❄️HVAC System Applications
Duct System Pressure Analysis
Static Pressure Measurements
AHU total static pressure: 2.5 inH2O
In Pascal:
In kPa:
In PSI:
Supply pressure: 1.2 inH2O
Return pressure: -1.3 inH2O
Normal operating range
Fan Selection Criteria
Office building: 10,000 CFM, 1.8 inH2O
Static pressure:
Fan power:
Motor size: 5 HP (with safety factor)
Operating cost: $0.12/kWh × 3.7 kW
Daily cost: $10.66 (24 hrs)
Annual electricity: $3,891
Refrigeration System Pressures
R-410A Operating Pressures
Summer operation: 95°F ambient
Suction pressure: 118 PSIG
In bar:
Discharge pressure: 400 PSIG
In bar:
Pressure ratio:
Normal operating range
Chilled Water System
20-story building water pressure
Building height: 200 ft
Static pressure:
In bar:
System pressure: 125 PSIG
Safety margin:
Adequate system pressure
🔧Hydraulic System Applications
Industrial Hydraulic Systems
Hydraulic Press Calculations
100-ton press: 3000 PSI system pressure
Required force:
Cylinder area:
Cylinder diameter:
In bar:
In MPa:
Use 10" cylinder at 207 bar
Mobile Equipment Hydraulics
Excavator boom cylinder sizing
Lift capacity: 15,000 lbs
System pressure: 5,000 PSI
In bar:
Cylinder area:
Cylinder bore:
Rod area: 0.6 in² (typical)
2" bore, 7/8" rod cylinder
Pressure Testing & Safety
Hydrostatic Testing
Pressure vessel test requirements
Operating pressure: 150 PSI
Design pressure: 200 PSI (safety factor)
Test pressure:
In bar:
Hold time: 30 minutes minimum
Leak rate: <1% pressure drop
Test at 1.5× design pressure
Relief Valve Sizing
System protection calculation
System pressure: 3,000 PSI
Relief setting:
Pump flow: 25 GPM
Relief capacity: 25 GPM at 3,300 PSI
In bar:
In L/min:
Set relief 10% above system
🌤️Weather & Environmental Monitoring
Atmospheric Pressure Monitoring
Barometric Pressure Trends
Weather station readings interpretation
Standard: 1013.25 mbar = 29.92 inHg
High pressure: 1030+ mbar = 30.4+ inHg
Low pressure: <995 mbar = <29.4 inHg
Hurricane: <900 mbar = <26.6 inHg
Record low: 870 mbar = 25.7 inHg
Altitude correction needed for sea level
3-4 mbar change = weather change
Altitude Pressure Effects
Mountain weather station at 1500m
Sea level: 1013.25 mbar
At 1500m:
Pressure drop:
In inHg:
Boiling point: ~90°C (instead of 100°C)
Oxygen: 84.5% of sea level
Significant altitude effects
Water System Pressures
Municipal Water Supply
City water tower height calculation
Desired pressure: 60 PSI
Height needed:
In meters:
In bar:
Tower height: 140 ft minimum
Distribution loss: ~10 PSI
150 ft tower for 50 PSI delivery
Well Pump System
Deep well pump sizing
Well depth: 200 ft
Static head:
Friction loss: 15 PSI
Tank pressure: 40 PSI
Total head:
In bar:
Pump rating: 150+ PSI
🔬Scientific & Laboratory Applications
Vacuum Systems
Laboratory Vacuum Levels
Different vacuum applications
Aspirator vacuum: ~25 Torr (3.3 kPa)
Rotary pump: 0.1 Torr (13 Pa)
Diffusion pump: 10⁻⁶ Torr (0.13 mPa)
Ion pump: 10⁻⁹ Torr (0.13 μPa)
In % vacuum:
• 25 Torr = 96.7% vacuum
• 0.1 Torr = 99.99% vacuum
Higher % = better vacuum
Vacuum Distillation
Solvent removal under reduced pressure
Atmosphere: Water boils at 100°C
At 100 mmHg: Water boils at 51°C
At 25 mmHg: Water boils at 24°C
Pressure in kPa:
• 100 mmHg = 13.3 kPa
• 25 mmHg = 3.3 kPa
Antoine equation for exact values
Lower P = lower boiling point
High Pressure Applications
Autoclave Sterilization
Medical equipment sterilization
Temperature: 121°C (250°F)
Gauge pressure: 15 PSIG
Absolute:
In bar:
In kPa:
Steam saturation curve relationship
15 PSIG at 121°C kills spores
Diamond Anvil Cell
Extreme high pressure research
Research pressure: 100 GPa
In bar:
In PSI:
Compare to Earth's core: 360 GPa
Atmosphere comparison: 1M× atmospheric
Material physics studies
Extreme pressure = new phases
❌Common Pressure Conversion Mistakes
Mistake #1: Confusing Gauge and Absolute Pressure
Wrong: Vacuum pump pulls 25 PSI (impossible!)
Right: Vacuum pump achieves 10 PSIA (4.3 PSI vacuum)
Always specify if pressure reading is gauge (PSIG) or absolute (PSIA)
Mistake #2: Wrong Mercury Density
Wrong: Using water density for mmHg conversions
Right: Mercury is 13.6× denser than water
1 inHg ≠ 1 inH2O. Mercury column is much heavier per inch.
Mistake #3: Ignoring Temperature Effects
Wrong: Pressure stays constant when temperature changes
Right: Gas pressure increases ~0.37% per °C (constant volume)
Gay-Lussac's Law: P₁/T₁ = P₂/T₂ for ideal gases
🎯Professional Pressure Measurement Tips
Measurement Best Practices
- Zero calibration: Check atmospheric reference regularly
- Temperature compensation: Account for ambient effects
- Static vs dynamic: Different for flowing systems
- Elevation correction: Adjust for altitude changes
Safety Considerations
- Pressure ratings: Never exceed component limits
- Burst pressure: Typically 4× working pressure
- Relief devices: Required for closed systems
- Hydrostatic testing: 1.5× design pressure minimum
Industry Standards
- HVAC: inH2O for low pressures, PSI for high
- Process: Bar or kPa in metric countries
- Hydraulics: PSI or bar depending on region
- Scientific: Pascal (Pa) or Torr for vacuum
Instrumentation Selection
- Range: Use middle 2/3 of gauge for accuracy
- Media compatibility: Chemical resistance important
- Process connection: NPT vs metric threads
- Environmental: Temperature and vibration limits