온수난방 시스템설계

Overview

Hot water heating systems circulate heated water through pipes to radiators, convectors, or radiant panels to provide space heating. They are widely used in residential and commercial buildings.

System types:

• Two-pipe system — separate supply and return pipes
• One-pipe system — common supply/return loop (series connection)
• Reverse return — equalized flow resistance across all terminals

Heat Output Calculation

The heat carried by hot water:

$Q = \dot{m} \cdot c_p \cdot \Delta T$

where:

• $Q$ = heat output (kW)
• $\dot{m}$ = mass flow rate (kg/s)
• $c_p$ = specific heat of water ≈ 4.18 kJ/(kg·K)
• $\Delta T$ = temperature difference between supply and return (K)

In terms of volume flow rate:

$Q = \dot{V} \cdot \rho \cdot c_p \cdot \Delta T$

where $\dot{V}$ = volume flow rate (m³/s) and $\rho$ ≈ 983 kg/m³ (at 60°C).

Flow Rate for Required Heat Output

$\dot{V} = \frac{Q}{\rho \cdot c_p \cdot \Delta T}$

For a Δ$T$ of 20°C (80/60°C system):

Pipe Sizing

Pipes are sized based on:

• Flow velocity — typically 0.5–1.5 m/s for quiet operation
• Pressure drop — typically 100–400 Pa/m for small systems

Recommended Pipe Velocities

Application	Velocity (m/s)
Residential branches	0.5 – 0.8
Residential mains	0.8 – 1.2
Commercial branches	0.8 – 1.2
Commercial mains	1.0 – 1.5
High-rise risers	1.5 – 2.5

Pipe Pressure Drop

The Darcy-Weisbach equation for pressure drop in a pipe:

$\Delta p = f \cdot \frac{L}{D} \cdot \frac{\rho v^2}{2}$

where:

• $f$ = friction factor (dimensionless)
• $L$ = pipe length (m)
• $D$ = pipe inner diameter (m)
• $v$ = flow velocity (m/s)

Expansion Vessel Sizing

Water expands when heated. An expansion vessel absorbs this volume change:

$V_{vessel} = \frac{e \cdot V_{system}}{1 - \frac{p_0}{p_f}}$

where:

• $e$ = expansion coefficient of water over the temperature range
• $V_{system}$ = total system water volume (L)
• $p_0$ = pre-charge pressure (bar absolute)
• $p_f$ = safety valve set pressure (bar absolute)

Design Temperatures

System Type	Supply (°C)	Return (°C)	ΔT (°C)
Conventional radiator	80	60	20
Low-temperature radiator	55	45	10
Underfloor heating	35 – 45	25 – 35	10
Fan coil units	45 – 60	35 – 50	10

Applications

• Residential central heating
• Commercial building HVAC
• District heating networks
• Industrial process heating
• Snow melting systems