Static Pressures in HVAC Systems
Static pressures in HVAC system avoids the tops of the systems drying out
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The static pressure of most HVAC systems consists of the height of the building plus a cushion at the top of the building. In general the cushion is around 5 - 10 psi (lb/in2), 10 - 20 ft (3 - 6 m) water column.
Static Pressure as Head
The static pressure can be expressed as head:
hs = hb + hc (1)
where
hs = static head (ft, m)
hb = elevation at the top of the building (ft, m)
hc = cushion required at the top of the tallest building (ft, m)
Static Pressure as PSI
It is often common to express static pressure in psi:
ps = hb + hc / (144 / γ) (2)
where
ps = static pressure (psig)
γ = specific weight of water at the operating temperature (lb/ft3)
Example - Static Pressure in a Water Heating System
The static pressure of a 190 ft tall building and cushion head 10 ft can be expressed as head:
hs = (200 ft) + (20 ft)
= 220 ft
If the water operating temperature is 60oF with specific weight of 62.4 lb/ft3 - the pressure in psig can be expressed as:
ps = (200 ft) + (20 ft) / (144 / (62.4 lb/ft3) )
= 95.3 psig
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Related Topics
- Heating - Heating systems - capacity and design of boilers, pipelines, heat exchangers, expansion systems and more
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