Expansion Tanks

Overview

Engineering reference data for Expansion Tanks in standard organizations.

Key Formulas

ISO Standard

$ISO \; 9001:2015$

Quality management systems.

ASTM Standard

$ASTM \; E8$

Standard test methods for tension testing.

ANSI Standard

$ANSI/ASME \; B16.5$

Pipe flanges and flanged fittings.

Variables

Types of Expansion Tanks

Expansion tanks are categorized into three main types for heating, cooling, and HVAC systems:

• Open Tanks: Directly exposed to atmospheric pressure. They require location above the highest system element and can lead to corrosion due to air absorption.
• Closed Compression Tanks: Pressurized tanks that can be subdivided into:
  • Adjustable expansion tanks (air evacuation/injection by automatic valves)
  • Pump-pressured cushion tanks (water injection/evacuation)
  • Compression tanks with closed gas volumes (pre-pressurized gas cushion)
• Diaphragm Tanks: Use a flexible diaphragm to separate system water from the air cushion, reducing corrosion and maintenance.

Design Considerations

• Safety Factor (k): Typically approximately 2 to account for uncertainties.
• Atmospheric Pressure (pa): Standard value of 14.7 psia.
• Corrosion Prevention: Open tanks absorb air, increasing corrosion risk; closed or diaphragm tanks are preferred for longevity.
• Location: Open tanks must be installed at the highest point, exposing them to freezing risks in cold climates.

Example Calculation: Open Expansion Tank

For a system with 1000 gallons of water heated from 68°F to 176°F, and a safety factor of 2:

$$V_{\text{et}} = k \cdot V_w \left( \left( \frac{v_1}{v_0} \right) - 1 \right)$$

Substituting values:

$$V_{\text{et}} = 2 \times 1000 \, \text{gallons} \times \left( \left( \frac{0.01651 \, \text{ft}^3/\text{lb}}{0.01605 \, \text{ft}^3/\text{lb}} \right) - 1 \right) = 57 \, \text{gallons}$$

This yields a minimum expansion volume of 57 gallons.

References

• Original Source