Dual Path Outside Air Systems
Throughout this text, our examples have shown the outside ventilation air being mixed with return air before being processed and supplied to the building. This mixing method works well in cooler, dryer climates. This does not work as well in warm/hot, humid climates. The reason is very simple: the main cooling coil cannot remove enough moisture without overcooling the whole air stream. What is required is high moisture removal without full cooling.
An effective way around this problem is to use a dual path system. The outside air comes in through a separate, dedicated cooling coil before mixing with the return air. This dedicated outdoor air coil has two functions.
Dehumidification: The system is designed and operated to dehumidify the outside air to a little below the required space-moisture content.
Cooling: The system cools the outside air to about the same temperature as the main coil, when the main coil is at maximum cooling.
When the system is in operation, the fully cooled outside air, say 20%, mixes with 80% return air before it reaches the main cooling coil. The mixture is equivalent to the full airflow, substantially dehumidified and 20% cooled. The main cooling coil now provides the required extra cooling that the system needs, and a modest, achievable, requirement for dehumidification.
The challenge of providing adequate dehumidification at an acceptable cost is an ongoing challenge in moist climates. The dual path method described above is one of the many ways available to tackle the challenge of removing moisture without overcooling.
The Next Step
This chapter has been all about all-air systems that serve many zones. In many cases systems with separate water heating and or cooling can be very effective. For instance, in a very cold climate, it is often more comfortable to provide a perimeter hot water heating system and use the air system for cooling, ventilation air supply, and fine temperature control. This also allows the air system to be turned off when the building is unoccupied, even though the heating sys- tem must remain on to prevent over-cooling or freezing.
In the next chapter, Chapter 8 we will consider water systems and how they coordinate with air systems we have discussed in this chapter and the previous one.