1. The gas cushion in the tank absorbs the increased volume of water as the system temperature rises.

Since water is essentially incompressible, the volume increase due to heating could rupture the piping or cause the relief valve to open, causing a loss of system water.

Replacement of this lost water after the system cools off adds dissolved chemicals and oxygen which can rapidly accelerate scaling and corrosion.

Of course, the system piping also increases in volume as the system temperature goes up, so sizing methods that use the net expansion of the system, or the difference between the water expansion and the piping expansion, should be used to determine the size of the compression tank required for the system.

2. The compressible gas cushion in the tank allows for the establishment of the proper "cold fill pressure," or the pressure established at the top of the system during initial filling, to allow for venting of air, prevent boiling at maximum temperature and provide sufficient pressure at the pump suction to avoid cavitation.

3. The compression tank acts as a pressure reference for the system.

The pressure at any point depends on the height of the column of water above the gauge, the cold fill pressure, any expansion which has occurred due to heating and the remaining pump pressure head at that point.

The pressure at the point where the tank connects to the system cannot be changed by the action of the pump, so it becomes a convenient point for analyzing system pressure changes.

4. The compression tank may also act as a receiver for air separated from the system water during normal operation.

There are two categories of tanks:

  Standard tanks use special fittings to connect the tank to the system (Figure 1).

  Other tanks that keep the gas and water from contact with each other by means of an impermeable diaphragm or bladder (Figure 2).

The first tank type is connected to the air separation device in such a way as to allow air bubbles to rise by buoyancy into the tank, where they stay trapped by the tank fitting.

In systems that use bladder or diaphragm tanks, the air is simply vented from the air separator.

All closed systems need to have a properly sized tank to carry out these tasks. Advice on sizing tanks is available from Fluid Handling representatives, the ASHRAE Handbook and Bell & Gossett’s ESP-PLUS.

Reprinted from TechTalk January 1998