Repurposing the Energy from Hazardous Exhaust
In general, all of these fumes are exhausted (and sometimes filtered) into the atmosphere. As such, none of the air is recirculated in the general HVAC system, and the system needs to bring in fresh air from outside to make up for the imbalance. Conditioning un-tempered air can be very costly though, so HVAC design engineers often look at potential energy saving measures.
In my first post on energy recovery I note that the most common tools for recovering lost energy are energy wheels and heat plates. Hazardous exhaust, however, forces engineers to completely isolate the exhaust and supply air streams, rendering energy wheels and heat plates unsuitable.
When dealing with hazardous exhaust, engineers will often refer to a form of energy recovery known as runaround loops. In essence, these loops place a heat exchanger coil in the isolated exhaust and supply air streams and then pipe them together. A pump circulates glycol fluid between the two coils and transfers heat from the exiting exhaust. This preheats the fresh air (without exposing it to the exhaust) and reduces the load on the primary heating system.
Figure 2 - Concept of a runaround coil, courtesy of Konvekta
Recovering heat from lab exhaust applications sometimes produces huge benefits. Many lab processes produce higher heat temperatures than usual room air temperature, so there is plenty of heat available for recovery.
Figure 3 - Design Schematic of Lab Exhaust Heat Recovery (https://www.adehvac.com/)
Runaround loops have been around for decades too, but they usually aren't considered for non-hazardous exhaust applications. The reason for this is that their efficiencies aren't as high as energy wheels, they have a higher upfront cost, and they require more maintenance than the non-hazardous methods.
Huge advancements in recent years, however, have helped put the runaround loop back into the mix. In fact, we recently studied an application in Buffalo, NY that engineered a runaround loop providing 90% of the total year round heat from recovered energy alone. Details are available here (warning: technical content).
Runaround loops have long been limited to hazardous exhaust applications, but that may all be about to change. With the increased energy saving of recent iterations, glycol loops are likely to grow more popular in non-traditional applications, taking runaround loops from a position of semi-obscurity to the forefront of the energy repurposing field.