BEST PRACTICE LOW LOSS HEADERS
FIGURE 3 Low loss header
1.5m/s
25mm pipe ∆P = 960Pa/m
80mm header ∆P = 4.3Pa/m
A
Flow in load circuit = 0.04kg/s or 5% of total fl ow
Boiler 2m ∆P = 8.6Pa Load
B
0.85kg/s Pump
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 ∆P = 960Pa/m
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
A Boiler B Load
Flow in load circuit = 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
www.cibsejournal.com
February 2014 CIBSE Journal 59
GORNJAK / SHUTTERSTOCK
0.1m ∆P = 96Pa
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