Component 205: Steam Nozzle by HVACSIM+
General Description
The steam nozzle calculates a mass flow rate and exit steam
temperature, given the downstream pressure and the upstream steam stagnation
temperature and pressure. The model assumes an adiabatic, isentropic process,
takes condensation into account, and uses real gas correlations to determine
the steam properties.
Nomenclature
A - nozzle exit cross sectional area
H - enthalpy
P - pressure
S - entropy
T - temperature
V - specific volume
w - mass flow rate
x - quality
Subscripts
e - exit
f - saturated liquid
g - saturated vapor
o - stagnation (upstream)
r - reference
sat - saturation temperature
Mathematical Description
The input pressure Po and Pe are first converted from gage pressure to
absolute pressures by adding the second parameter, Pr, which represents the
absolute pressure corresponding to zero gage pressure. The entropy, inlet
enthalpy, saturation temperature, saturation entropy, and outlet temperature
are then determined by calling the steam property function subprograms
described in chapter 3.
S = So = Se = SS(Po,To)
Ho = HS(Po,To)
Tsat = TSATS(Pe)
Sg = SSATS(Tsat)
Te = TPSS(Pe,S)
Condensation occurs if Sg is greater than So. If condensation does occur, then
the concept of quality is used to determine the exit properties.
Sf = SSATW(Tsat)
Vf = VSATW(Tsat)
Vg = VSATS(Pe,Tsat)
Hf = HSATW(Tsat)
Hg = HSATS(Tsat)
x = (S - Sf) / (Sg - Sf)
Ve = Vf + x*(Vg - Vf)
He = Hf + x*(Hg - Hf)
If condensation does not occur, then the outlet properties are determined as
follows:
Ve = VS(Pe,Te)
He = HS(Pe,Te)
The steam mass flow rate can now be calculated. The factor of 1000 in the
following equation converts the enthalpy from kJ/kg to J/kg.
w = (A/Ve)*[2*(Ho - He)*1000]^0.5
Component 205 Configuration
Inputs Description
1 To - upstream stagnation temperature (C)
2 Po - upstream stagnation pressure (kPa, gage)
3 Pe - exit pressure (kPa, gage)
Outputs Description
1 Te - exit temperature (C)
2 w - steam mass flow rate (kg/s)
Parameters Description
1 A - nozzle exit cross sectional area (m^2)
2 Pr - absolute pressure corresponding to zero gage pressure
Reference:
1. HVACSIM+ Building Systems and Equipment Simulation Program Reference
Manual (NBSIR 84-2996)
Daniel R. Clark
United States Department of Commerce
National Institute of Standards and Technology
Gaithersburg, Maryland 20899-0001