Minimum flow through the hot water boiler in Software

Drawing QR Code in Software Minimum flow through the hot water boiler

As indicated earlier, the boiler manufacturer whose boiler is under consideration must provide information as to minimum flow rate and, in some cases, the maximum change in flow rate These two factors may or may not be important The boiler manufacturer should guarantee the boiler against thermal shock under specific flow conditions Thermal shock is avoided by providing a minimum flow rate in the boiler Minimum flow rate is maintained by providing a circulating pump on the boiler or by primary-secondary pumping Other manufacturers depend on bypass valves rather than the boiler or primary pump It is the boiler manufacturer s responsibility to provide detailed pumping and piping instructions for the boiler installation to ensure that thermal shock does not occur in the boiler

Boilers are classified by their flow requirements to ensure that adequate flow is maintained in them The boiler manufacturer should always be consulted for minimum flow and temperature requirements

Most fire tube and cast iron boilers require substantial flow under most load conditions on the water system It is partiularly important to have the actual boiler manufacturer of these types to state the minimum allowable flow at various loads and water temperatures

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Minimum flow Water tube boilers, manufactured of steel or copper tubes, often require a minimum flow that is specified by their manufacturer No flow requirement Many new condensing boilers do not require any minimum flow This, of course should be certified by the manufacturer, whether at full or minimum load on the system

1973 Locating pumps at boilers The question always arises as to the location of the pump with respect to the boiler There are some accepted rules in the HVAC industry that have no foundation The selection of the pump location with respect to the boiler must be determined by the physical conditions of the actual application For example, 1 The pumps should be located, if possible, on the cooler return water to the boiler This enables the pump internals such as the mechanical seal to have a longer useful life 2 Locating the pumps on the cooler return water also does not expose the pumps to the air released from the water in the boiler (see Table 25) 3 The static pressure of the building determines the required operating pressure of the boiler If putting the pump on the inlet to the boiler increases the design pressure of the boiler, either the boiler construction should be changed or the pump should be located on the discharge of the boiler Often, boilers constructed of steel can be equipped with pressurerelief valves set at pressures required by the installation with the pump installed on the return water connection to the boiler Location of the pump with respect to the boiler is a decision that must be made by the designer, taking in consideration all the conditions that exist on a particular installation

198 Reset of Boiler Temperature versus Zone Reset A controversy exists in the HVAC industry over the relative merits of boiler temperature reset and the reset of zone temperatures It is important that this be addressed in this chapter, since many designers are losing sight of what achieves lowest possible energy consumption in a hot water system Operation of the boilers at the lowest acceptable

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