Three tips for designing VRF and ACB systems

Learning objectives

• Understand the basics of specialty heating and cooling systems. 
• Learn about variable refrigerant flow and active chilled beam systems.  
• Know three key things when specifying zone-based systems.  

Centralized variable air volume systems have become the ubiquitous system of choice for a no–frills HVAC design. There are ways to optimize VAV systems for higher efficiency, but many engineers opt to move to a zone-based system.  

Zone–based heating, ventilation and air conditioning systems have the drawback of distributing more equipment throughout the building; however, they allow for much greater control, which results in greater efficiency. Two such systems are variable refrigerant flow and active chilled beam systems. There are unique considerations for designing with these particular zone-based systems engineers must be sure to think through. Here are the top three: 

1. Design based on capacity, not airflow.
2. Latent load must be handled with ventilation air.
3. Efficiency is managed by zone control. 

Capacity, not airflow 

Many engineers have gotten used to HVAC load calculations resulting in a required amount of airflow, but that airflow assumes a certain temperature in order to satisfy the actual peak sensible load. VRF and ACB systems should be sized based on the peak sensible capacity, not the airflow they can provide. VRF systems vary the leaving–air temperature from the unit as a result of modulating the refrigerant to match the required space load. They also can modulate the airflow in tandem.  

ACB systems provide cooling by inducing space air across a cooling coil using the Venturi effect. The actual airflow in the room is the combination of the primary air and induced air, which is defined by a beam’s induction ratio. The ACB’s cooling coil surface temperature is kept above the space dew point temperature to mitigate condensation issues resulting in warmer supply air than variable air volume system. Similar to how refrigerant flow is modulated to a VRF indoor unit, the chilled water to an ACB is also varied. 

Latent load by ventilation 

In VAV systems, ventilation air is often the largest source of latent load. Ironically, the ventilation or primary air is the main mode of controlling latent load when designing VRF and ACB systems. VRF systems at least have a backup plan by employing a drain pan to catch condensate if the latent load were to get out of control.  

ACB systems have no such insurance policy. VRF IUs have very little latent cooling capacity, and ACB are designed to be sensible only cooling devices, which is why they don’t have a drain pan. This requires the primary air to serve double duty. It has to provide fresh air for the occupants while being dehumidified enough to absorb any latent load in the space without getting out of comfort ranges. This means the engineer must ensure that not only is the volume of ventilation air in compliance with ASHRAE Standard 62.1-2016: Ventilation for Acceptable Indoor Air Quality but given the dew point temperature, it also can meet the latent load. 

Efficiency by zone control 

The most efficient piece of equipment is one that is off. Centralized systems require the entire system to be on when any zone needs heating or cooling. Of particular consequence is reheat energy from having zones with coincident heating and cooling needs. Reheat energy is caused when one zone needs cold air and another needs hot air. The centralized systems can supply one temperature of air, so the total supply air volume is cooled to the required supply air temperature for the zone needing cooling.  

Then more energy is expended to heat the air back up at all the zones without a cooling load. More than double the amount of energy is expended to ensure all the zones in the centralized system are satisfied. 

Systems with heating and cooling at the zone, like VRF and ACB systems, have greater control. Heating and cooling can be cycled on and off independently to meet that specific zone’s needs rather than letting one zone drive the entire system. Having control at the zone completely eliminates the need for reheat. These systems don’t get their greatest efficiency from state-of-the-art technology. They are on only when they need to be. 

Knowing how to design to design a diverse array of HVAC systems is a basic requirement of the modern, high-performance engineer. Being cognizant of these three design considerations will make VRF and ACB designs more successful. 

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