The program TubeCalc allows for the calculation of pressure drop in closed solar thermal collector circuits which can then serve as a basis on which the dimensioning of circulation pumps can occur. The solar thermal circuit is defined by four separate user input sections; standard tube, corrugated tube, a selection from thirteen standard piping components (return elbows, tube size expansion/reduction and one-way valves to name a few) and the simple definition of a solar absorber. Each section has a range of standard commerical products available to chose from as well as the ability to enter the part specifications manually. Furthermore, the definition of heat exchanger fluid properties of density and viscosity for water, ethylene-glycol and propylene-glykol are both temperature and concentration dependent. Based on all of the above, the systems friction and fluid flow characteristics are ascertained and displayed with an overview from each circuit section.
An additional comment concerning the definition of corrugated tube (german: Wellrohr) is required. The three parameters pitch, depth and width are illustrated in the figure below. Allowed depth is between 0 and 10mm. The pitch can account for spiraled and non-spriraled corrugations between 3 and 30mm. The width, which would normally be half the pitch (sine wave), can be modified where non-symmetrical corrugations are considered.
The newtonian flow conditions inside a tube with circular section depends on the inner diameter of the tube (di), the volume flow of the fluid (dot(v)) and its kinematic viscosity (nu). Thus we obtain the average velocity in the tube (bar(w)) and Reynolds number as such:
,
.
Using Reynolds we then find the friction coefficient (lambda) in a laminar regime with
,
and for a turbulent regime using the implicit Colebrook equation;
.
The pressure drop of a system involving friction coefficients (summed over components) is then obtained with the help of dynamic pressure:
Pressure coefficients for the following components can be added to the system according to the following parameters:
| # | Component ID | English name | German name | Figure (parameters in blue) | Description |
|---|---|---|---|---|---|
| 1 | Return elbow (180° in-plane) | Doppelkrümmer |  | ||
| 2 | Double 90° elbow (90° out-of-plane) | Raumkrümmer |  | ||
| 3 | Double 90° step (in-plane) | Etagenkrümmer |  | ||
| 4 | Elbow (variable angle) | Kreiskrümmer |  | One can make use of this component to model a coil heat exchanger (e.g. 10 coiled heat exchanger has a rotation angle of 3600°) | |
| 5 | Pipe strain-absorber (loop-form) | Lyraglatrohrbogen |  | ||
| 6 | Pipe strain-absorber (u-form) | U-Bogen |
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| 7 | Throttle valve | Drosselklappe |
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| 8 | One-way valve | Rückschlagventil |
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| 9 | Ball and socket valve | Kugelhahn |
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| 10 | Pipe size expansion | Rohrerweiterung |
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| 11 | Pipe size reduction | Rohrverengung |
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| 12 | Flow separation & recombination (45°) | Rohrverzweigung & wiedervereinigung (45°) |
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| 13 | Flow separation & recombination (90°) | Rohrverzweigung & wiedervereinigung (90°) |
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