Building services engineers would not normally strike you as superstitious sorts. So why is it that so many HVAC system designs incorporate aspects which are not so much about technology and are more about ‘touching wood’? We seem hard-wired to be over cautious, over-sizing components, and building in extra capacity that experience tells us is unnecessary.
Perhaps it is all about delivering a trouble-free system. In less energy aware days, and when equipment was less reliable, it was a sensible strategy. But these old habits can lead to inefficiency and waste being designed into HVAC systems from the outset. And today, when sustainability is king and inefficiency costs, we can’t afford to deliver systems that underperform throughout their lifetimes.
So what old habits do we tend to cling to and are we asking for trouble if abandon these safety nets?
It is very common to encounter HVAC pump installations incorporating full-duty stand-by, with all the carbon (and physical) footprint that entails. Removing back-up completely would be unwise, but a valid alternative is frequently overlooked. Parallel pumping (covering the duty with two smaller pumps) can, in many applications, effectively halve the carbon footprint relating to the manufacture of the equipment itself, and halve the kWs installed.
System designers often discount this alternative because they assume that in a parallel pumping scenario, if one pump fails, the remaining pump will only cover 50% of the duty. This is not so. Half the installed kW does not equal half the capability.
For example, in an application with a system design duty of 200 l/s at 200 kpa, a pump with a 75kW motor would typically be specified with a second 75kW motor pump installed as full duty back-up and standing idle. A total of 150kW installed.
In a parallel pumping alternative, duty would be split between two smaller pumps, each fitted with 37kW motors (a total of 75kW installed compared to 150kW).
In the event of a failure of one of the pumps, the remaining pump however is capable of delivering 167 l/s which is 83% of the full design duty while the other pump undergoes maintenance (see Figure 1).
As the majority of systems require full design duty very rarely, the impact on the system will be minimal during maintenance. In addition to reducing carbon footprint, the two 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, thus increasing uptime.
Capacity based control
The full duty stand-by discussion is part of a larger one about control strategies.
In space heating, for example, we are dealing with a part load application. Typically we size the plant for design day conditions – capacity based control. But these conditions are rarely experienced in temperate climates. So it makes more sense to try to match boiler load to the building demand rather than to the capacity of the boiler– demand based control. It might require a significant shift in thinking, but so often this proves to be the key that unlocks greater energy efficiency savings.
Embrace new ideas
There’s a fine line between sensible levels of caution and getting left behind. If you’re about to install a like for like replacement pump/boiler/system in a refurbishment project then alarm bells should ring. Relatively simple measures, such as moving to variable speed pumping, or installing enhanced control of key system components such as boilers and pumps, can deliver significant energy savings.
The same applies to system operating temperatures. In North America and the UK, low temperature hot water heating systems were traditionally designed with a supply temperature of 80°C and a return of 70°C but it pays to challenge this thinking. In Continental Europe 80°C supply and 60°C return was the norm. This doubling of the system ?t immediately halves the flow required.
For example, in a system with a heat load of 240kW, with a ?t of 10°C, the flow required would be 5.7l/s. With a ?t of 20°C the flow required would be 2.85l/s. This has a dramatic effect on pipe sizing, pump sizing, friction losses, electrical power consumption and lifecycle cost.
It is true that the higher ?t reduces the mean temperature from 75 to 70°C. Today, however, building air tightness and thermal insulation are vastly improved so there is little impact on emitter size.
Over-sizing of pumps
We all know that oversizing pumps offers a security blanket for the system designer, but the costs of a system under-performing throughout its lifetime can be colossal. A quick life-cycle calculation will quickly prove the folly of building in unnecessary capacity.
There is another potential area of waste however that engineers may not be aware of, because it is driven by some equipment manufacturers. When optimising 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 trims impellers as a matter of course, but the same cannot be said for all pump manufacturers.
To standardise their product ranges, 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 oversizing of the pump.
At commissioning stage the problem intensifies, 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!
Finally, a practical point about other parts of the installation. The use of 3D graphic modelling (see Figure 2) frequently identifies opportunities to reduce system pipework. Unnecessary civil engineering work can also be avoided by locating vertical inline pumps in the pipework rather than building concrete inertia bases.
There are also system components available which perform multiple functions, reducing the number of components and connections required which in turn reduce physical footprint and installation costs. Armstrong Integrated’s Suction Guide, for example, creates optimum flow conditions at the pump inlet with a single component, eradicating the need for the conventional Y Strainer, flanges, nuts, bolts, gaskets and suction spool piece.
So in summary, the key to delivering more sustainable HVAC systems may lie in shrugging off old habits and applying new thinking for a new kind of customer.