MHS Boilers Ultramax 115, Operating & Maintenance Manual, Page: 31 / 46

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Example of a low velocity header with isolating valves and an expansion tank.
Mounting the low velocity header vertically has additional advantages: the upper section
functions as an air separator and the lower section serves as a dirt separator.
When air heaters (for ventilation or air treatment) are included in the system it is generally desirable
to have a small ∆T over the air heaters. Because of this, the water flow rate through the whole
secondary circuit is usually greater than that through the boilers. The low velocity header must
be so dimensioned that the water speed does not exceed 0.5 m/s. In this case the diameter of the
low velocity header must be calculated on the basis of the water flow through the secondary circuit.
Because the volume of the water in the secondary system is greater than that in the primary
circuit (boiler), there will be a water circulation in the opposite direction to that of the primary
circulation through the low velocity header. A mixed temperature will then exist which is lower
than the supply temperature from the boiler. The regulation system will react to this and will open
the regulator functions (valves, etc.) in the system. Generally, the temperature of the water supply
from the boiler(s) will need to be corrected to obtain the desired temperature in the connected
circuits.
Systems with a separate flow header and a return header
Flow headers in combination with return headers are often used in renovation projects. Several
circuits operate with mixing valves or diverting valves. In both cases a low velocity header or a
bypass is necessary.