Balancing of a Water and Air System (PDF

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System balance becomes more important in terminals with a large temperature difference.
Less water flow is required, which reduces the size of pipes, valves, and pumps, as well as
pumping costs. A more linear emission curve gives better system control. If flow varies by
more than 5% at design flow conditions, heat transfer can fall off rapidly, ultimately causing
poorer control of the wet-bulb temperature and potentially decreasing system air quality.
Heat Transfer at Excessive Flow
Increasing the flow rate above design in an effort to increase heat transfer requires careful
consideration. Figure 3 shows that increasing the flow to 200% of design only increases heat
transfer by 6% but increases resistance or pressure drop four times and power by the cube of
the original power (pump laws) for a lower design ∆t. In coils with larger waterside design ∆t,
heat transfer can increase.
Hydronic Pressure Measuring Instruments
Manometer U-tube
The U-tube manometer has primarily been used for measuring pressure drops across terminal
heat exchangers and flow meters. There is no calibration required and other instruments can
be verified against the manometer.
Digital Manometer for Water Measurement
There are several manufacturers of digital manometers and each has specific functions it can
perform. The standard requirements of the digital manometer is it can read in English units or
metric, has a broad range of pressures, can display in pounds per square inch (psi), foot head
(R), inches of water column (in. wc), and kilo Pascal’s (kPa). Each manometer should have
auto zeroing capabilities.
The manufacturer’s pressure rating and calibration instructions should be followed. When
used to measure pressures, the calibration shall be verified by using a U-tube manometer or a
calibrated pressure gauge. The calibration verification can also be accomplished by using a
dead weight kit.
The accuracy of the gauge reading on differential pressure shall be ±2% of reading ±1
digit.
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