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 ∆P = 960Pa/m A
Flow = 0.425kg/s if load circuit
Boiler ∆P = 1,920Pa 2m ∆P 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
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