DOE-2 Reference Manual Version 2.1 - DOE2.com

2. CURVE-FITThe CURVE-FIT instruction exists both in SDL and PDL. It is describedhere because the first opportunity to use the CURVE-FIT instruction occurshere in the SYSTEMS program.Often it is necessary to specify input data to the program in the form ofa curve, or equation. The term "curve" in this connotation can be a line, aplane, a curve, or a curved surface. The program has built into it certainequipment performance curves, which are stored in the form of equations. Theseare the default performance curves (or equations). Often however, a user willselect, for simulation, a piece of equipment with a performance curve thatvaries from, or does not fit, the default performance curve. It therefore isnecessary to input data to override the default curve and establish a newcurve to suit the user's chosen equipment. This is one of the purposes of theCURVE-FIT command. Another is to provide a method to input data where defaultcurves do not already exist. If the user is satisfied with the default valuesin the program (or is uncertain about the default values at this time) he mayskip over this section and go on to ZONE-AIR. The user should know that thiscapabl lity does eXlst ln the program and that he may return to this section atany future time should it become necessary.The object is to establish new equations in the program. Since mostvendor-supplied equipment information is provided in the form of curves ortabulated data, not equations, the user must assist the program in determiningthe new equation from the available curves and tabulated data. Even if theequation is known (and it is different than the default equation, should oneexist) the user must specify the new equation using the CURVE-FIT instruction.CURVE-FIT is used to specify data points or coefficients so that the programcan determine and use the equation so defined. The particular equationwill then be associated by keyword to a particular packaged unit, piece ofequipment, etc., by the user. Again, this instruction will be used only whena user finds that curve equation coefficient default values in the program areinappropriate for his use.There are two methods of input reflecting the two ways of defining acurve, given the form of the curve (linear, bi-linear, quadratic, bi-quadratic,or cubic). These are (1) by giving a list of up to 20 sets of coordinates ofpoints on the curve, or (2) by giving the coefficients of the equation. Forexample, the following simple linear curve may be defined(1) as linear, with coordinate points of the form (x,z): (0,3)and (10,8); or(2) as linear, with coefficients of the form z = a + bx: a = 3 andb = 0.5, where z = f(x).IV.180 (Revised 5/81)