Deep Thoughts by Deepinder Singh, CEO at 75F
When it comes to restaurants, you have an RTU, a makeup air unit and exhaust hoods. The amount of air that gets pulled into the RTU through its outside air damper increases the air inside the restaurant. With outside air coming in and exhaust hoods expelling air out, the building pressure is dynamic and always shifting. So as pressure within the restaurant grows, there’s a hinged relief flap built into the RTU that enables excess air (positive pressure) to flow back outside. This exists to help balance the building’s pressure and prevent doors from flying open. The problem with these flaps is they’re not very accurate, they tend to stick and generally don’t work.
Economizers have no concept of what’s going on in the building. They only turn on when thethermostat calls for cooling. Let’s say the desired indoor temperature is 72°F and the actual temperature is 73°F. In a normal scenario, your thermostat requests cooling and that signal travels to the RTU, which holds the economizer. It would interpret the signal coming in from the thermostat as a request for cooling, even if the outside air is 60°F and ideal for free cooling.
Based on the enthalpy, the system would calculate that it would be less expensive to bring in free cooling, so the damper opens 100%. It would open a full 100% because typical economizers have no concept of how much cooling is required, they are simply fully open or fully closed as long as the mixed air temperature is above a minimum (typically 55°F). The economizer is now fully open and there’s a whole lot of outside air flowing in. So let’s hope that hinged flap functions properly, or you’ll be left with excess positive pressure, forcing your doors to swing open. Even when the flap does operate, there’s always going to be a fair amount of excess pressure built in, which is the same amount of pressure it takes to open that flap. Then as soon as the indoor temperature goes back to 72°F, the economizer will turn off. As described, typical economizers are 100% open, then 100% closed, all the time. So you end up having a huge amount of pressure in the building and then a sudden drop. This is how they work especially with mixed air temperatures of 55°F temperatures and above. If the mixed air temperature is below 55°F, some of them will modulate so that the air slowly comes to be 55°F. This would typically happen only when the outside air is around 35°F. However, if the outside temperature drops below 35°F, there’s a good chance you don’t need the free cooling anyway. Buildings mostly use free cooling when the outside air is 55-70°F.
In the case of 75F, we understand the building loads and we mix the indoor and outdoor air together to exactly satisfy the load, resulting in a space that is more consistent and comfortable. Instead of sayinglet’s fully open the damper and then fully close it, 75F takes the overall average open percentage and simply opens the damper that percentage. For example, if an economizer is needed to be on for 10 minutes and then off for 15 minutes, that would mean it would be in use 40% of the time. So 75F opens the damper 40% for the full 25 minutes, alleviating the yoyo pressure scenario we ran into earlier. Now, a lot of outside air isn’t rushing in all at once. This leaves you with a system that is more stable.
Restaurants in particular, must be mindful of pressure as their exhaust hoods expel a lot of gas, requiring new air to come in. That air has traditionally been replaced by a makeup air unit, which is a dedicated outdoor air system (DOAS). The problem is that these units are quite expensive, the installation is rather costly and they run 24/7. You’ve now got an RTU which is running for regular operations and a makeup air unit which runs solely for the purpose of tying back to the exhaust hoods. Finally, people said why not use an RTU as a makeup air unit and just keep its outside air damper open about 50% to compensate for the exhausted air? There is a problem with this theory however. Having the hood on constantly is not economical, because first thing in the morning, it’s turned on at one fixed speed and then it expels a set amount of CFM all the time. The fryers are not yet turned on, cooling is unnecessary as there are no guests and now you’re wasting a lot of energy. So restaurant owners started installing temperature sensors and infrared smoke sensors in exhaust hoods.
These sensors assess the exhausted air temperature to determine the size of the actual cooking load. As the cooking load increases, theexhaust speed ramps up to increase the amount of air being pushed out. The unfortunate part is that the RTU is never informed. So as this cooking load increases, the RTU has a fixed percentage opening and you end up going back to the same scenario of having negative pressure.
IS THERE A WAY TO CORRECT RESTAURANT PRESSURE IMBALANCE?
To correct this issue of excess pressure, 75F monitors indoor and outdoor pressure before opening the outside air damper the perfect percentage, to ensure pressure is forever maintained. This allows the hood to runindependently. Even more importantly, the facility manager no longer needs to arbitrarily set the outside air damper to minimum value based on expected exhaust loads. When it comes to the cool air brought in by the 75F economizer, we can limit it based on pressure if needed. However, limiting free cooling would be extremely rare since we generally don’t need to open the outside damper fully (see previous note on our modulation). This prevents positive pressure, which would result in the doors blowing open. In a restaurant, the pressurization piece is used both for positive and negative pressure.
IoT for verification. One of the most difficultthings with standard economizers is verifying the operation of the units. Economizers are notorious for not being maintained and having dampers that stick, sensors that go bad or actuator motors that fail. One of the really cool things that 75F does, is it uses its sensors to actually verify that the economizing action is happening. This can be done by looking at the outside air temperature, the indoor air temperature and the mixed air temperature (the temperature of the inside air once it has been mixed with the cool air from outside). Not only do we use this to make sure that the economizer is working, we can actually calculate just how much energy we save, in real time!
Lastly, enthalpy sensors go bad quite frequently. This occurs especially when they are exposed to outside air (especially hot, humid air), which they need to be to function. So 75F uses live weather feeds from reliable sources such as meteorological stations and airports instead. This IoT innovation eliminates one of the most common points of failure to the traditional economizers operation and allows the 75F solution to perform more reliably, delivering essential energy savings and superior IAQ.
In summary, for an economizer to be effective, it’s got to factor in a building’s temperature, humidity, pressure, air quality, thermal envelope and forecast weather. Today 75F has the mostadvanced outside and inside air optimization system in the world. IoT and cloud computing allows us to track these factors and use them for making proactive, smart decisions that heighten comfort and productivity. All the while, this smart solution lowers energy consumption and our carbon footprint.