The Myth of Pressure Independent VAV Terminals

 

Reprinted from ASHRAE Journal August 1989, Used with permission.

by Gil Avery

 

There is no question that VAV systems are an economical means of providing multiple zones of control from a single air handling unit, but there is increasing opposition to these systems by multi-users such as the armed services and large building management companies.

Many of the problems and complaints stem from improperly sized VAV terminals furnished with pressure independent (P.I.) controls. This practice is a ” black eye” on our industry, an industry that almost universally assumes that the pressure independent feature will atone for oversized terminals, poor duct design, and sloppy supply duct pressure controls. This same industry would frown on installing a 6″ modulating steam valve in a 6″ line yet it condones installing a 6″ VAV terminal in a 6″ branch duct.

The net free area is based on a casing I.D. 1/8″ less than nominal and less the area of the butterfly damper. (1/2″ x casing I.D.)

  • Note A – Non standard sizes needed to fill voids in capacity ranges.
  • Note B – Capacity equals to 3582 F.P.M. (Vel. Press. = 0.8″ W.G.) x the net free area.
  • Note C – Capacity equals to 1266 (Vel. Pres. = 0.1″ W.G.) x the net free area.

Very few VAV terminal manufacturers have published performance data on pressure dependent (P.D.) terminals without the air flow sensor, therefore all calculations in this paper are mathematical and are not based on tested units. The capacities shown in Fig. 1 are representative of the typical performance of P.D. terminals, but should not be used for design. The capacities are based on valve free area (inside diameter area less damper area). Actual performance will vary with the manufacturer since casing thickness, shaft sizes, blade designs, etc. will not be the same. Note that the performance of the 5″ valve with 0.8″ pressure drop is about the same as an 8″ valve with a 0.1″ pressure drop.

The pressure drop through the typical VAV 90 deg butterfly air valve that discharges into a rectangular plenum is approximately the velocity pressure through the valve free area. The down stream flow is turbulent and the velocity is virtually zero; therefore, there is no regain.

Since the drop across the valve is velocity pressure (Pv) across the valve opening, the damper on any oversize unit will only open far enough so that the velocity pressure through the partially open valve plus the duct losses will equal the trunk duct static pressure.

The pressure (P) ratio for any terminal is the ratio of the terminal velocity pressure at design CFM with the damper wide open, to the pressure that is available (See Fig. 2).
The curve in Fig. 2 is a plot of the maximum damper rotation verses “P” ratio.

VAV MYTHS

MYTH : VAV Terminals can be sized for lower pressure drops if P.I. controls are used.

FACT: Most VAV systems are designed for trunk duct static of at least 1″ W.G., since it would be difficult to maintain anything less than this on trunks serving multiple terminals even though a Static regain duct design was used. The 8″ terminal in Fig. 3 is the same size as the branch duct. The terminal velocity pressure with the damper wide open is a low 0.1″ W.G. and the “P” ration is 0.1 / (1 – 0.2) or 0.125

The curve in Fig. 2 shows that the damper will stroke a maximum of 50 degrees.

The 5″ terminal handling the same CFM is sized for the drop available and has a “P” ration of
0.8 / 1 – 0.2 or 1 The damper will stroke through the full 90 degrees.

Obviously the 5″ terminal is the better selection. A terminal sized in this fashion does not require pressure independent controls. If the duct pressure increases 50% to 1.5″ the maximum air flow would only increase about 22% and if the pressure doubled the flow would go up about 41% but only until the valve is repositioned by the room thermostat.
Because the actuator on the 8″ P.I. terminal only stokes the damper through about 1/2 the rotation, the gain in the control loop almost doubles. Small changes in room temperature or duct pressure will produce large changes in terminal air flow. This destabilizes the system. Many VAV systems with oversized P.I. terminals actually suffer from the pressure, “Domino” effect. If the pressure on one terminal increases, the P.I. controls close the damper thereby increasing the pressure on the other terminals that also start closing. The duct static pressure controller finally takes over and starts reducing the duct static and the cycle begins again in reverse.

This coupled with the hysteresis in the valve actuator linkage and in the pressure reset controller linkage, leads to more instability.

MYTH: Larger VAV terminals reduce the noise level of the system

FACT: Properly sized P.D. terminals generate about the same noise that oversized P.I. terminals do. The 8″ P.I. terminal in Fig. 3 has a 1″ drop across the valve, the same as the 5″ P.D. terminal when it starts to open. As the valves in the 5″ and 8″ terminals modulate, the pressure drop across both will be the same since the available pressure is the same for similar flows. If noise is a problem, it should be reduced by attenuation, not by oversizing the terminal, thereby degrading the control system.

MYTH: P.I. controls are a necessity for re-heat type systems.

FACT: Sizing P.I. re-heat terminals is critical because of the extremely low velocity pressure signal that is available when the unit is in the re-heat mode. Assuming that the re-heat air flow is 1/2 the maximum flow, the velocity pressure of the 8″ terminal in Fig. 3 @ 200 CFM is only 0.026″ W.G., not enough pressure for reliable operation of the velocity controller. In contrast the velocity pressure of the 5″ terminal at 200 CFM is still a usable 0.19″ W.G. If P.I. VAV reheat terminals must be used, the air valves should be sized as small as possible and with a “p” ratio as close to 1 as possible.

MYTH: DDC will resolve all the problems with oversized P.I. VAV terminals.

FACT: Direct digital control may provide some improvement in the system operation in that it will display the instability of the air system and action can be taken to correct the problems. Re-linking the air valves so that the valves rotate less than 90 deg, while the actuator stokes fully, may help. For example, the valve rotation on the 8″ P.I. terminal in Fig. 3 could be re-linked to open 50 deg instead of 90 deg. Re-linking the valves has the adverse effect of increasing the linkage hysteresis and re-linking does nothing to improve the performance of the air flow sensor.

Properly sized P.D. terminals not only eliminate most of the system instability associated with P.I. terminals but also provide the following benefits:

  • Lower first cost. The installed cost of P.D. terminals can be as much as 30 – 40% less than oversized P.I. terminals.
  • Easily understood simple controls.
  • Less maintenance.
  • No velocity sensors to get dirty.
  • Less leakage in the closed position.
  • Undivided control system responsibility. The control contractor can furnish all of the P.D. controls. Controls of various manufactures are generally furnished on P.I. terminals.

This discussion not only applies to variable air flow systems but to variable water flow systems as well. Balancing valves and flow controllers are not required on branches to correctly sized coil valves. Any drop allocated for balancing devices and flow controllers should be taken across the modulating valve.

DEFINITIONS

  • Pressure Independent – Valve
    actuator controlled by velocity controller. The set point of the velocity controller is reset by the zone thermostat.
  • Pressure Dependent – Value actuator controlled by the zone thermostat.