An introduction to proportional pressure in water boosting systems
Find out how proportional pressure benefits commercial buildings with more dynamic losses than static losses – such as airports, hospitals and shopping malls.
In many heating and chilled water systems, proportional pressure control mode is the preferred control standard. And with good reason. In HVAC systems, only dynamic losses from pipes, bends and components are taken into account. Here static losses or lift can be disregarded.
In water boosting systems in commercial buildings, however, constant water pressure has become the norm, even though there are many dynamic losses in these types of buildings as well, rather than just static pressure losses.
In this course, we’ll introduce you to proportional pressure in water boosting systems and how it can bring savings to commercial buildings.
But before we go into further detail, let’s refresh two key terms: static pressure losses and dynamic pressure losses. Simply put, static pressure losses are independent from the flow rate, while dynamic pressure losses are highly dependent on the flow.
During the sizing process of a water boosting pump, it’s crucial to calculate static height or lift to determine the static losses, while the dynamic losses have to be calculated as well. Static height equals the measured elevation from the booster set and to the highest tapping point in the building. Dynamic losses depend on the water consumption. So, when the water consumption and flow is high, the dynamic losses in the pipework and fittings increase. At zero flow, there are no dynamic losses.
Traditionally, constant water pressure is used in water boosting systems in tall commercial buildings, because there’s a large amount of static height. As seen here, the pump pressure or head, shown with the red line, is constant, regardless of flow and dynamic losses. But a water booster pump can be optimised by adapting to proportional pressure – even in tall buildings. Let’s take a closer look.
As we see here, when the pump operates in proportional pressure control mode, it adapts its performance to the required pressure, which is determined by the flow rate. So, even though the savings of operating in proportional pressure control mode may seem modest in buildings with a large amount of static head, they are still worth the effort.
Many commercial buildings, however, do not belong in the ‘tall buildings’ category. Buildings such as hospitals, shopping malls and airports cover large areas, but only few storeys. And for commercial buildings such as these, there is no doubt that proportional pressure is the way to go.
Imagine an airport. A typical airport covers large areas, but has a rather modest height. And when it comes to water boosting, this is quite significant as it essentially means that the static and dynamic loss ratio is switched around in comparison to tall commercial buildings. As this graph illustrates, proportional pressure becomes even more relevant in buildings such as these, and the potential savings are substantial.
Another important factor is the water pressure and flow at the tap. In systems operating in constant pressure control mode, the water flow at the tap isn’t constant. In fact, the water flow through the tap increases when the overall system flow decreases. This is because the taps are subject to pressure variations, which depend on the system’s dynamic losses – losses that are dependent on flow. Let’s look at an example.
If a system’s water flow is reduced. The available pressure at the tap will increase from, for instance 100 to 200 kPa and the water flow at the tap increases from 0.25 to 0.33 l/s – a 32% percent increase in water flow. So, when overall system consumption is reduced, dynamic losses are also reduced. And if the pump head remains constant, it leads to an increased pressure at the tap, ultimately resulting in an unnecessary overspend of water.
On the other hand, systems operating at proportional pressure can adjust the pressure to what is required. So, once the system flow is reduced, the pump’s pressure or head is reduced accordingly. That way, water pressure at the tap is more or less constant, resulting in nothing more than a marginal overspend – again highlighting key advantages of proportional pressure.
There are in fact two types of proportional pressure control modes: linear and squared. Linear proportional pressure control is characterised by the pump adapting its performance proportionally according to the flow. Squared proportional pressure control, however, simulates real system conditions. In this example, the red area highlights the potential energy savings achieved through linear proportional pressure, while the blue area illustrates even further savings that can be made with squared proportional pressure.
To sum up, proportional pressure control – both linear and squared – in water booster systems bring great energy savings as well as water savings. And while the savings are more evident in commercial buildings with few storeys such as airports and hospitals, applying proportional pressure control in water boosting systems in any commercial building will bring savings.