For a fan coil unit, static pressure and residual pressure are the same idea: 10 Pa equals about 1 m of duct.
As a rule, 5–6 Pa per metre matches real supply runs. Static pressure is measured with the fan running and the outlet closed (no dynamic pressure), and equals total pressure.
Residual pressure is what remains after the fan, coil and filter subtract their own resistance. It helps you size the connecting pipework.
For fan coil connections, duct resistance is small (under 1 Pa/m); you mainly consider the local resistance of the outlet and return.
Most makers take 1 Pa/m for a standard duct at normal speed (main 5–7 m/s, branch 4–5 m/s). For fittings they allow: return plenum 15 Pa each, supply plenum 40 Pa each, filter 10 Pa each, outlet 10 Pa each.
These are typical estimates for normal cases. Static pressure is measured with the fan running and the outlet closed.
Dynamic pressure appears once the outlet opens, from the moving air: dynamic = 0.5 x density x velocity². In practice velocity is fixed by design, so dynamic pressure is constant. The duct resistance is overcome by static pressure, which is why makers quote static pressure, not outlet residual.
One point worth stating: does cooling capacity rise or fall as static pressure Pj increases? They are linked, and so is static pressure to airflow.
1. The key factor for fan coil cooling capacity is the coil heat-transfer coefficient K: K = 1 / (1/(P·V^m·xi^n) + 1/(S·W^r)), where ^ means exponent.
Here K is the coil coefficient, V the face velocity, xi the wet-bulb factor, W the water velocity; P, m, n, r are test coefficients and exponents. P, m, n, r are known; W is fixed; the variables are V and xi.
Over a range, face velocity V affects xi nearly linearly, so we fold xi's effect on K into V.
For one model size, the static pressure Pj can be set by trading dynamic for static. Higher static pressure means lower fan speed n, and lower n means lower face velocity V.
So for a fan coil: cooling capacity Q, coil coefficient K, face velocity V and static pressure Pj relate as Q ∝ K ∝ V ∝ 1/Pj.
2. Airflow L ∝ face velocity V. The link between static pressure and airflow has two cases.
First, within one static-pressure limit, say 30 Pa, static pressure and airflow are proportional. The 30 Pa is the maximum the unit gives; airflow is the top fan speed. Lower speeds give less airflow and pressure. One motor changes speed by changing power, so here Q ∝ K ∝ V ∝ Pj.
Second, across different static-pressure limits, say 30 Pa and 50 Pa, static pressure and airflow are inversely related. At the same motor power, trading dynamic for static gives higher static pressure but lower airflow.
Total pressure = static + dynamic (static depends on air density). Dynamic = 0.5 x density x velocity². Residual = total pressure − resistance of all in-system parts. Duct pressure is read from the fan curve against the duct resistance curve; see a fluid mechanics text.
1. Static pressure is the ambient air pressure, such as 96.0 kPa or 101.325 kPa quoted in books.
2. Dynamic pressure = 0.5 x air density x velocity x velocity.
3. Total pressure = static + dynamic.
4. Residual pressure = dynamic pressure at the outlet minus the calculated dynamic pressure (the usual leave-some-margin figure).
Diffusers are tagged by neck size and velocity, so their effective area factor is 1. Louvres are tagged by face size, so use about 0.7. Diffusers can be sized by throat, so no factor is needed; area is 100%. A maker's factor is based on the outer frame. Software sizes by throat, so the effective area factor is 1. Diffuser effective area factor is usually 0.5; double louvre about 0.7; single louvre about 0.8.