Economizer Components Supplemental Information

Chapter 3: Economizer Components Supplemental Information
3.6. Supplemental Information
3.6.1. Dampers Damper Selection Damper Sizing
3.6.2. Actuators Piston Actuators Gear Train Actuators With Crank Arm Drives Gear Train Actuators With Shaft Concentric Drives Linear Actuators Installation and Commissioning Issues
3.6.3. Sensing Elements Temperature Sensors Freezestats Enthalpy Switches and Sensors
3.6.4. Pressure Sensors and Switches Pneumatic Pressure Transmitters Electronic Force Based Differential Pressure Transmitters Electronic Flow Based Differential Pressure Transmitters Transmitter Installation and Sensing Line Considerations Flow Sensors
3.6.5. Blank-off plates
3.6.6. Air Blenders and Baffle Plates

Table of Figures
Figure 3.9: Parallel and Opposed Blade Dampers
Figure 3.10: Parallel Blade Damper Characteristics
Figure 3.11: Typical Pneumatic Piston Actuator and Positioning Relay
Figure 3.12: Hydraulic Type Piston Actuator
Figure 3.13: Electric Gear Train Actuators
Figure 3.14: Electric Gear Train Actuator, Shaft Centerline Mounting
Figure 3.15: Linear Actuator
Figure 3.16: Piston actuator linkage arrangements
Figure 3.17: Averaging sensor application
Figure 3.18: Typical two position enthalpy switch
Figure 3.19: Enthalpy switch operating curves for a typical switch
Figure 3.20: Enthalpy transmitters
Figure 3.21: Pneumatic low differential pressure transmitter and controller
Figure 3.22: Typical flow based pressure sensor
Figure 3.23: One pipe pneumatic transmitter operation
Figure 3.24: Typical stand-alone process controller
Figure 3.25: Typical force based differential pressure transmitter
Figure 3.26: Pressure Measurement with an Open Calibration Port
Figure 3.27: Typical math function type signal conditioner
Figure 3.28: Typical static pressure sensing probes
Figure 3.29: Packaged outdoor air measurement and control assembly
Figure 3.30: Typical Air Blender
Figure 3.31: Blank-off plates and baffle plates help alleviate a stratification problem

3.6. Supplemental Information
Most economizer and mixing sections include some or all of the following components.
1   Dampers to control the flow and mixing of the air streams and regulate minimum outdoor air.
2   Actuators to power the dampers.
3   Sensors that provide the inputs to the control loops, interlocks, and safety circuits that control the temperatures, pressures, and flows associated with the economizer process.
4   Blank-off Plates to adapt dampers that are smaller than the duct.
5   Air Blenders and Baffle Plates to promote the mixing process.
The following sections will discuss these items in greater detail.
3.6.1. Dampers Damper Selection
Dampers are provided in the mixing section to modulate the outdoor air and return flows as required by the economizer cycle. There are two common blade arrangements used for these control dampers, opposed blade and parallel blade, shown in Figure 3.9.
The blades of a parallel blade damper remain parallel to each other throughout the rotation cycle. This arrangement allows the damper to direct the air as it moves through them, which can be an advantage in a mixing situation. However, the dampers require a higher pressure drop through them to achieve a linear control characteristic compared to opposed blade dampers.
Opposed blade dampers have a linkage arrangement that causes pairs of blades to rotate towards each other as the damper actuates. As a result, the air stream experiences very little change in direction as it passes through the damper. When compared to a parallel blade damper with identical dimensions and the same air flow across it, an opposed blade damper will achieve a linear flow characteristic with less pressure drop.
Figure 3.9: Parallel and Opposed Blade Dampers

In both arrangements, the damper blades are mounted on a shaft that allows the blades to rotate around their long axis. For multi-blade assemblies, there is typically one blade or shaft that is designed to be the drive blade or shaft to which the actuator is connected. The other blades in the damper are driven by a linkage system from the drive blade. There are typically two arrangements for this linkage system.
The linkage drives the blades by rotating the shaft and is completely concealed in the damper frame. This arrangement keeps the linkage and pivot points out of the air stream, which can be an advantage from a pressure drop standpoint as well as a maintenance stand point if the air stream is dirty. However, if linkage maintenance is required, it is often more difficult due to the concealed location inside the frame.
The linkage drives the blades through a linkage that extends from blade to blade. In this arrangement, the shafts provide support and a pivot point, but generally do not transmit power from the actuator to the blades. The drive linkage is more accessible in this arrangement, but is also exposed to the air stream, which can add pressure drop and reduce the life of pivot points if the air stream is dirty.
In addition to the blade-oriented considerations discussed in the preceding paragraphs, the actual configuration of the blade itself should be considered when the damper is selected. There are two general blade designs available in the HVAC market currently.
Flat Plate This configuration is the standard offering of most manufacturers and consists of a flat plate secured to the damper shaft, usually with some sort of bend or break folded to the length of the blade to ensure rigidity.
Airfoil In this configuration, an extruded blade with a streamlined profile is used. An airfoil provides a rigid assembly that is resistant to flutter at high velocities and helps to ensure good blade seal compression. Thus, these dampers are often have a low damper leakage rating. The streamlined shape results in a significantly lower pressure drop as compared to the flat plate design; often by as much as half for identical damper geometries at identical flow rates. This lower pressure drop can be an advantage or a disadvantage depending upon how the damper is applied as will be seen in the next paragraph. Damper Sizing
For most control applications, it is desirable to achieve some sort of linear relationship between damper position and damper flow. For example, the damper should reduce the flow through it by 50% when the damper has rotated 50% closed. To achieve this, it is necessary for the damper to have a significant pressure drop relative to the system that it serves. Figure 3.10 illustrates the damper characteristic curves for a parallel blade damper at a variety of pressure drop ratios (a) where a is the ratio of system pressure drop to damper pressure drop. Opposed blade dampers have similar characteristic curves, but require less pressure drop to achieve the same linear control.
Ideally, the mechanical designer or controls contractor should have sized the dampers to achieve a nearly linear characteristic. Unfortunately, this step is often neglected and commissioning problems are often the result. Generally, the problems will fall into the following categories: Poor mixing due to lack of velocity of the air streams. This topic is discussed in Chapter 3, Section 3.1.2: Economizer Free Cooling.
Poor controllability due to a non-linear relationship between damper position and flow.[1]
Figure 3.10: Parallel Blade Damper Characteristics
Alpha is the ratio of system pressure drop to damper pressure drop. An alpha of 200 means the damper pressure drop is 1/200 of the pressure drop of the entire system. As seen from the graph, a parallel blade damper needs to have a significant pressure drop across it relative to its system in order for the control to be nearly linear (50% stroke = 50% flow). The curves for opposed blade dampers are similar, but require less pressure drop to achieve the same linearity of control. (Developed from data in the ASHRAE Handbook of Fundamentals)

As a general rule, achieving a linear damper characteristic in most systems will require damper face velocities in the range of 2,000 to 2,500 fpm for flat plate type dampers and 2,500 to 3,000 fpm for airfoil type dampers. Based on this information a commissioning agent can quickly assess a system for potential mixing and economizer control problems by simply dividing the design flow rate by the damper face area. If the result is a velocity outside of the ranges indicated above, the commissioning agent should spend extra time on the testing and tuning of the economizer cycle.
As can be seen from the preceding discussion, the details of damper blade configuration can have a significant impact on the performance of the system the damper is installed in. Other details related to damper design, installation and construction with significant efficiency and/or other operational implications include:
Document the details to successfully achieve design intent An amazing number of commissioning problems occur simply because the dampers are not installed correctly. There have been instances where multi-section parallel blade dampers were assembled with different blade rotations for each section; in other words, the blades in one section were horizontal and rotated downward as they closed, in the next section horizontal but rotating upward as they closed, in the next section the blades were vertical and rotated to the left. In other instances, it was virtually impossible to maintain actuators and linkages due to the installed configuration of the damper assembly. To avoid this sort of confusion, mixing damper configurations should be detailed in the design, preferably by the designer but as an alternative, by the control contractor, prior to installation. The detail should show configurations, number of sections, blade rotations, actuator locations, actuator linkage arrangements, blank-off panel locations and all other items required to install the dampers as required to achieve the design intent.
Provide adequate access Each control damper (not just fire and smoke dampers) should be provided with an adequately sized access panel (or panels) that is unobstructed by surrounding systems, equipment, and building structure. This panel should be located so that when a technician is working through it, they can reach all damper components that are inside the duct, including linkage, bearings jamb seals and blade seals. (A 12″ x 12″ access panel in a 72″ x 24″ duct located 25 feet in the air over a motor control center and 3 feet upstream of the control damper is an access panel in name only.)
Vertical blades need to have thrust bearings Usually, but not always, the manufacturers catch this. Without them, the operation of the damper will wear out the jamb seals very quickly at a minimum. It is also possible that the blades will start to push into the degraded seals as they wear leading to other operation problems.
Limit blade length Blade lengths in excess of 48 inches are undesirable, especially for non-airfoil, non-extruded blades. At high velocities, the longer blades can flutter, and getting good blade seal compression as required to achieve the rated leakage rates can become a problem.
Select dampers that securely attach the shaft to the blade There are a variety of techniques used to secure damper blades to shafts. Approaches vary from set screws to keyed arrangements. As a general rule the more positive connection provided between the damper blade and shaft by a slot and key or hexagonal shaft in a hexagonal hole is highly desirable. Less positive arrangements like set screws can often become loose over time, resulting in loss of damper functionality.
On occasion, the configuration of the air handling unit and the building make it desirable to combine some of the code-dictated fire and smoke isolation damper requirements with the economizer functions. This approach can have several advantages including:
Reduced cost due to multiple functions being served by one device.
Reduced energy requirements due to the elimination of one pressure drop generating element from the air stream.
Reduced space requirements due to the need to only install one device.

As a result, some of the economizer damper assemblies are fire and smoke rated assemblies and the control signals to them must accommodate these functions in addition to the more conventional economizer functions. These applications deserve special attention from the commissioning agent to:
Ensure that all life safety control functions take precedence over environmental control functions, regardless of operating modes. For example, the smoke control cycle should have priority over the economizer cycle.
Ensure that all life safety functions are implemented in a manner that does not threaten to harm the air handling system due to excessive positive or negative pressures or air hammer effects. Air hammer is presented in Chapter 11: Distribution.
Ensure that the integrity of the fire and smoke damper assemblies are maintained during the installation and start-up process so that the agency listing of the dampers is not violated.
Additional information regarding fire and smoke dampers can be found in Chapter 15: Management and Control of Smoke and Fire.
3.6.2. Actuators
The actuators used for HVAC control dampers generally fall into the following four categories, which are discussed in detail in this section.
Piston actuators
Gear train actuators use an electric motor with reduction gearing to rotate and output shaft that then moves the damper via crank arms on the actuator and damper shaft and an intervening linkage system.
Gear train actuatorsuse an electric motor with reduction gearing arranged to mount with their output shaft concentric with the damper shaft, thereby eliminating a crank arm and linkage system.
Linear actuators use an electric motor with reduction gearing to drive a jack screw which drives a crank arm on the damper shaft directly with out an intervening linkage system. Piston Actuators
Figure 3.11: Typical Pneumatic Piston Actuator and Positioning Relay

(Image courtesy of the Kele Associates web site)
From a mechanical standpoint, piston actuators are generally the simplest, consisting of a housing, a piston and diaphragm, a spring, and a shaft through which force on the piston is transferred to the damper crank arm. They are typically actuated by pneumatic air pressure, with a maximum pressure of 30 psig or less in commercial applications. The spring constant determines the range over which the damper will actually stroke as the actuating pressure varies.
There are variations on the pneumatic actuator concept seen in the commercial HVAC market that are electrically powered. One fairly common design uses a small hydraulic pump to circulate oil from a reservoir to a chamber behind the piston. A variable size orifice controls how quickly this fluid can bleed back into the reservoir from the piston chamber. The orifice size is controlled by the control signal. As the size of the orifice is decreased, pressure builds up in the piston chamber and the piston extends. As the size of the orifice increases, pressure bleeds off and the actuator spring causes the actuator to retract. In another variation developed for remote sites with no control air source, a small self-contained compressor generates air pressure for use by a conventional pneumatic actuator. A fairly rare actuator design that may be seen on older projects uses a wax that has a high thermal coefficient of expansion and a heater. The heater output is varied by the control signal, which causes the wax to expand and move the piston.
Figure 3.12: Hydraulic Type Piston Actuator

(Image courtesy of the Kele Associates web site)
Pneumatic piston actuators are often sequenced by selecting spring ranges appropriately so that one actuator strokes fully before the next actuator begins to stroke. However, forces generated by airflow acting on the damper blades in the HVAC system can feed back through the actuator shaft and work against the spring. The forces can shift the spring range significantly and significantly alter the sequencing, which can lead to energy waste.
For example, if a control system designer wanted to sequence a chilled water valve with the economizer dampers based on discharge temperature, the designer might specify a spring range of 5-8 psig for the economizer dampers with a normally closed outdoor air damper and a normally open return damper. To sequence the chilled water valve properly, they might specify an 8-13 psig range for the valve actuator with a normally closed valve. On paper, the outside air damper would be fully open by the time the signal reached 8 psig. At this point, the chilled water valve would start to open, thus guaranteeing the full utilization of free cooling using the outdoor air prior to using chilled water.[2] However, if the forces on the damper blades shifted the spring range 1 or 2 psig, the chilled water valve might start opening before the outdoor air damper was fully open, thus using chilled water for cooling when it may have been possible to serve the load with outdoor air. To solve this problem, a device called a positive positioner or positioning relay is installed on the actuator.
The positive positioner is a position controller that has its own source of supply air. The device senses the position of the damper shaft with adjustments for start point and span. The positioner applies pressure to the actuator piston in order to get the piston to move as the control signal varies in the specified range. For example, if you wanted to be sure that a damper moved exactly over a 5-8 psig range, then you would install a positioning relay on the damper and set it to have a 5 psig starting point and a 3 psig span. In operation, when the control signal to the positioning relay reached 5 psig, the positioner would begin to apply main air pressure to the actuator piston until it detected shaft motion, even if it took 7 or 8 psig to make the damper start to move. The positioner would then proportionately move the damper shaft in response to an increase in control signal to ensure that the shaft was fully extended by the time the control signal reached 8 psig (the sum of the 5 psig start point plus the 3 psig span value). Gear Train Actuators With Crank Arm Drives
Figure 3.13: Electric Gear Train Actuators

(Image courtesy of the Johnson Controls website)
These actuators represent older electronic technology in which a small single-phase motor (usually the shaded pole type) is used to drive a crank arm through a gear train. The gear train actuator can interface with a variety of signals including electric floating contact type controllers, variable resistance type controllers, two position signals, and common automation system outputs like 4-20 milliamps, 1-10 vdc or pulse width modulation. The type of interface is usually determined by an interface circuit board in the actuator or an interface module mounted to the actuator. Settings on the circuit boards or independent modules allow these actuators to be sequenced in a manner similar to that described for pneumatic actuators discussed in the piston actuator section. Most models can be equipped with a spring return feature to guarantee a fail-safe position on power failure.
Figure 3.14: Electric Gear Train Actuator, Shaft Centerline Mounting

(Image courtesy of the Siemens website) Gear Train Actuators With Shaft Concentric Drives
These actuators represent newer technology that developed in response to the widespread use of DDC control systems in the commercial buildings industry. The actuators avoid some of the potential linkage problems associated with piston actuators and gear train actuators driving through crank arms by mounting the actuator directly to the damper shaft, with the output torque applied directly to the shaft. Since the DDC market drove the development of these actuators, they can usually be interfaced directly to common DDC outputs. Sequencing with other actuators is usually accomplished by settings incorporated directly into the actuator. Most of these actuators are offered with spring return options for fail safe positioning on power failure.
Several manufacturers combine the actuator in the same housing as their VAV terminal unit controllers. This tends to make the installation costs lower with less start up problems as compared to systems that require that the VAV unit damper actuator be mounted and wired independently from the controller. But, it can increase the replacement cost when the unit fails since the entire assembly needs to be replaced, not just the failed component.
Figure 3.15: Linear Actuator

(Image courtesy of the Tyco website) Linear Actuators
These actuators are more common in the industrial market than the commercial market but are sometimes found serving applications such as inlet guide vanes where a lot of actuating power is required. They typically consist of a jackshaft coupled with a motor of some sort and can accept most of the common output signals available from a DDC system. Installation and Commissioning Issues
Generally, there are several commissioning issues related to installation of damper actuators as follows:
Sizing By reading the fine print on the damper leakage curves, one may discover that the preload torque required to achieve the leakage rate is higher than the torque required for actuation. Actuator selection, sizing, and set-up need to take the pre-load torque into consideration and the commissioning process should include verification that the field installation meets all applicable requirements.
Linkage Arrangement Piston and linear type damper actuators need to be mounted a manner that allows the linear motion of the actuator shaft to be converted to rotation at the damper shaft. Usually this is accomplished via a linkage system that includes crank arms, extensions and swivels. Actuators should be mounted to maximize the linearity between actuator stroke and blade position and also maximize the torque available to the damper from the actuator through the damper stroke. The kinematics associated with the linkage arrangement are often not well understood by the field personnel installing the actuators, thus issues related to this can show up as a commissioning problem. Figure 3.16 illustrates how the relationship between actuator stroke and damper bland position varies for a piston actuator.
Figure 3.16: Piston actuator linkage arrangements

Note 1: A + indicates that the percentage of stroke leads the percentage of blade rotation.
Note 2: Only the actuator force applied perpendicular to the crank arm creates rotation and torque.
Similar considerations apply to linear actuators as well as gear train actuators that drive the damper via a crank arm and linkage. Actuators that concentric to the shaft and drive the damper shaft directly eliminate these issues by virtue of their mounting and drive arrangement.
Multiple section dampers with multiple actuators need to have identical actuators with identical linkage arrangements for all sections to make the characteristic for all of the sections consistent. Consistent damper section installations also make the performance of the entire assembly predictable based on the performance of one section. If the outdoor air damper and the return damper are multi-sectioned dampers controlled by the same signal, then all actuators for both sections should be identical with identical mounting arrangements for the same reason.
Actuating Speed and Power Most electric actuators have operating times that can vary from 30 to over 90 seconds to move through a full stroke. The slow speed is due to the small motors, which must be geared down significantly to deliver the necessary torque required to actuate common HVAC devices. Pneumatic actuators, on the other hand, can have full stroke actuating times that are extremely fast and can deliver tremendous power due to the amplifying effect of air pressure over a large diaphragm area. For instance, a six inch diameter piston damper actuator with 20 psi air acting on it can easily deliver over 500 pounds of force at the shaft, even after compressing the damper spring. These characteristics can be advantages or disadvantages depending on the application.
Rapid actuation speeds are desirable and even necessary if tight, reliable, and stable control is to be achieved on some systems. Usually, this is expensive and difficult to achieve electrically using commonly available technology in the HVAC industry. This item is worthy of review by the commissioning agent in the design phase. During start-up or in a retro-commissioning environment, changing from electric to pneumatic actuation may represent a viable solution to a start-up or operational problem related to system response time.
Rapid actuation can be undesirable in cases where air or water hammer could be the result of a rapidly closing valve or damper.[3] In situations where this is a possibility, the slower actuating times associated with electric actuators may be an advantage.[4] If pneumatic actuators are installed at locations where they could generate air or water hammer, then it may be necessary to slow them down by using restrictors or metering valves in the circuit serving them. The commissioning agent needs to be aware of these issues and requirements and design functional test sequences to ensure that they are properly addressed.

Sequencing As described earlier in this section, proper sequencing of actuators is often essential for efficient system operation, but cannot be solely guaranteed by the specifications and requirements of the contract documents. The functional testing performed during the commissioning process should include verification of proper sequencing under all operating modes. If sensors are installed at the proper locations in the system, trending can be used to allow the commissioning agent and operators to verify the persistence of the original sequencing. Smart alarms triggered by conditions that are not logical (a temperature rise across a heating coil that has its valve commanded closed for instance) may also be desirable.
From a historical perspective, pneumatic actuation and control systems where quite common in the commercial buildings industry. In the past decade, the wide acceptance and implementation of DDC control systems coupled with the development of shaft centerline drive type actuators have made electronic/electric actuation system more common in the commercial HVAC market. The electric/electronic actuators often can interface directly with the DDC system where as pneumatic actuators require some sort of signal converter[5]. In addition, pneumatic actuators require a source of clean, dry air. On sites where this is not already available, the use of pneumatic actuation will require that this equipment be installed, operated and maintained. However, if a large operating torque is required and/or quick (less than a minute) actuation speed is necessary, then pneumatic actuation often represents the most cost effective and reliable option.
3.6.3. Sensing Elements
Sensing elements perform critical tasks in ensuring proper economizer operation. The following paragraphs discuss sensing requirements particular to economizers. Temperature Sensors
Figure 3.17: Averaging sensor application
The mixed air sensor is in the red circle. The spiraled sensing element is the freezestat. This single point sensor will have a difficult time averaging the temperature in this 6 ft by 10 ft mixing plenum.

Selection and proper configuration of the temperature sensors used by the economizer cycle can have a very significant impact on its ability to perform as intended. Since even a well-designed mixing plenum can have some temperature stratification over its cross section, selecting and installing a temperature sensing system that averages the temperature across the entire cross section is an important aspect of the overall system design. There are a surprising number of projects encountered by commissioning agents where the first step in solving the economizer operating problems is the replacement of a single point sensor with an averaging sensor. The picture in Figure 3.17 is from one such project. In this instance, the poor sensor configuration coupled with independent heat transfer element control loops was causing the system to do 10 F of unnecessary preheating.
Specifying averaging sensors for economizer applications is a good first step, but it does not fully address the issues related to measuring temperatures in a mixed air plenum. The following items should also be considered:
1  The temperature that will ultimately be achieved by the complete mixing of a stratified air stream is a function of both the temperatures and the mass flow rates of the different temperature layers. The information sensed by averaging temperature elements is purely temperature and is effectively immune from the influences of mass flow rate[6]. If the nominal face velocity over all portions of the averaging element is the same, then the average temperature indicated by the element is probably a reasonable measurement of the temperature that will be achieved in the air stream when it is completely mixed. However, if there are significant differences in the velocity over certain areas of the averaging element, then the average temperature indication sensed by the element may not accurately reflect this final mixed temperature. For example, when mixing a teaspoon of water at 200 F water with a gallon of 100 F, you would not expect the mixed temperature to be 150 F, even though this is the average of the two temperatures being mixed. Similar considerations apply to airstreams in mixing plenums.
If the true mixed temperature is higher than what the averaging element predicts, there may be comfort control problems since the system will not be delivering air at the required temperature. In some systems, the refrigeration equipment may operate to make up for this difference even though the system could achieve the required setpoint by using additional outdoor air in the economizer cycle, thereby negating some of the energy benefits of the cycle.
If the true mixed temperature is lower than the value indicated by the averaging element, then there could be problems with nuisance freezestat trips or coil failures. In some systems, the preheat coil will be activated to warm the air up to the required setting despite the fact that the system could achieve this temperature by using less outside air on the economizer cycle. Again, this wastes energy and negates some of the benefits of the cycle.
2  Stratification patterns and their associated velocity profiles can change with the operating point of the economizer. As the outdoor air and return air dampers modulate due to changes in setpoint, return, and outdoor air conditions, the temperature and velocity profile produced at the discharge of the mixed air plenum can change. Systems that mix well under some operating conditions may stratify under other conditions. Other systems may exhibit similar profiles regardless of the operating point. The Temperature Traverse Test and the High Turndown Ratio Test help identify temperature and flow patterns in the mixed air plenum and target solutions to related problems.
3  Large plenums may require multiple sensors to accurately reflect what is occurring in the plenum. The mixed air plenum stratification test included in this chapter can be used to determine the best configuration for the mixed air sensing elements under different operating conditions. It may be necessary to perform this test as part of new construction or retro-commissioning to optimize the configuration of the sensing elements so they provide repeatable, reliable information under all operating modes.
4  The mounting system used for the averaging elements should allow them to be easily reconfigured during the commissioning process if necessary. It should also support the sensing elements in a manner that allows them to accurately reflect the air stream conditions, free from other influences such as heat conduction from coils or other supporting equipment and radiant temperature effects from high temperature heat transfer elements like steam coils. Radiant temperature effects can be felt upstream as well as downstream of a heat transfer element. If you can stand at the mixed air sensor location with the palm of your hand facing a nearby high temperature coil, and can feel the radiant heat from the coil with your hand, then the element is probably being influenced by the coil. Freezestats
Freezestats are limit controls that are installed to protect the coils in an air handling system from being subjected to sub-freezing temperatures. Typically, they are arranged to shut down and lock out the system and a manual reset is required to resume normal operation. Contrary to popular opinion, the long sensing elements that they are provided with are not really averaging elements, even though they are visually similar. The sensing elements on most freezestats will respond to the coldest temperature seen by any one or two foot segment. This is desirable because it only takes a small stream of subfreezing air to freeze and rupture a tube in a coil.
Even though freezestats are not averaging elements, the same considerations outlined previously for the installation of averaging elements apply to the installation of sensors on freezestats. In fact, it is often desirable to string the freezestat element along with the mixed air temperature-averaging element so that they both are subjected to the same conditions. This allows the mixed air sensing element to reliably generate a pre-freezestat trip alarm that can be used to prevent an unscheduled outage. The advent of DDC technology has lead to the development of software-based freezestats. At first glance, this approach appears to offer some advantages in terms of lower installed cost and ease of operation. However, there can be some problems with this approach that can very quickly negate any of its benefits.
1  Usually the first cost savings is achieved by eliminating a device; i.e. the independent freezestat and its associated wiring to the starter. This means that the mixed air sensing element becomes the input to the freezestat control code and, in a sense, the mixed air sensor ends up trying to protect itself from its own failure.
2  Since the operation of the freezestat is now dependent on the operation of the DDC controller, a controller failure eliminates the freezestat protection. In some cases, a controller failure will not necessarily shut down the unit, so it is possible for a system to end up on line and operating without the intended protection in place.
3  Since the freezestat function in this approach is part of the controlling software, it is a part of the automation. Usually, the automated output that starts and stops the fan is wired to the Auto position of the Hand-Off-Auto selector switch on the fan. If the fan is placed in Hand , then the freezestat protection is eliminated from the circuit. An independent device can be wired into the common side of the selector switch so that it provides the necessary level of protection regardless of the selector switch position.
For these reasons, the best approach is to simply not use software based freezestat (or a software based safety of any type for that matter). If the software-based system is employed on a project, it needs to be very carefully designed and commissioned to prevent the types of problems outlined above from occurring.
Repetitive freezestat trips? No problem with the correct tools and equipment!
On one project, the owner wanted their mechanics to be able to rapidly identify a freezestat shutdown of the air handling equipment and demanded that the freezestat be wired into the control circuit only on the auto side of the hand-off-auto selector switch. This would allow the mechanics to quickly place the unit in hand and see if it ran when it was off for some unexplained reason. If it ran, then they knew they had a freezestat trip, the anticipated most likely problem. If the unit didn t run, they would focus their troubleshooting efforts elsewhere. The consultant reluctantly agreed with the owner s demand with the stipulation that the hand position be designed to spring return to the off position to prevent an operator from inadvertently walking away and leaving the system operating in hand with no freezestat protection. During the first winter of operation, the designer was called to the project site to investigate and determine why a coil had been frozen in one of the air handling units equipped with the spring return type switch. When they walked up to the starter, the cause of the problem became immediately obvious. A creative mechanic (who didn t understand the control system) had solved the problem with that pesky selector switch that wouldn t stay in the hand position by clamping the jaws of a crescent wrench to the actuating tab on the selector switch.
Most people think of a freezestat as providing protection for systems with water coils and would not consider installing one on a system that is equipped with direct expansion cooling and gas fired furnaces for heating. Generally, this is a reasonable assumption. But there have been instances where the plumbing and sprinkler piping in a building was frozen because such an air handling system remained in operation with an economizer that failed wide open during extreme weather over a weekend. Since there are no water coils to protect in this situation, the freezestat can be installed in the discharge duct where it is relatively immune from the effects of poor mixing and the related problems with nuisance freezestat trips, yet will provide protection against freezing the building plumbing and sprinkler piping. Enthalpy Switches and Sensors
To perform the enthalpy-based economizer changeover function described in Chapter 3, Section Enthalpy Based Interlock, enthalpy switches or transmitters may be used.
Two Position Enthalpy Switches Figure 3.18 illustrates a typical two-position enthalpy switch. The different scales on the switch correspond to different near-constant enthalpy states. By coordinating the scale selected with the requirements of the project, the switch can be used to provide a relatively low cost enthalpy based economizer change-over signal. The switches need to be mounted in an accessible location that subjects the sensing element to a free-flowing outdoor air stream yet protects the element from exposure to water and direct and reflected sunlight. This location can introduce some operating anomalies into the system due to lack of air flow when the unit is not in operation. For instance, minor air leakage back out of the unit from the building when the unit is off may cause the sensor to think the ambient conditions are suitable for cooling in the summer when they are not. When this unit first starts, it may try to operate on the 100% outdoor air cycle until the outdoor air flowing through the intake system can bring the local environment at the switch location into alignment with the actual outdoor conditions. This process usually happens fairly quickly, but it can be misleading to the commissioning agent during testing.
Figure 3.18: Typical two position enthalpy switch

(Image courtesy of the Honeywell website)
Protecting the enthalpy switch from dust is also desirable, but usually difficult to accomplish because most recirculating systems do not filter the air until after the mixing plenum. For sites with a 100% outdoor air unit, the enthalpy switch can be protected by locating it downstream of the first set of filters but upstream of any heat transfer elements. This enthalpy signal can be used to pilot the other units on the site. For this approach to be successful, the unit where the switch is installed must be one that runs at all times that any of the other units piloted by the signal run. Without air moving across the filters and to the enthalpy switch, the signal from the switch will not necessarily reflect the ambient conditions.
The switch settings correspond to a set of near-constant enthalpy lines on a psychrometric chart. Many of the manufacturers include a small chart with the operating curves as a part of the switch installation literature or they may print the chart on the switch housing. However, in the event that the installation instructions are not available, then it is fairly easy to construct the operating curves from a blank psychrometric chart and the tables included in the catalogs. Figure 3.19 is an example of a chart constructed in this manner. Another option is to go to the internet and download the installation instructions since many manufacturers now provide this information in the form of .PDF files. The following link contains the information for the switch depicted in the curves below;

Figure 3.19: Enthalpy switch operating curves for a typical switch
If the installation information with the enthalpy switch operating curves is not available, operating curves can be drawn on a psychrometric chart using the information typically included in the supply catalog. Once you have made the curve for one type of switch, you can just keep a copy with your test equipment. For this set of curves, if the switch was set on scale D, then it would be in economizer mode for any condition left of curve D and out of the economizer mode for any conditions to the right of curve D.

Notice how the operating curves in Figure 3.19 are really not constant enthalpy lines. They parallel the lines reasonably well for the upper portion, but fall away from running parallel, from the 50% point to their lower end. This curvature should be taken into consideration when determining the appropriate switch setting.
Figure 3.20: Enthalpy transmitters

(Image courtesy of the HyCal web site)
Enthalpy Transmitters Figure 3.20 depicts an analog enthalpy transmitter. These units provide an analog output that is directly proportional to enthalpy. They can be wired as an input to the control system and the information they provide can be used for various enthalpy-based control decisions such as enabling or disabling the economizer mode. In general terms, the same mounting and servicing considerations apply to enthalpy transmitters as apply to enthalpy switches, although there are some designs available that are suited for direct or nearly direct exposure to the outdoor environment.
Enthalpy Calculations Today s control systems generally are capable of performing mathematical operations on their variables. This opens the door for calculating enthalpy using psychrometric equations contained in the ASHRAE Handbook of Fundamentals or the ASHRAE Brochure on Psychrometrics. Both of these documents contain psychrometric equations that can be solved for enthalpy if other parameters are known. Usually, the required inputs include dry bulb temperature, relative humidity or dew point, barometric pressure and elevation. For most applications, assuming a constant elevation and barometric pressure for the site will provide a satisfactory level of accuracy. As a result, the calculation can be reduced to one based on dry bulb temperature and humidity or dew point. For instance, outdoor air temperature and humidity are measured, then outdoor air enthalpy can be calculated and used for enthalpy based economizer interlocks assuming a return air enthalpy. Space or return air temperature and humidity sensors can be used to calculate space or return air enthalpy, and economizer changeover strategies based on differential enthalpy can be implemented.
3.6.4. Pressure Sensors and Switches
Pressure switches are also used in some systems to provide a safety interlock to prevent over or under pressurization of the duct or air handling unit casing in the event of a control failure. This is discussed in greater detail in the Chapter 3, Section 3.5: Control Strategies.
While pressure sensors are not used directly by the economizer cycle, they are used in some control strategies to modulate the relief air dampers, which relates to the economizer function. This is discussed in greater detail earlier in Chapter 3, Section 3.1.3: Building Pressure Control. When this approach is employed, building static pressure is measured and the system is arranged to modulate the relief dampers to maintain a slightly positive pressure in the building.