David Hearne,Market Sector Manager for Yorkshire Fittings,looks at how commissioning valves can aid consulting engineers when designing modern heating and cooling systems for commercial buildings.
In today’s modern commercial buildings there is an ever increasing need for flexibility.Developers looking to let office space,for example,are seeking the maximum opportunities to impress potential clients by requiring that space is designed to offer open plan and individual offices within a single floor.
Temperature management can be controlled through the flow rate of heated and chilled water to a terminal device,such as a fan coil unit,heating beam,radiant panel or radiator.In modern office buildings,terminal devices are typically housed in the ceiling voids,with their output (ie heated or chilled air) controlled by remote devices.In an open plan space,terminal units will be housed at intervals to manage the temperature across the floor.However,the challenge for consulting engineers is to consider the office tenant, who may require individual offices to be constructed subsequent to the initial construction contract.In this case,if the terminal unit is not located in the space where the office is,the user of that office will not have the benefit of being able to control the temperature in their own environment.This is a problem that consulting engineers need to overcome.
The solution has been to install additional terminal units with a lower flow rate for smaller spaces such as partitioned offices.
However,low flow rates present their own problems,as it is difficult to measure them – the “signal” - accurately.Their increased use therefore presents a number of challenges for both consulting engineers and valve manufacturers,who are seeking to provide suitable heating and cooling solutions to meet these differing – and potentially changeable – space requirements.
Measuring low flow rates
There are two options available to overcome the difficulty in measuring low flow rates when commissioning a modern heating and cooling system.The first is the traditional orifice plate,the second a Venturi measuring device,both of which use pressure differential measurement.
The traditional orifice plate has been for many years the standard measuring device for all systems.However,in recent years these have started to face increased problems due to the smaller variation between flow rates for individual terminal devices.For example, consultants will often select flow rates of 0.01 l/s (litres per second)and 0.011 l/s for many heating terminal devices,due to the increased insulation factors within the building construction.This means the signals generated are almost identical,allowing little allowance for error to maintain an accuracy of +/-5 percent.
The orifice plate works by forming an obstruction within the pipe system with the primary purpose of creating a pressure differential that can be measured.This allows the flow rate to be calculated regardless of the impact on the losses in the system.
The alternative is to introduce a Venturi measuring device.
Venturis increase the flow rate and signal flow)measured without the associated losses customary with the traditional orifice plate.
The Venturi functions by compressing and accelerating the water through a profiled nozzle(wasp waisted orifice)and allowing the water to expand in controlled conditions through the trumpet.This creates a more energy efficient flow path whilst allowing the flow rate to be measured.
In application a Venturi can achieve significantly higher signals than an orifice plate and,in association with a regulation and isolation valve arrangement(such as Yorkshire Fittings’ Proflow Venturi),offers equal or lower pressure losses than the traditional orifice plate and double regulating device.
To illustrate the differences,in a traditional orifice plate in conjunction with a regulation device,a typical flow of 0.015 l/s would generate a signal of 1kPa (kilo Pascal)with a loss of 0.96kPa. However,a Venturi with a regulation device and with the same typical flow would offer a signal of 2.25kPa with an overall loss of 0.62kPa.Should the next terminal device have a designed flow of 0.017 l/s,the orifice plate device would generate a signal of 1.28kPa - a minimal increase - whilst the Venturi would increase to 2.9kPa.
The relationship between signal and loss continues throughout the size range.Typically many Venturi’s are selected with signals in the band of 1-60kPa,offering the contractor the benefit of using a line sized valve,whereas a traditional orifice plate would need to be down sized through the use of a reducer.
For example,should a flow of 0.2 l/s be selected for a 25mm pipe,an orifice plate at this size would produce a signal of 0.7kPa.
This falls outside the required signal minimum of 1kPa and would therefore not be acceptable.However,a Venturi would generate a signal of 5.93kPa,which is well within the preferred measuring range.This feature offers the contractor significant cost savings as there is no need to purchase or install reducers in a Venturi system.
In addition to their significant benefits in terms of signal measurement and associated losses,Venturi’s also offer increased accuracy.Traditional orifice plates tested to,or manufactured in accordance with, BS 7350 offer an accuracy of +/-5 percent.
However Venturi’s can be flow tested to +/-1.5 percent,with a typical accuracy of +/-3 percent,further improving the balance across a low flow system.
Installation
Whilst valve manufacturers can advise consulting engineers, contractors and installers of the benefits which various types of measuring device can offer,all of these are inherently linked with their correct and accurate installation.
The traditional orifice plate requires upstream and downstream lengths of straight pipe to be installed either side of the valve (without fittings)to maintain its flow accuracy; and this also applies to a Venturi.However it has been demonstrated with Venturi’s such as Yorkshire Fittings’ Proflow Venturi,that these lengths are available incorporated into the valve body without interfering with the performance of the measuring function.This results in a valve which is smaller than the traditional orifice plate solution, aiding
installation space planning and manoeuvrability.
Incorporated lengths mean that Venturi’s can be connected to all new modern pipe materials without the need to apply a correction factor – something that is commonly required with an orifice plate installation.This feature removes some responsibility from the contractor to install the products correctly. In cases where orifice plates are used,any installation errors would only be identified once the system has been prepared for commissioning.
Application
Venturi technology has been used for over two centuries.
However,it is only in recent years that its application has extended to be suitable for the building services sector.With a climate control system now being almost a given in new build and major refurbishment projects,the increased importance of technology such as the Venturi commissioning valve can only continue.
