District Heating Calculator

This calculator is designed to assist in the selection of the pipe sizes in district heating networks. It provides a built in diversity factor for the domestic hot water demands but simply adds all of the various heating loads. As the calculator is aimed at domestic district heating networks every unit should have a domestic hot water cylinder or instantaneous heater but there is the facility to add a heating load that does not have a DHW load.A quick start sheet is provided with the programme but you need the heat losses for the buildings and the type of DHW cylinder for each unit.

Select the distribution flow temperature for the system and then for each unit adjust the heating return temperature if required, enter the trench length, and select the pipe type and bore. If your selection produces too high velocity the programme will automatically select a suitable size. After all units are entered click "All units", then in turn select each entry, enter the trench length to the next tee, and click "Join" on two lines that you wish to join together. After the last two lines are joined click "Save" to save the data as a PDF file wherever you choose. When you have joined all of the available lines the "Save" button will be enabled.

The pdf file includes an estimate of the short term peak (STP). This is an estimate of the peak half hour demand which can be used for a calculation of the minimum accumulator size.

The formulae used are listed below.

heat = flow x specificHeat x density x (flow'C - return'C); pressureDrop = f(L/D)rhoV

The friction factor f is calculated from the Reynolds Number (rhoVD/mu) and relative roughness (e/D) using Serghide's formula.

The formula for the diversity factor for the domestic hot water loads is SUM{(2^(1/n))x(2^(1/n)-1)} - 1.

Where n = the total number of units that have domestic hot water. All but the largest DHW load are averaged.

STP = htgkWwithoutDHW + totalDHWkW +(totalhtgkW - htgkWwithoutDHW)* (1 - diversityFactor/DHWcount).

A sample layout and worked calculation are provided to help to understand the procedure.

Thank you for your interest,

George Fletcher