BEST PRACTICE LOW LOSS HEADERS
header will be in the reverse direction (figure 7), achieving a constant return temperature.
To achieve continuous condensing operation in condensing boilers, the return temperature must be below 54°C at 10% excess air, while 90% latent heat recovery does not occur until the return temperature is 34°C. The flow temperature to the secondary circuits will be diluted by the recirculating flow along the header, while the secondary flow mixed circuit temperature can be calculated by nodal analysis, as can temperatures in any low loss header system. One condensing boiler manufacturer recommends that Qs should be 30% greater than Qp at full load. A measure sometimes employed to avoid
reverse flow along a horizontal circulating header is to insert a non-return valve in the centre of the header between the last secondary circuit flow take-off and the first secondary circuit return (figure 8). Unfortunately. this does not produce a sound engineering solution because the header is split into separate flow and return headers producing the primary/secondary circuit interactions the header is intended to avoid when Qs> Qp.
System pressurisation To prevent unintended primary-primary, primary-secondary or secondary-secondary circuit interactions, no point on the header should be at positive pressure with respect to any other point on the header by more than a few Pa. The header should be designed as the neutral point of the hydraulic circuit with system pressurisation connected as shown in figure 5, 6 and 7. Pump inlets should be connected to the header, which places them on the return to boilers and on the flow to each secondary circuit. Placing pumps in any other configuration will result in unwanted circulation between circuits with unpredictable and unintended consequences. There is a need for clear design guidance
on how to determine the length of low loss headers and the spacing of pipe connections on the header, and to identify any other critical design rules not covered in this article. To this end, CIBSE would like to identify any fundamental research that has been carried out, or to receive details of any published technical advice from other than a boiler manufacturer. Readers are invited to e-mail their comments to consultants@
campbellpalmer.com. CJ
DAVID PALMER is director of the Campbell Palmer Partnership and a member of CIBSE Council
FIGURE 7 Low loss header configured for constant return temperature
Qp
VSD
Circuit 1 Qs1
Condensing boiler
Condensing boiler
Condensing boiler
Circuit 1
Forward flow at bottom of header
Pressurisation
For condensing boilers Qs 30%> Qp with reverse flow along the header producing a constant return temperature
FIGURE 8 Inappropriate use of a non-return valve
Qp VSD
Circuit 1 Qs1
VSD
Gas/LPG/Oil boiler
VSD
Circuit 3 Qs3
Gas/LPG/Oil boiler
DO NOT INSTALL a non-return valve
Circuit 2 Qs2
Circuit 1
Gas/LPG/Oil boiler
Circuit 2 Circuit 3
Qs = Qs1 + Qs2 + Qs3 Qp > Qs to ensure forward flow along header
Pressurisation
62
CIBSE Journal February 2014
www.cibsejournal.com
Forward flow
Intentional reverse flow VERTICAL HEADER
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80