Using a PLC, I will make summer/winter changeover in an HVAC system. I have incoming and outcoming temperature (celcius) and RH (%)numbers. How can I make this switchover (=What is the formula of enthalpy)? What hystheresis (%) do you use to switch from one state to the other?

Thanks a lot

 

Here's a short BASIC program for calculation of enthalpy from dry bulb temperature and releative humidity.

10 REM ENTHALPY CALCULATION
20 REM Assumes standard atmospheric pressure (14.7 psi), line 110
30 REM Dry-bulb temperature in degrees F (TEMP)
40 REM Relative humidity in percentage (RH)
50 REM Enthalpy in BTU/LB OF DRY AIR (H)
60 T = TEMP + 459.67
70 N = LN( T ) : REM Natural Logrithm
80 L = -10440.4 / T - 11.29465 - 0.02702235 * T + 1.289036E-005 * T ^ 2 - 2.478068E-009 * T ^ 3 + 6.545967 * N
90 S = LN-1( L ) : REM Inverse Natural Logrithm
100 P = RH / 100 * S
110 W = 0.62198 * P / ( 14.7 - P )
120 H = 0.24 * TEMP + W * ( 1061 + 0.444 * TEMP )

Hope this helps.

Zvi
[email protected]

 

Automatic season changes can be a real source of problems especially if you live in a area where conditions can change on a daily basis from winter to summer and back again if you believe the formulas. Also you equipment has to be ready and able to make the change ( cooling towers have to be clean and chillers ready to run etc. ) I suggest you contact a professional in the HVAC business in your area for advice and implementation.

James Bouchard

 

Hi. My name is Cheah.

I saw your calculation on the enthalpy. how can i convert it to kj/kg from btu/lb? Pls advise. I multiply it by 2.33 but didn't correspond to psy. chart.

Can you help.

Rgds, Cheah