ASHRAE Journal -November 2012 - 48

and the exhaust airflow is controlled based on a hood’s exhaust temperature (more accurately the temperature difference between hood exhaust and space temperature) a dilemma is revealed: which exhaust temperature (or hood) should be used as a control signal for DCV without dampers? The hood with the highest exhaust temperature would be the safest bet, but this would require a more sophisticated control algorithm (not the case for many DCV suppliers) and will still end up with a higher total exhaust airflow compared to DCV with dampers. In some cases, a fixed “leading” hood is assigned, and its exhaust temperature is used to control exhaust airflow for the whole system in DCV systems without dampers.

120 100 80 Exhaust Fan Speed (%) 60 40 20 0 5/13/12 Exhaust Fan VFD Speed Without Dampers 5/14 5/15 5/16 5/17 5/18 5/19 5/20 5/21 5/22 Average Airflow = 86% of Design

Figure 6: Case study without balancing dampers installed.
Estimated Savings System Heating
(Therms)

Future of DCV Systems

Cooling
(kWh)

Exhaust Fan
(kWh)

Supply Fan
(kWh)

As evidence shows, the cooking activity sensor DCV With is an important component of an efficient DCV 1,133 6,435 32,554 10,851 Dampers system. However, this is not the most effective DCV Without 623 3,539 15,697 5,232 way to identify appliance status. Taking a signal Dampers directly from the cooking appliance is a more efDifference 510 2,896 16,857 5,619 fective way to detect appliance status (cooking, idle or off). Most modern cooking appliances are Table 3: Energy savings comparison with and without balancing dampers. equipped with programmable logic controllers (PLCs) that already know appliance status. Establishing comCompared to DCV systems with cooking activity sensors, munication between the appliance and DCV controller is all systems that use only temperature sensors can have significant that is needed. lags in response time; more than two minutes in the evaluated As noted previously, cooking equipment and CKV are a cases of the open-vat fryer and griddle. Not detecting cooking kitchen’s primary energy consumers. The term “demand con- in a timely manner results in loss of C&C, which allows heat trol ventilation” implies that hood exhaust is modulated based and cooking effluent to spill to the kitchen space. Any savon demand by cooking appliances under the hood. Cooking ings associated with fan energy can quickly be offset by an appliances define overall kitchen energy consumption because increased load on cooling and heating equipment. CKV energy consumption is, to a large extent, driven by apWhen temperature only systems are used, setpoints must pliances being used, and their status defining DCV exhaust be calibrated for a given application (appliance combination). airflow. However, DCV doesn’t optimize the energy consump- This is key to ensuring the system operates as intended. Intion of the source: the cooking equipment. appropriate setpoints can result in hoods that run at design The next step in the development of an energy-efficient airflow constantly (an expensive exhaust-only hood) or idle kitchen is implementing a demand-controlled kitchen (DCK) continuously (allowing spillage to occur). The setpoints also strategy, where appliances are controlled based on cooking should be reset for winter and summer to account for variation demand and communicate their status to DCV to minimize in kitchen space temperature, unless a space temperature senCKV energy consumption. Indeed, how many times have you sor is used for automatic reset. seen a range with all burners on and no pots on it or a tripleUsing automatic balancing dampers listed per UL Standard stack conveyer oven with all stacks on and just one conveyer 710 for DCV systems with multiple hoods connected to a single being used? When we implement a DCK strategy with energy- exhaust fan significantly improves system energy efficiency. The efficient cooking appliances that are integrated with a DCV energy savings for a four-hood system can be double when comsystem (controlled based on cooking schedule and demand), pared to an identical DCV system without balancing dampers. we will have a truly energy-efficient kitchen.

Conclusions
Commercial kitchen DCV systems can offer great energy savings to the end user when properly implemented. Care should be taken to ensure the proper DCV system and sensor type are selected for a given appliance lineup.
48 ASHRAE Journal

References

1. NFPA. 2011. NFPA Standard 96-2011, Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations. National Fire Protection Association. 2. ICC. 2012. 2012 International Mechanical Code. International Code Council. ashrae.org November 2012

ASHRAE Journal -November 2012

Table of Contents for the Digital Edition of ASHRAE Journal -November 2012