Page 150 - Boiler_Operators_Handbook,_Second_Edition

Special Systems 135

Codes have changed and in every state and province all sure to prevent flow of water through the boiler during
hot water boilers must have a low-water cutoff. If your this period. Sometimes the valve is fitted with a spring
boiler is equipped with a low-water cutoff its operation rather than using weight. Don’t put another type of
should be confirmed during fill-up and before the burn- valve in its place or it may not work.
er is fired. Since service water heaters are seldom fitted As the hot water flows out the top of the tank it is
with a gage glass this test assumes a water level. Turn replaced by cold water entering the bottom of the tank.
control power on before filling the boiler; the burner The interface level raises until it is above the level of the
should not start. Then constantly monitor it to note that upper temperature control switch (TC1). Contacts on
the burner starts to fire once the level reaches the low- TC1 close to start the pump. Auxiliary contacts on the
water cutoff and closes its contacts. Then, immediately pump motor starter close to bypass the TC1 Contacts so
shut off the water supply and open the drain. If the low- the pump will not stop when the TC1 contacts open. The
water cutoff is operating properly it should immediately auxiliary contacts also permit burner operation.
stop burner operation. If it doesn’t secure the burner Whenever hot water demand does not exceed the
yourself and see to it that it’s fixed. Secure the drain turn capacity of the boiler the system continuously repeats
the power off and fill the system completely venting at the operation described above. The pump and boiler
all points before restoring power. start, heat a volume of water equal to the volume of the
When stable operation is achieved the throttling storage tank between TC1 And TC2, then stop and wait
valve (TV) should be adjusted to achieve the desired until that volume of hot water is consumed.
outlet temperature as indicated by the thermometer (T2) When service water demand exceeds the capacity of
at the boiler outlet. Throttling of that valve is normally the boiler the difference between hot water demand and
required to restrict the rate of water flow through the boiler capacity is made up by hot water flowing out of the
boiler to get the desired hot water temperature. If the storage tank and cold water entering the tank. The tank
valve is open too far the flow exceeds the design flow supplies all the hot water until the level is above TC1 then
rate and boiler outlet water temperature is too low. If the the hot water from the boiler and the hot water stored in
valve is throttled too much the boiler will heat the water the tank combine to serve the hot water demand.
excessively and the burner will start short cycling on the Whenever the service water demand exceeds the
operating limit (OL). capacity of the boiler the elevation of the interface in-
Think about it, what’s a Btu? If the heater is fired creases. Provided the high demand does not continue
at a constant rate (most are) then there is a consistent until the hot water stored in the tank is consumed the
output in Btu. Since the water flow is constant (the tank boiler will continue to fire until the storage tank is once
is a detour for any water that isn’t used in the system) again filled with hot water down to the level of TC2,
the water temperature rise should be constant. completing a boiler operating cycle.
Provided the demand for hot water does not exceed Under unusual circumstances of sustained high
the capacity of the boiler, hot water will enter the tank demand for hot water the reserve in the storage tank is
faster than it flows to the building. Therefore some of the consumed. Thereafter the water leaving the system will
water heated by the boiler remains in the tank, mixing be a mix of cold water passing up through the tank and
with and displacing the cold water. Once the volume of hot water produced by the boiler. Hopefully this will
the tank above the inlet pipe from the boiler is filled with never be the case in your plant. If it frequently is, sug-
hot water an interface forms between the hot and cold gest a larger tank, larger boiler, or a combination because
water because the cold water is denser than the hot water. there’s a hazard associated with it that is not desirable.
Boiler operation continues and hot water displaces You may wonder why there are two temperature
the cold water in the tank until the level of the interface switches on the tank. Tests I performed indicate the
drops to the level of the lower tank temperature control interface in a storage tank has a temperature gradient
switch (TC2) to terminate heating operation. The open- of 5 to 10 degrees per inch depending on turbulence. A
ing of contacts on the lower tank temperature control system with a single temperature control would cycle
switch interrupts operation of the pump and burner to on and off frequently as the interface rises and falls dur-
complete a heating cycle. ing each cycle. Each time the burner starts and stops a
During the period when the circulating pump and purge is performed that, despite its purpose of safety,
burner are shut down the building is supplied by hot cools the boiler and water with purge air. Provision of
water from the tank. The weight of the check valve on two temperature controls properly spaced (more on that
the pump discharge provides sufficient differential pres- later) can significantly reduce losses and wear and tear