BSEE


The focus of this arcle is commercial heang/chilled water systems over 70kW output as described in BS6644 and all systems using category 4 fluids where fully automac filling devices are the norm


subsequent ‘topping up’ of the system water and the absorption of volumetric expansion. The great majority of commercial heating and chilled systems use water as their transfer medium. The advantages of using water, it’s an excellent conductor, its readily available at a relatively low cost and inherently stable. The disadvantages are, its non-linear expansion, its ability to carry dissolved oxygen and it has a relatively low boiling point (at atmospheric pressure). All of these features should be addressed at the design stage.


I uHistorical open vented system


Historically open vented systems were commonplace. The advantages of an open vented system being, they were generally reliable, easy to install and provided even pressure throughout the systems temperature range. The Disadvantages being, restrictions on system design, corrosion, air locking, pumping over, and drawing air in via the vent pipe. In addition, there is increased system evaporation due to being open to atmosphere.


‘ Modern


building design almost invariably renders open vented systems obsolete


’ uModern sealed system


The advantages of using a sealed system include, freedom of design, finer levels of control, reduction in corrosion (extending life expectancy of the system) and decreased system evaporation. The disadvantage being, a more complex design/selection is required. The minimum information required for the sizing of expansion vessels on sealed systems: uThe cold fill pressure (this is dependent on the relative position of the fill equipment within the


8 BUILDING SERVICES & ENVIRONMENTAL ENGINEER OCTOBER 2019


The issues of the open vented systems were addressed by the introduction of the sealed system, where the water content is sealed from the atmosphere. This system dispensed with the open feed and expansion tank and used purpose designed vessel/s to deal volumetric expansion. The system water is now introduced in to the system by way of a filling device. These devices range from simple filling loops and pumped pressurisation units to direct type filling devices.


n any heating or cooling system where water is the thermal transfer fluid (Hydronic systems) the following points need to be addressed: an introduction of the initial charge of system water, suitable backflow protection,


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The filling of commercial heating and chilled water systems over the years


building, the height of the system and any specific product requirements).


uSystem volume / Connected load uMaximum system design pressure uSystem flow and return temperatures uExact composition of any water and glycol mixture to determine the density and expansion characteristics of the fluid


An important note


The sizing of expansion vessels is by definition an imprecise science due to the many possible variations, i.e. water content, pressure, temperature and the effect of additives. For the vast majority of systems the tried and tested expansion vessel is the default design, as they are relatively low cost and extremely reliable with no external power supply required.


The most important factor to bear in mind is that the sizing of the expansion vessel is paramount. An oversized vessel is of no detriment to the system, whereas an undersized expansion vessel will result in excessive system pressures, manifested by system shut downs and alarm conditions.


All expansion vessel manufacturers will have a maximum vessel acceptance factor but all may differ. The vessel acceptance factor is an indication of the maximum amount of expansion that a given vessel can accept enabling longevity and safety and this is normally around 25 per cent. So as a result a 100 litre vessel is designed to accommodate around 25 litres of expanded water. Therefore manufacturers generally err on the side of caution when sizing expansion vessels and where applicable, always select the next vessel size up.


Methods of filling the system, incorporating suitable backflow prevention


In the first instance water has to be introduced into the heating/chilled circuit. Sealed systems cannot obviously use an open vented tank, water has to be introduced in a controlled way. This can be a complex decision as the designer needs to be aware of the requirements of the current regulations and the service and maintenance demands of the selected equipment. BS 6644:2011 states that a sealed system shall be provided with a means for initial filling that is acceptable to the local water supplier. Water Supply (Water Fittings) Regulations 1999 and Scottish Water Byelaws 2004 define system fluid risk categories. The method of filling should be appropriate to the type of system and should satisfy the requirements for backflow protection of the wholesome water supply.


Fluid risk category 4 (commercial and industrial heating systems)


Fluids in category 4 represent a significant health hazard because of the concentration of toxic substances, including any fluid which contains chemical and carcinogenic substances, or pesticides (including insecticides and herbicides), or environmental organisms of potential health significance.


Typical applications of a category 4 fluid is water in primary circuits and heating systems in other than a house, irrespective of whether additives have been used or not, this also includes commercial process applications.


Commercial filling devices (fluid risk category 4) fall into three categories (all of these devices incorporate the means of preventing backflow contamination to comply with the current water regulations):


RPZ valves or filling apparatus incorporating them - When filling fluid risk category 4


Safety valves


Safety valves must be installed on all sealed systems. These are installed to protect the system should the pressure exceed the valve rating. Modern building design almost invariably renders open vented systems obsolete. Sealed systems are now commonplace as they offer total flexibility of design and add to the potential of remote management. Sealed systems are generally easier to install, occupy less building space and support best practice in design.


We recommend introducing the Mikrofill 3 into a ‘neutral’ point on the system, ideally a low loss header. This eliminates the potential of a false system condition generated when installed too close to the suction side of a circuit pump.


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heating/chilled systems the RPZ can be used to protect the wholesome water supply from backflow contamination.


However RPZ valves do require additional controls to act automatically. In addition, the RPZ valve is subject to the legal requirement of a costly annual verification check. Tank Fed Pumped units - These units utilise a small cold water feed tank and a suitable pressurisation pump/s. In addition these will include switches or transducers to control water pressure and set alarms where necessary. Cased or uncased versions are available with differing levels of sophistication. This type of unit is not suitable for initial filling and can normally only be used for ‘topping up’.


Direct type device (Mikrofill 3) - Purpose designed fluid risk category 4 filling device which can be connected to the wholesome mains/boosted cold water supply. These are inclusive of all controls and can be used to initially fill and ‘top up’ with no annual verification check required. A point often overlooked when packaging pressurisation units and expansion vessels together is that the units ‘coldfill’ and the expansion vessel’s nitogen/air charge need to be in equilibrium. For example, a system with a coldfill of 1.5 bar requires the PU unit set at 1.5 bar and the relative vessel/s nitrogen/air charge set at 1.5 bar. With a potential final working pressure of 2.5 bar the system pressure now enters the expansion vessel therefore increasing the nitrogen/air charge pressure to equalise with that of the system working pressure.


All of the aforementioned units must be used in conjunction with a suitably sized expansion vessel/s. Water contains dissolved gases including oxygen that will react with the different metallic materials within the system and cause light internal surface corrosion. If the system exhibits leaks, the continual introduction of wholesome water can result in premature failure of the system components. This problem is easily monitored by using modern fill equipment that can record the volume and regularity of water introduced into the system. Some units have integral flood protection where safety interlocks will interrupt the plant if large amounts of water are being lost; these features not only protect plant but also reduce water wastage.


Safety devices for sealed systems include high and low pressure switches which monitor the pressure in the system. If the set pressures are exceeded they will shut down the source of heat/cooling.


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