanges, nuts, bolts, gaskets and suction spool piece. Armstrong FloTrex valves provide similar opportunities for the discharge outlet, carrying out the functions of a check valve, throttling valve and shut-off valve with just one component. In addition to reducing carbon footprint, multi-function components of this type can greatly reduce the physical footprint and installation costs. For example, traditional base mounted end suction pumps may take up to three times more floor space than the Series 4300 equivalent.
In conclusion, measures such as those discussed, with the capability of reducing the lifetime carbon impact of the pumping system, will become increasingly important in the future. Whilst discussion at present is focused on motor efficiency, the next phase for European legislation (the Extended Product Approach) will see the HVAC industry move away from the issue of component-specific efficiency and focus on standards for overall system efficiency.
The author is director of product solutions at Armstrong
6 August 2012