BEST PRACTICE LOW LOSS HEADERS
Placing pumps in any other configuration will result in unintended and unwanted circulation between circuits with unpredictable and unintended consequences
FIGURE 4 Low loss header
Primary glow
Secondary flow
Vertical header Secondary return Primary return Sludge trap Drain cock
FIGURE 5 Correctly configured low loss header and associated circuits
Qp
Automatic air valve
show load circuits connected as in figure 5, where separate flow and return headers are connected to the secondary side ports on the low loss header, and not along the header itself. This allows a relatively short low loss header to be mounted vertically while the flow and return headers can be mounted horizontally. However, current UK design practice is to allow multiple primary and secondary connections on the circulating header. In order to accommodate multiple header connections, circulating headers are mounted horizontally which, in turn, requires sufficient flow velocity to prevent sludge deposition in the header. Unwanted primary/secondary circuit interactions may then occur because this arrangement does not provide the primary/secondary hydraulic isolation desired.
Gas/LPG/Oil boiler
Qs Horizontal flow header
Gas/LPG/Oil boiler
Gas/LPG/Oil boiler
Constant flow temperature vs constant return temperature The direction of water flow along the header depends on the ratio of primary flow rate (Qp) to secondary flow rate (Qs). If Qp>Qs, flow will be in the forward direction (figure 5). Most designs require water to arrive at each secondary circuit at the same temperature it has left the boiler – that is, at a constant flow temperature. This, in turn, requires Qp>Qs under all operating conditions to ensure that reverse flow, and the resulting flow temperature dilution, cannot occur. Three individually pumped boilers are
Horizontal return header Pressurisation
Qp > Qs to ensure forward flow along header
FIGURE 6
Unintentional reverse flow along header
Qp
Gas/LPG/Oil boiler
Qs Horizontal flow header
Gas/LPG/Oil boiler
shown which, operated in sequence, could achieve an overall turndown of 12:1 or better. Hence, to satisfy the condition Qp>Qs at low loads, flow modulation of the secondary circuits is required with an overall turndown ratio of at least this value. If all, or most, of the secondary flow is on a single constant temperature (CT) circuit, this will require both large and small capacity variable speed secondary pumpsets, while splitting circuits into a number of smaller pumped circuits can avoid the cost and control complexity of large parallel pumpsets on a single circuit. On variable temperature secondary
Gas/LPG/Oil boiler
Horizontal return header Pressurisation
If Qs > Qp reverse flow occurs along the header
circuits, the mixing valve will achieve the necessary modulation. It is particularly important that Qp>Qs at all times when multiple secondary circuits are connected because reverse flow along a header can occur (figure 6). When Qp>Qs, the mixed return temperature to the boilers at the bottom of the header will vary depending on load, but if a fixed, and low, return temperature is required – for example, for condensing boilers – Qs>Qp at all times and the flow along the
60 CIBSE Journal February 2014
www.cibsejournal.com
Reverse flow VERTICAL HEADER VSD Circult 1 VSD Circult 2 VSD Circult 3 Circult 3 Circult 3 Circult 2 Circult 2 VSD Circult 3 Circult 1 Circult 1 VSD Circult 2
Forward flow VERTICAL HEADER VSD Circult 1
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