VART and TART
VART and TART are methods of measuring the average return temperatures in a district heating system based on volume and temperature.
VART - Volumetric Average Return Temperature
The return temperature is averaged over the volume of water and is used to measure impact on plant efficiency.
TART - Time Average Return Temperature
The return temperature is averaged by time and is used to measure heat loss from pipework. Heat loss from pipework is proportional to the temperature of the pipework and the flow within the pipework is irrelevant.
The return water sits at 50C for 23 hours in a day with a very low flow rate of 1 cubic metre per hour, and rises to 70C for one hour at a flow rate of 100 cubic metres per hour. Exaggerated figures used to demonstrate the principle.
The heat loss from the pipework will 23/24 times the heat loss at 50C, plus 1/24 times the losses at 70C, based on time. The TART is therefore 50.8C
The effect on plant however will be 24 cubic metres at 50C returning, plus 100 cubic metres at 70C. The one hour where flow rates are higher will have more of an impact on plant as volume of water is important - this is what needs to be reheated. The VART works out to 66.1C