Pressure holding pumps are be used to maintain a constant pressure in a closed heating or air-conditioning system.
A pump-based pressure holding system is comprised of one or more pumps connected in series. The pumps can be fitted with a speed regulator, e.g. frequency converter, to maintain correct pressure within the system. In this case a pressure expansion tank is not necessary because the pumps themselves are able to regulate the pressure.
Pressure holding systems using speed-regulated pumps are superior to systems that employ expansion vessels. In the latter case, a constant-speed pump that maintains a constant flow can give rise to pressure surges when the pump starts and stops. Furthermore, a pressure holding system requires less physical space since there is no expansion vessel.
A pressure expansion tank combined with a pressure holding pump can be used to remove air from process water. A small volume of water is filled into the container, which is pressureless. When the pressure falls to atmospheric pressure, most of the air is released. A similar volume of water is pumped back into the system by a pressure holding pump to maintain the desired pressure in the system.