#!/usr/bin/php
<?php

   /*
    *   Heater calculation for cheap hot air (un)soldering machine
    *
    *   (C) 2006, Daniel Rozsnyo
    *             <daniel@rozsnyo.com>
    *
    */

    // input
    $fanCFM       = 5;          // CFM, cubic feet per minute
    $temp         = 270;        // deg.C
    $diameter     = 50;         // mm

    // physical constants
    $heatCapacity = 1.005;                                      // J/(g.K)
    $weight       = 1.2759;                                     // kg/m^3
    $roomTemp     = 20;                                         // deg.C
    
    // measurement unit conversion constant
    $feet         = 0.3048;                                     // meters
    
    // math constant
    $pi           = 3.14159265;
    
    // normalization
    $cubicFeetInM3= pow($feet,3);                               // m^3
    $fanM3s       = $fanCFM * $cubicFeetInM3 / 60;              // m^3/s
    $diameter     /= 1000;                                      // mm -> m
    $radius       = $diameter/2;                                // m
    
    // calculation of heater
    $fanKgs       = $fanM3s * $weight;                          // kg/s
    $degrees      = $temp   - $roomTemp;                        // K
    $power        = $fanKgs * $heatCapacity * 1000 * $degrees;  // J/s = W
    
    // calculation of speed
    $area         = $pi * pow($radius,2);                       // m^2
    $speed        = $fanM3s / $area;                            // m/s
    
    echo "Heater must have ",round($power,2)," Watt\n";
    echo "Air flow will be ",round($speed*100,2)," cm/s (",round($speed*3.6,2)," km/h)\n";
    
    /*
        The 5 CFM fan, 270 deg.C soldering temperature and 50mm diameter results in:
        
            Heater must have 756.46 Watt
            Air flow will be 120.18 cm/s (4.33 km/h)
            
    */
    
?>