7.3 Air and Water Distribution: AHUs, VAV vs. CAV, Pumping Systems, Fans, and Economizers

Air and Water Distribution Systems

Generating cold water or hot air is only half the battle; that thermal energy must be transported to the occupied zones. Distribution systems—comprising fans, ductwork, pumps, and piping—are responsible for this transport. In many buildings, the fans and pumps consume more annual electricity than the chillers themselves. Therefore, mastering distribution efficiency is a core competency for any Certified Energy Manager.

Air Handling Units (AHUs) and Volume Control

An Air Handling Unit (AHU) is the large metal box that houses the fans, heating/cooling coils, filters, and dampers required to condition and distribute air. The fundamental design of the AHU and its terminal units dictates the system's energy profile.

Constant Air Volume (CAV) vs. Variable Air Volume (VAV)

Historically, many buildings utilized Constant Air Volume (CAV) systems. In a CAV system, the supply fan runs at a constant, full speed all the time. To control the space temperature when the cooling load drops, the system either cycles the compressor on and off, or more egregiously, it uses a "reheat" coil to heat the cooled air back up before it enters the room. This simultaneous cooling and heating is a massive waste of energy.

Modern buildings utilize Variable Air Volume (VAV) systems. In a VAV setup, a Variable Frequency Drive (VFD) controls the speed of the main supply fan. Each zone has a VAV terminal box equipped with a motorized damper. As the cooling load in a specific room decreases, the thermostat signals the VAV box damper to close slightly, reducing the volume of cold air entering the room. As multiple VAV boxes close off, the pressure in the main duct rises. A pressure sensor detects this and commands the VFD to slow the main supply fan down.

The energy savings from VAV systems are profound due to the Fan Affinity Laws. The laws state that power consumption varies with the cube of the fan speed. If a VAV system reduces fan speed by 20% (operating at 80% speed), the power consumption drops to (0.8)^3, which is 0.512, or roughly 51% of full power. A 20% speed reduction yields a 49% energy savings!

Air-Side Economizers (Free Cooling)

An air-side economizer is an arrangement of dampers in the AHU that allows the system to draw in up to 100% outside air. When the outdoor temperature and humidity are lower than the return air from the building, the economizer opens the outside air damper and exhausts the return air.

This provides "free cooling" because the mechanical chillers or compressors can be shut off or dialed back significantly. Economizers are particularly effective in dry, temperate climates. They can be controlled via:

• Dry-Bulb Control: Compares outdoor temperature to return temperature.
• Enthalpy Control: Compares the total heat (temperature and humidity) of outdoor air to return air, which is much more accurate and prevents bringing in cool but excessively humid air.

Hydronic Pumping Systems

Water is an excellent heat transfer medium, carrying significantly more thermal energy per unit of volume than air. Chilled water and hot water are distributed using centrifugal pumps. Just like air systems, water distribution can be constant volume or variable volume.

Primary-Secondary Pumping

Many large chiller plants use a primary-secondary pumping arrangement.

• The primary loop circulates water through the chillers at a constant volume. Chillers historically required constant flow to prevent the water inside their evaporator tubes from freezing.
• The secondary loop circulates water out to the building's AHU cooling coils. This loop utilizes VFD-controlled variable speed pumps and two-way control valves at the coils. As cooling loads drop, the two-way valves close, system pressure rises, and the secondary pumps slow down, yielding massive pump energy savings via the pump affinity laws (which are identical to the fan affinity laws).

Modern chiller designs and advanced controls have allowed for Variable Primary Flow (VPF) systems, eliminating the secondary pumps entirely and varying flow directly through the chillers, offering even greater savings.

Hydronic Heat Transfer Equation

Just as we have a sensible heat equation for air, we have an equation for calculating heat transfer via water. This is vital for analyzing chiller plant performance or sizing heat exchangers. The formula is:

Heat Transfer (Q) = 500 * GPM * dT

Where:

• Q = Heat transfer in Btu/hr
• 500 = A constant derived from the weight of a gallon of water (8.33 lbs), the specific heat of water (1.0 Btu/lb-F), and minutes in an hour (60). (8.33 * 1.0 * 60 = approx 500)
• GPM = Flow rate in Gallons Per Minute
• dT = Temperature difference between the supply and return water (°F)

Worked Example: A chilled water loop supplies water to an AHU at 44°F and returns it at 54°F. A flow meter indicates the water flow rate is 300 GPM. What is the cooling load in Btu/hr and Tons?

First, calculate the temperature difference (dT): dT = 54 - 44 = 10°F

Next, use the hydronic equation: Q = 500 * GPM * dT Q = 500 * 300 * 10 Q = 1,500,000 Btu/hr

To convert to Tons (divide by 12,000 Btu/hr): Tons = 1,500,000 / 12,000 = 125 Tons

By leveraging the hydronic equation alongside chiller kW inputs, a CEM can continuously calculate the real-time kW/ton efficiency of a central plant.