Circuits is the first station of the hydronic chain. Each circuit is its own record, with its own plant, pipe networks, pumps and expansion vessel, and every station after it works from the fluid, temperatures and flows set here.
Set up a circuit
- 1
Choose the project under System, or choose New project to start one. A hydronic project opens here, and so does an HVAC project with water plant.
- 2
Give the circuit a Circuit name, then choose What the circuit carries and the Fluid. For ethylene or propylene glycol, enter the Concentration, by mass.
- 3
Enter the Supply and Return temperatures. They must differ: the difference is the circuit's ΔT, and the fluid's properties are read at their mean.
- 4
Under Terminals, choose Add a terminal for each load the circuit serves. Give each a label and either its Load or a Fixed flow from its own selection. A fixed flow wins, and the load follows from it.
- 5
Where a terminal's own pressure drop at design flow is known, enter it under Its ΔP, with the selection sheet it came from under Source. Pipe Sizing uses it to find the index circuit.
- 6
Choose Save new circuit, or Save circuit for one already saved. Anything still missing is listed under the button, and nothing is saved until it is filled in.
What Circuits works out
Each terminal's flow follows from its load and the circuit's ΔT, with the fluid's density and specific heat at the mean temperature. Terminal flows lists each terminal's load and flow and where it came from. Circuit totals gives the circuit's load, flow, ΔT and mean temperature, and Fluid properties used the density, specific heat, viscosity and freezing point behind them.
The Circuit ledger lists every circuit on the project. The one you have open shows its figures as you type, before it is saved. Its status reads Calculated when every terminal resolved, Check when the figures carry a warning, Not judged when no figures could be worked out, and Input refused when an entry itself is wrong.
Water and glycol
- Water's properties are built in, from IAPWS-IF97, and need nothing from the reference library.
- Ethylene and propylene glycol properties come from the glycol solution properties in Kelvin's reference library. Kelvin interpolates inside that table and never beyond it: a concentration or temperature outside it gives no properties, and the circuit says why.
- The concentration is by mass and is used as entered. Convert a figure in volume % before you enter it.
- If the fluid would freeze above the supply temperature, the circuit carries a warning. A mean temperature at or below the glycol's freezing point gives no properties at all.
- Where the table gives the freezing point near your concentration only as a bound, Kelvin shows it as a ceiling (≤): the true freezing point is at or below it.
- Where the table marks a concentration as one not to use, Kelvin passes that warning on with the properties.
If a glycol's table isn't in Kelvin's library, the flows wait for it. See what missing reference tables mean.
Diversity
A Diversity factor brings the circuit total below the sum of its terminals. Kelvin applies it only with its Source of the factor, so it won't save a factor without a source, or a source without a factor. Leave both blank and the total is the sum of the terminals.
Static pressure
Static pressure is the circuit's standing (fill) pressure at the pump suction. Pumps uses it only for the NPSH check: a closed circuit's pump sees friction alone, so the standing pressure adds nothing to its head. Expansion & Volume asks for the tank's own fill and maximum pressures.
When the flows don't come out
- A terminal that can't be worked out shows the reason in Terminal flows instead of a zero, and the totals then cover only the terminals that resolved. On an HVAC project the usual cause is an air system with no saved result.
- If the supply and return temperatures are the same, Kelvin says so and works nothing out until they differ.
- A terminal's pressure drop isn't saved without its source: each one cites the selection sheet it came from.
Note
Saving a circuit makes its plant, pipe networks, pumps and vessel stale, because all of them are worked out from it. The version it replaces stays in the circuit's history.