Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
FaultModel:Fouling:Chiller reduces the nominal capacity of the chiller. Referring to the Engineering Reference, the fouling factor is applied to the chiller capacity and the chiller COP. If the chiller has adequate capacity for the current load, the "Chiller Evaporator Cooling Rate" will be identical with or without the fouling factor. If the chiller is delivering the same load, but at a reduced COP, then the electric power consumption will be higher and the heat rejection to the condenser will be higher.
The fouling factor is only applied whenever the fault model Availability Schedule is > 0. This much works as stated in the Input Output Reference.
Where it gets tricky (and the Input Output Reference is wrong) is that the Severity Schedule is applied as a divisor to the nominal Fouling Factor, not as a multiplier. Let's assume the Fouling Factor is 0.8 with a Severity Schedule that varies from 1.0 to 1.2. The 1.2 value implies 20% more fouling, so the fouling factor will be 0.8/1.2 = 0.67. A Severity Schedule value of 0.8 would imply 20% less fouling, or 0.8/0.8 = 1.0 = no fouling. As long as you know how the number will be used, then you can decide what to put in the schedule.
Also, if the Severity Schedule is zero, then the nominal Fouling Factor is used. And if the resulting computed fouling factor is >1.0, then 1.0 is used (no fouling).
I've added this to issue #6717 to correct the Input Output Reference description for Severity Schedule which is simply wrong here and in the other fouling fault models.
