Page 140 - Boiler_Operators_Handbook,_Second_Edition

Special Systems 125

a result of expansion of the building and adding boil- is any large volume of air in the system it will create
ers without changing the expansion tank, will not have differential pressures that can prevent or limit system
enough room in the expansion tank. All systems are flow (Figure 4-2) because the pump wasn’t designed to
normally fitted with a make-up pressure regulator that overcome that differential. The pump in Figure 4-2 was
admits city water to maintain a certain minimum pres- designed to pump the water around the system. Once
sure in the system and a relief valve that will drain off air accumulates in the radiator to produce a condition
water when the pressure builds. where the water drains to the boiler the pump has to
Open and simple closed expansion tanks are fitted push the water up to the radiator and frequently doesn’t
with a gauge glass so you can see the water level and have the ability to do it. Opening the vent on the radia-
know what’s going on. Bladder type tanks do not pro- tor allows the pressure in the expansion tank to push the
vide any indication of level unless special instruments water up to displace the air. Air in water systems can
are provided or you have a good ear and can get to the create all sorts of problems.
tank to tap your knuckles on it. I prefer a simple closed If there’s not enough in the system, and a makeup
tank because, in addition to knowing what’s happening regulator isn’t provided, you will have to add water to
in the system by looking at the water level, you can add the expansion tank manually in order to to restore the
a low water cutoff to any tank mounted above the boiler operating level.
for primary protection in the event of a loss of water.
The tank low level cutoff can’t work alone because
steam can be generated in the boiler to displace water
in the tank so you don’t get a low water indication at
the tank. That’s why you need a low water cutoff on the
boiler and why a low system pressure alarm switch, shut-
down if the plant isn’t attended, is a necessity as well.
Unlike steam plants the fluid in a hydronic heat-
ing system doesn’t move around on its own. You’ll find
I swap the words water and fluid around when talking
of hydronic systems. That’s because many of them use a
glycol mixture, not just plain water. The glycol changes
the boiling point of the fluid so you need another set of
tables besides the steam tables but they otherwise work
the same.
Steam will readily flow from one point to another
Figure 4-2. Differential produced by air in hydronic
with a very little difference in pressure. A hydronic heat-
system
ing system is full of water with the only pressure varia-
tion being the elevation at a particular point. There may One neat thing about hydronic systems is they’re
be a little thermosyphoning going on where lighter hot easy to measure. Given the definition of a Btu all you
water is lifted up as heavier cold water drops down to need to know is the temperature in, temperature out,
displace it but it’s never enough for heating any reason- and the flow rate to know how many Btu’s a boiler is
ably sized system. You might find what we call a gravity putting out or how much a particular piece of equip-
system in a house where the pipes are large enough to ment is using. That’s true at any instant anyway. It’s
allow the liquid to move around but I doubt if you’ll another story when you want to get average or total
see it elsewhere. So, for most installations there’s no readings.
pressure differential to force the heated water out of the The flow rate has to be close to the rating of the
boiler and to the load. circulator There are pressure drop curves (Figure 4-3) in
That’s why every hydronic heating system has the instruction manuals for most equipment so you can
circulators. Circulators are pumps that push the water read the pressure drop through a coil and read the flow
around the hydronic heating system. They’re not sized off the curve. I prefer a differential gauge but using the
to fill the system, nor capable of pushing the water up same gauge on both connections will give you a fairly
to the highest level in a system. They are selected to accurate differential; just reading both installed gauges
overcome the resistance to flow through the system at assumes they’re identically calibrated and they almost
the designed flow rate and that’s all they do. If there never are. Two weeks ago I saw two gages on the same