A qualitative decision support system was developed for reasoning on cold box separation process. The qualitative effects from the manipulation of variables were identified from numerical simulation data. The qualitative relations were expressed as binary or ternary qualitative relations by using sign algebra. Two approaches to qualitative modelling were compared. The direct method is more straightforward and results to a compact model. The linear synthesis method is systematic but difficult to apply except in simple cases. The recommended procedure is the combination of these methods.