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OPINION BEST PRACTICE LOW LOSS HEADERS


HEADERS L


Talking


Low loss headers play a vital role in ensuring that boilers operate at a constant fl ow rate, so why is there such a lack of design guidance? David Palmer sets out some rules


An immediate consequence of low fl ow velocity is the potential for sludge and debris to collect in the header


ow loss headers, often referred to as common headers, are advocated as design best practice because they enable boilers to be controlled in their


own constant fl ow rate circuit while fl ow rates in load circuits vary. The preferred hydraulic circuit arrangements in CIBSE Guides B and H show boilers and load circuits connected by a low loss header. However, there is an absence of guidance on how to design them. Coupled with a lack of understanding of how they should operate, this can result in the very circuit interactions and boiler controllability issues the designer is attempting to eliminate.


Why they are needed The principal function of a low loss header is to provide hydraulic isolation between both primary (containing heat generators) and secondary (load) circuits: water will fl ow in a closed circuit only if there is a pressure difference across it. The following are simplifi ed examples to illustrate the benefi t of a low loss header. Figure 1 shows a boiler without a low loss header. It comprises a boiler and pump in a


1.5m/s 25mm pipe ΔP = 960Pa/m A


Flow = 0.425kg/s if load circuit


Boiler ΔP = 1,920Pa 2m ΔP 1.920Pa Load


primary circuit, and a secondary circuit attached across the primary pipe. If the primary circuit has a fl ow rate


of 0.85kg/s at a velocity of 1.5m/s in a 25mm pipe, the pressure loss will be 960Pa per metre length of primary pipe (ignoring bends and fi ttings). If the secondary circuit connection points A and B are 2m apart, a pressure difference of 1,920Pa will appear across the secondary circuit. Then, if the


pressure loss in the secondary circuit is also 1,920Pa, 50% of the fl ow will pass through the load. However, if points A and B are close


together, say 100mm apart, the pressure difference across the secondary circuit will be only 96Pa, producing less fl ow in the load circuit (fi gure 2). However, even with the secondary fl ow and return pipes close together, an unwanted fl ow of 22% of the primary fl ow can still be produced in the load circuit. A low loss header achieves hydraulic


isolation by reducing the pressure loss along the header to a very low value. Figure 3 shows the same circuit but with an 80mm


B 0.85kg/s Pump


FIGURE 1 Circulating header with secondary connections


58 CIBSE Journal February 2014


header, which reduces the pressure drop to 4.3Pa/m at a fl ow velocity of 0.1m/s. With the secondary circuit connections again spaced 2m apart, the unwanted fl ow in the load circuit is now just 5% of the primary fl ow. At realistic secondary circuit pressure losses the unwanted secondary fl ows by using a low loss header will be 1% or less of the primary fl ow


www.cibsejournal.com www.cibsejournal.com Boiler B Load


and a pump is required in each


secondary circuit to


achieve the required fl ow. An immediate consequence


of low fl ow velocity is the potential for


sludge and debris to collect in the header. For this reason, a low loss header should always be mounted vertically with a sludge trap and drain cock at the bottom (fi gure 4). The lowest connection point on the header must be above the level at which sludge collects.


Header length and separation distances Header design information is provided by


1.5m/s 25mm pipe ΔP = 960Pa/m


FIGURE 3 Low loss header


1.5m/s 25mm pipe ΔP = 960Pa/m


80mm header ΔP = 4.3Pa/m


A Flow in load circuit = 0.04kg/s or 5% of total fl ow Boiler 2m ΔP = 8.6Pa Load B


Pump 0.85kg/s


many boiler manufacturers who recommend header length and connection spacings for their own boilers. Some general rules inferred from their data are:  The greater the output of the boilers, the larger the header diameter and the longer the header needs to be


 The greater the system design temperature drop, the lower the fl ow rate, the smaller the header diameter and the shorter the header


 The greater the total load, the more widely spaced the secondary connections need to be


 For a constant fl ow temperature system on the load side, the secondary connections should be inside the primary connections


 For a constant return temperature system on the load side, the secondary fl ow connection should be above the primary fl ow connection. Boiler manufacturers’ guides mostly


Velocity in header = 0.1m/s DESIGN RULES


Key considerations include:  Is the system to be designed for constant fl ow or constant return temperature?


 Do all of the load circuits operate at the same temperature, or is there a mix of higher temperature (such as air handling units) and lower temperature (underfl oor heating or oversized radiators) circuits?


 Are multiple boilers to be used on the primary side?


 Is a biomass boiler to be included in the boiler mix?


 What turndown ratio will result on the primary side of the header?


 Will all load circuits operate at the same temperature difference?


To achieve true hydraulic separation a low loss header and its associated circuits should be designed as follows:  Rule 1. For constant fl ow temperature operation, the fl ow along the header must always be in a forward direction, requiring Qp >Qs at all times, and the secondary fl ow connection must be sited below the primary fl ow connection as in fi gure 5


 Rule 2: For constant return temperature operation Qs> Qp and the secondary fl ow connection must be sited above the primary fl ow connection as in fi gure 7


Flow in load circuit = A 0.19kg/s or 22% of total fl ow


 Rule 3: The fl ow velocity along the header should not exceed 0.15m/s at full load. A rule of thumb is that the header diameter should be at least three times that of secondary fl ow connection


 Rule 4: The header should be mounted vertically to trap sludge at the bottom, while air in the system will rise to the top from where it can be removed


 Rule 5: The header should operate at neutral pressure with the suction (inlet) side of all pumps connected directly to the header


Pump 0.85kg/s


FIGURE 2 Circulating header with closely spaced secondary connections


 Rule 6: System pressurisation should be directly onto the header with the pressurisation connection below the primary return and above the sludge trap


February 2014 CIBSE Journal 59


At the ASHRAE winter Conference in New York


Critical connections In response to David Palmer’ s ‘ Talking headers’ article in the February 2014 issue of the CIBSE Journal, I have the following observations, primarily concerning CHP connection. CHP systems are less tolerant of temperature increase than boilers – particularly when used with 82 C design -fl ow systems – due to engine -cooling limitations. Most headers are installed with system F&R connections


from the header, as Figure 8 (without the NRV s) in the


article. If the system fl ow Qs is 30% > than the primary heat - source fl ow Qp, system 3 will short circuit the heat source by recirculation down the header. I therefore believe the heat -


source F&R connections should be at the centre of the header, to achieve a common mixed condition. The arrangement shown in Figure 5 & 6 avoids this by ensuring a common mixed condition to and from the systems to the header. However, the 30% imbalance suggested with Qs 30% > Qp, can cause problems where CHP systems are connected and system dT and return temperature are critical. The recirculation along the header could require increased primary heat -source temperatures, to achieve the required system - fl ow temperature, above CHP maxima for full heat recovery. Conversely, if Qp is > Qs,


as often occurs due to variable system fl ow as control valves


www.cibsejournal.com


close down, constant primary fl ow involves short circuit of fl ow temperature to the heat - source inlet, resulting in return (inlet) CHP temperatures above the maximum for heat recovery. For relatively small CHP heat


supply, this problem could be avoided by installing the CHP in a series shunt line off the common system return, before the header connection. The pre -heating of the return


may compromise condensing boiler performance, but the boilers will have a less signifi cant top -up/back -up role. Otherwise, careful design and fl ow control may be required to ensure that Qp and Qs are matched to minimise recirculation impact. David Hague, Cogen Solution


David Palmer replies: The article ‘Talking headers’ is the fi rst of two articles on header design focused on the connection of boilers to headers. The second article will cover the more complex matter of connecting biomass boilers, other renewables sources and CHP units to headers. The matter raised by David Hughes will be addressed in the next article.


CIBSE Journal welcomes readers’ input, whether it be letters, opinions, news stories, events listings, humorous items, or ideas and proposals for articles. Please send all material for possible publication to: editor@cibsejournal. com, or write to Alex Smith, editor, CIBSE Journal, CPL, 275 Newmarket Road, Cambridge, CB5 8JE, UK. We reserve the right to edit all letters.


(see page 20), the society’s president, Bill Bahnfl eth, highlighted the fact that the focus on energy effi ciency had distracted the industry from its other major responsibility to provide a high standard of indoor air quality (IAQ). In the rush to plug fabric leaks


and minimise heat losses, IAQ has been left behind. As Bahnfl eth said: ‘We have halved the energy consumption of buildings… but are still doing more or less the same things with IAQ’. He also noted that, in the US, no legislation is planned in this area , despite governments, worldwide, regularly acting to tackle pollution of outside air. For politicians, air becomes invisible as soon as it enters a building. Improved energy standards


under Part L of the Building Regulations have made buildings increasingly airtight, which means they need a higher level of ventilation. While this is clearly stated in Part F of the regulations, many buildings fall short of the required standards. BSRIA reports that 89% of the


10,000 air -tightness tests it has carried out met the standards set by Part L, proving that building airtightness standards have risen to meet the energy challenge. However, it has also measured serious problems with the quality of mechanical ventilation with heat recovery (MVHR) systems designed to deliver the required number of air changes


to an occupied space without compromising the energy -saving design. It poses the question: ‘ Are we ventilating right?’ Or are we suffocating occupants in order to reduce our energy bills? Building ventilation systems


should meet the basic brief of reducing overheating and maintaining good IAQ all year round without increasing operating and capital costs – or driving up carbon emissions. Developments in the design


In the rush to minimise heat losses, IAQ has been left behind


of natural ventilation systems, in particular, mean it is possible to signifi cantly reduce life-cycle costs while improving the IAQ. However, it does require some extra effort and installers must be trained to regard the ventilation as an integral part of the whole building.


That effort will pay dividends because there is strong evidence that naturally ventilated buildings have lower levels of airborne contaminants. We don’t have to choose; we can


produce healthier buildings that are also cheaper to run. See Nigel Ingram on MVHR – page 30.


Ruskin Air Management is a market leader in air distribution, and fi re and smoke control. It combines the Actionair, Air Diffusion and Naco brands. The companies work together to provide complete HVAC solutions for the built environment.


SPONSORED BY RUSKIN


BREATHE EASY – HEALTHY BUILDINGS CAN BE EFFICIENT


Indoor air quality has been neglected in our push for energy-saving buildings, but Ruskin’s David Fitzpatrick says they are not incompatible goals


March 2014 CIBSE Journal


25


0.1m ΔP = 96Pa


GORNJAK / SHUTTERSTOCK


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