Introduction to VPF chilled water systems

Introduction to VPF chilled water systems

In this module, you will learn about benefits of Variable Primary Flow systems and how Grundfos variable speed pumps based on Delta P control can help optimise HVAC systems to lower energy consumption.

Welcome to this ECADEMY module introducing high efficiency primary-only, known as Variable Primary Flow, or VPF, air conditioning chilled water systems using variable speed-controlled pumps.

In this module, we will discuss:
- What constitutes an air conditioning system;
- How greater pump control and efficiency has changed the nature of air conditioning systems; and,
- Why VPF operation is considered the standard for air conditioning systems today.

Let’s start by looking at a typical air conditioning system.
Chilled water systems provide cooling for many air conditioning and industrial processes. Regardless of size or complexity, every chilled water system is comprised of cooling loads, cooling equipment such as chillers, pumps, and control valves.

Requirements for optimised and efficient operation are increasing, as are compliancy requirements.  To meet these increasing demands, there is now much more focus on the pumps used, and their operation and control.
In particular, the increasing affordability and reliability of pumps with built-in variable speed drives makes system optimisation and efficiency gains easier to obtain.

If a single set of variable-speed pumps handles both plant and system-flow requirements, conditions of excess flow are minimised. Furthermore, the pump’s integrated control modes mean that a target differential pressure, Delta P, is easily maintained in the system.

This contributes to resolving probably the most serious challenge facing air conditioning systems: to maintain constant Delta T and avoid potential extra cost for chilled water production and distribution due to the higher pump and chiller energy usage caused by low Delta T.

As mentioned, VPF systems are now considered the standard for air conditioning systems. Potentially, they save costs, because they are:
- more energy-efficient than their alternatives; and,
- cheaper to install than equivalent primary or secondary systems.
Now, let’s look at how a VPF system actually achieves these savings. Essentially, the VPF system design eliminates the constant-flow chilled water pumps and uses the variable-speed pumps to circulate water throughout the entire chilled water loop.

By separating pump control from chiller design, the VPF design enables the pumps to fulfil their task: to maintain a target differential pressure, Delta P, at a specific point in the system, whereas chiller sequencing is about ensuring that the water is sufficiently cold to satisfy the building load.
The pressure difference needed in a system tends to decrease when the air-handler control valves open in response to increasing loads.
To keep the required Delta P in the system, the pump controller increases the speed of the pump. Conversely, when control valves close in response to decreased coil loads, the pump controller slows the speed to maintain the target Delta P.

If properly designed and installed, variable speed-controlled pumps can reduce energy and other operating costs significantly over the lifetime of the pumping system.

Let’s review the benefits of installing VPF air conditioning systems using variable speed-controlled pumps:
- Lower initial cost, because fewer pumps are required;
- Savings on the fittings, vibration isolation, starters, wiring and so on, which would otherwise be required for the secondary pumps;
- Fewer pumps require less space, which can result in substantial cost reductions, depending on the plant layout and space constraints; and,
- Significant cost savings from lower energy consumption and reduced operating costs over the lifetime of the pumping system.
That concludes our introduction to high efficiency VPF chilled water systems using variable speed-controlled pumps.