Water-based fluid system design: pipe sizing (Darcy-Weisbach, Hazen-Williams), flow rates, pressure drops, CDU selection for DTC cooling, pump curves, and ASHRAE TC 9.9 water class enforcement. Activates for cooling loop design, plumbing calculations, CDU selection, pump sizing, and hydraulic system validation.
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Water-based fluid system design: pipe sizing (Darcy-Weisbach, Hazen-Williams), flow rates, pressure drops, CDU selection for DTC cooling, pump curves, and ASHRAE TC 9.9 water class enforcement. Activates for cooling loop design, plumbing calculations, CDU selection, pump sizing, and hydraulic system validation.
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Design and validate water-based fluid systems from household plumbing to high-density data center cooling loops.
Activation: InfrastructureRequest type='cooling' or type='plumbing', any CDU sizing request, pipe sizing question, flow rate calculation, or pressure drop analysis.
Key capabilities:
Pipe sizing via Darcy-Weisbach (all fluids) and Hazen-Williams (water only)
Flow rate calculation from heat load (Q = Q_heat / (rho x Cp x DT))
Pressure drop analysis with fitting equivalent lengths
ASHRAE TC 9.9 water class selection (W1-W5) for data center cooling
CDU/DTC cooling selection and manifold topology
Pump curve analysis with NPSH verification and affinity laws
Safety boundary enforcement by safety class (residential through industrial)
ENGINEERING DISCLAIMER: All calculations must be verified by a licensed Professional Engineer before use in construction or installation. Local plumbing codes (IPC/UPC) and pressure vessel standards (ASME B31.9) impose requirements not captured here. User assumes all responsibility for verification.
Quick routing: Pipe sizing method -- see Pipe Sizing. ASHRAE water class selection -- see Data Center Cooling. Pump selection and NPSH -- see Pump Selection. Full derivations -- @references/pipe-sizing.md.
Pipe Sizing
Darcy-Weisbach (All Fluids)
The general pressure drop equation for incompressible flow in pipes:
DeltaP = f x (L/D) x (rho x v^2 / 2)
Variable
Definition
Units
DeltaP
Pressure drop
Pa
f
Darcy friction factor
dimensionless
L
Pipe length
m
D
Internal pipe diameter
m
rho
Fluid density
kg/m^3
v
Flow velocity
m/s
Friction factor f: Determined from the Moody diagram or computed via the Colebrook-White equation for turbulent flow:
This is implicit in f and requires iteration (3-4 iterations from f=0.02 starting point converge). The Swamee-Jain explicit approximation avoids iteration -- see @references/pipe-sizing.md.
Reynolds number: Re = rho x v x D / mu
Regime
Re Range
Friction Factor
Laminar
< 2,300
f = 64/Re
Transition
2,300 - 4,000
Avoid -- unstable
Turbulent
> 4,000
Colebrook equation
Pipe roughness epsilon:
Material
epsilon (m)
Common Use
Copper
0.0000015
Plumbing, chilled water
PVC
0.0000015
Drain, low-pressure supply
Galvanized steel
0.00015
Older installations
Cast iron
0.00026
Municipal, fire protection
Stainless steel
0.000015
Process, high-purity
For full Colebrook equation derivation and Moody chart interpretation -- @references/pipe-sizing.md
Hazen-Williams (Water Only)
Simpler empirical formula valid only for water at normal temperatures in turbulent flow:
v = 0.849 x C x R^0.63 x S^0.54
Variable
Definition
Units
v
Velocity
m/s
C
Hazen-Williams coefficient
dimensionless
R
Hydraulic radius (D/4 for full circular pipes)
m
S
Slope of hydraulic grade line (DeltaP / (gamma x L))
dimensionless
C coefficients:
Material
C Value
Copper
150
PEX
150
PVC
150
New steel
145
Cast iron
130
Old steel (20+ years)
100
When to use: Water systems only, turbulent flow, not valid for viscous fluids or non-circular pipe cross-sections. Simpler than Darcy-Weisbach but less accurate; acceptable for plumbing design and preliminary sizing.
Velocity Limits
Application
Min (ft/s)
Max (ft/s)
Rationale
Main distribution lines
4
6
Balance pressure drop vs noise
Branch lines
2
4
Noise reduction near occupants
Data center cooling
3
8
Higher acceptable in enclosed space
Suction piping
1
3
NPSH protection
Velocities below 2 ft/s risk sedimentation; above 8 ft/s risk erosion, noise, and water hammer.
Standard Pipe Size Selection
Process: Calculate minimum internal diameter from velocity limits at design flow rate, then select the next larger NPS (Nominal Pipe Size).
A = Q / v = 0.000958 / 1.83 = 0.000524 m^2
D = sqrt(4A / pi) = sqrt(4 x 0.000524 / 3.14159) = 0.0258 m = 1.02 in
Select next larger NPS: 1-1/4" (Schedule 40, ID = 1.38 in). Actual velocity: 3.3 ft/s -- within acceptable range.
From Fixture Units (Plumbing)
Per UPC/IPC Table 610.3 (demand conversion):
Fixture
Fixture Units (FU)
Water closet (flush valve)
10
Water closet (tank)
4
Lavatory (private)
1
Lavatory (public)
2
Kitchen sink
2
Shower
2
Bathtub
4
Dishwasher
2
Washing machine
4
Demand conversion: Total fixture units mapped to design flow rate via UPC demand curve (not linear -- accounts for diversity). Approximately: 1 FU corresponds to 1 GPM demand basis at the riser.
Pressure Drop Analysis
Equivalent Length Method (Fittings)
Convert each fitting to an equivalent length of straight pipe, then sum with actual pipe length for total pressure drop calculation.
Fitting
Equivalent Length (pipe diameters)
90 degree standard elbow
30D
90 degree long-radius elbow
16D
45 degree elbow
16D
Tee (flow through branch)
60D
Tee (straight through)
20D
Gate valve (fully open)
8D
Globe valve (fully open)
340D
Check valve (swing)
50D
Ball valve (fully open)
3D
Strainer/filter
50-100D (check vendor data)
System Curve
Total system pressure drop is the sum of all resistances:
Pipe losses: Darcy-Weisbach or Hazen-Williams applied to total equivalent length
Equipment losses: CDU, chiller, heat exchanger -- use vendor pressure drop curves at design flow
Elevation term: DeltaP_elev = rho x g x DeltaH (Pa), where DeltaH = height change in meters
Design margin: Size system to operate at 75% of available pressure differential to accommodate future expansion
Water Hammer Check
Surge pressure from sudden valve closure:
DeltaP_hammer = rho x c x DeltaV
Where c is approximately 1,400 m/s for water in rigid pipe. Keep surge pressure below the safety class limit (see Safety Boundaries).
Mitigation: slow-closing valves (5+ second stroke), surge tanks, relief valves, VFD-controlled pumps with soft ramp-down.
Data Center Cooling
ASHRAE TC 9.9 Water Classes
Class
Supply Temp Range
Typical Use Case
Economizer Hours/Year
W1
2-17C
Legacy data centers, high-density GPU
<500 (most climates)
W2
2-27C
Mixed air + liquid cooling
~500-2,000
W3
2-33C
Modern liquid-cooled, moderate climates
~3,000-5,000
W4
2-45C
Warm water cooling, high economizer use
~6,000+
W5
>45C
Hot water, heat recovery to district heating
~8,000+
Class selection: Higher W class enables more free-cooling hours and lower PUE. Verify server inlet air temperature remains within ASHRAE A-class (15-35C) for mixed air+liquid environments.
Full ASHRAE TC 9.9 water class specifications -- @references/ashrae-tc9-9.md
CDU Selection for DTC Cooling
The Coolant Distribution Unit (CDU) is the heat exchanger that isolates facility water from the server-level coolant loop.
Sizing: CDU rated kW >= rack peak heat load x 1.25 (design margin)
Direct-to-chip (DTC) parameters:
Parameter
Typical Range
Notes
Flow per cold plate
0.5-2.0 LPM
Verify with server/GPU vendor spec
Server-side pressure
50-150 kPa
CDU provides pressure isolation
Cold plate inlet temp
25-45C (W3/W4 class)
Depends on water class
Manifold topology
Per-rack supply/return headers
Quick-disconnect fittings
Leak detection requirements:
Zone
Sensor Location
Response
1 - Server tray
Overflow tray in each tray
Local alert
2 - CDU drain pan
Under each CDU assembly
Auto-shutoff CDU supply
3 - Raised floor
Floor-mounted leak cable/sensor
Room alarm + facility shutoff
4 - Building BMS
Aggregated from all zones
Facility-wide response
Secondary containment: Drip pan beneath each rack assembly minimum for data center class. Guttered containment for high-density installations.
Full CDU selection criteria -- @references/ashrae-tc9-9.md
Pump Selection
Operating Point
The operating point is the intersection of the system curve (parabolic: DeltaP = k x Q^2) and the pump H-Q curve (from vendor data).
Plot both curves on the same axes (flow rate vs head/pressure)
Operating point must fall within the pump's stable operating region
Avoid operating on drooping portions of the pump curve
Cavitation consequences: Noise, vibration, impeller erosion, loss of flow capacity, eventual pump failure.
Pump Affinity Laws (VFD Sizing)
Parameter
Scaling Law
Example (50% speed)
Flow Q
Q2 = Q1 x (N2/N1)
50% flow
Head H
H2 = H1 x (N2/N1)^2
25% head
Power P
P2 = P1 x (N2/N1)^3
12.5% power
VFD benefit: Reducing flow to 50% by reducing pump speed saves 87.5% of pump power. This is the primary justification for VFD on variable-flow cooling loops.
Redundancy Configurations
Configuration
Description
When to Use
Duty/Standby
Two pumps each at 100% capacity; one runs, one standby
Critical single-loop systems
N+1
N duty pumps + 1 standby with lead-lag rotation
Multi-pump parallel systems
2N
Fully redundant parallel systems
Highest criticality (Tier IV)
Switchover: Flow switch or differential pressure switch triggers automatic switchover within seconds. Lead-lag rotation distributes wear.
Full pump curve analysis and VFD selection -- @references/pump-selection.md
Safety Boundaries
Pressure and Temperature Limits by Safety Class
Parameter
Residential
Commercial
Data Center
Industrial
Max working pressure
80 PSI
150 PSI
150 PSI
300+ PSI (PE required)
Max supply temperature
60C
82C
55C (server side)
150C+ (PE required)
Leak containment
Drip pan
Sensor + shutoff
Sensor + shutoff + alarm
Engineered containment
Pressure testing
Visual inspection
Hydrostatic 1.5x MAWP
Hydrostatic 1.5x + leak-down
Per ASME B31 code section
Water hammer limit
Not calculated
25 PSI spike max
10 PSI spike max
Per surge analysis
Safety Warden Integration
This skill produces CalculationRecord objects with safety margin calculations. The Safety Warden reviews these and generates SafetyFinding entries:
Condition
Severity
Domain
Action
Calculated pressure > safety class max
blocking
pressure
Requires human review (PE)
Temperature > safety class max
critical
temperature
Flag for PE review
NPSH_available < NPSH_required
critical
pressure
Cavitation risk -- redesign suction
Missing leak detection (data-center class)
warning
containment
Recommend sensor installation
Industrial and data-center class findings with severity='critical' or 'blocking' always require PE review before construction.
