Question 24 CEL01 - Chief Engineer - Limited

The Correct Answer is C ### Explanation for Option C (Correct) In a typical single-zone HVAC system incorporating an economizer function (which the presence of outside air, return/recirculation, and exhaust dampers implies), the system must maintain a constant supply air volume while satisfying two constraints: 1) the total flow entering the mixing box (Outside Air + Recirculation Air) must equal the constant supply flow, and 2) the exhaust flow must equal the outside air flow required to meet minimum ventilation standards or, during economizer operation, equal the amount of return air being vented. During operation, the recirculation (return) air damper, the outside air (OA) damper, and the exhaust (EA) damper are interconnected, often using a single linkage or control strategy, typically involving a parallel relationship between the OA and EA dampers, and an inverse relationship with the recirculation damper (RA). The mixing box equation is: $\text{Supply Air} = \text{Outside Air} + \text{Recirculation Air}$ (Constant total volume). If the system increases the amount of outside air taken in (OA damper opens), it must simultaneously increase the amount of air being exhausted (EA damper opens) to maintain building pressure (Exhaust = Outside Air, assuming minimum ventilation rates are met or exceeded). Since the total supply volume is constant, increasing the OA volume necessitates a corresponding decrease in the recirculation volume. Therefore, **the more the outside air (OA) and exhaust (EA) dampers open (e.g., during full economizer cooling or minimum ventilation), the more the recirculation (RA) damper must close**, and vice versa (when cooling/heating loads require maximum recirculation, OA and EA close, and RA opens). This relationship, $\text{OA} \uparrow \text{ and } \text{EA} \uparrow \implies \text{RA} \downarrow$, confirms that Option C is the correct description of the damper relationship. ### Explanation for Incorrect Options **A) The more the exhaust and recirculation dampers are open, the more the outside air damper is closed, and vice versa.** This is incorrect because the exhaust damper position is directly linked to the outside air damper position (during economizer operation, $\text{Exhaust} \approx \text{Outside Air}$). They move together, not inversely. Opening the recirculation damper necessitates *closing* the OA and EA dampers, not opening the EA damper. **B) The exhaust, outside air, and recirculation dampers are all open or closed to the same degree for all operating conditions.** This is incorrect. The outside air (OA) and exhaust (EA) dampers move together, but the recirculation (RA) damper moves inversely. For example, when the system is operating on 100% outside air, OA and EA are 100% open, but the RA damper is 0% open. **D) The more the outside air and recirculation dampers are open, the more the exhaust damper is closed, and vice versa.** This is incorrect. Outside air (OA) and recirculation (RA) dampers cannot be fully open simultaneously if the supply volume is constant (as $100\% \text{OA} + 100\% \text{RA} = 200\% \text{Supply}$). Furthermore, when the OA damper is open (e.g., $50\%$), the exhaust damper (EA) must also be open (e.g., $50\%$), not closed. The OA and EA dampers operate in tandem.