Kele Blog

How Many Square Feet Does a Gas Sensor Cover?

Posted on by Kele Inc

That’s a common question. If there was a good, all-purpose answer to it, we’d make sure it was in every data sheet and on every Web page. Unfortunately, physical area isn’t the limiting factor in all cases.

The gases that mix well with air (CO, CO2) can sometimes be treated by area because they fill the space at a constant rate when they’re introduced, as long as barriers aren’t substantial. So for an open area, manufacturers will say their sensor has a “50-foot surveillance radius” or covers “5,000 square feet,” or something else that pretty much covers liability from the manufacturer’s perspective. That’s OK.

However, for gases that are either much lighter than air (methane, hydrogen, helium) or much heavier (most refrigerants, butane, propane, etc.), the placement of the sensor really has to be engineered.

If your customer has a chiller full of R-134A and it springs a leak, the gas will fall to the floor and spread just like water. If there’s a trough in the room, or a curb, or another obstruction, the gas will stop there and stack up until it flows over it. You need to observe the room and pick the best spot to put the sensor to catch the leak in the shortest amount of time. Low spots are the best bet, and the closer to the machine, the better. If there are multiple machines in a giant room, I wouldn’t get more than 25 feet from any given machine.

If you’re watching for methane (or natural gas) in a closed space, find the high spot for your sensor. Any leaking methane will head there first. Once you’ve found the high point, go find the building owner, the architect, the engineer, or someone in authority and ask why the heck they don’t have a vent in that closed room when there could be a potential leak.

Just think those gas sensor placements through before simply applying a square-foot-per-sensor rule to them. And call Kele—we’ll be glad to help.

How Much Propane is Left in My Grill Tank, Anyway?

Posted on by Kele Inc

Yes, we get questions from all directions at Kele tech support. This one started out as a level measurement problem, but it quickly turned residential in nature. Here’s how the Kele Engineering team handles it:

Somewhere on your propane tank is stamped the empty, or tare, weight. It’s usually on the protective collar around the valve with the designation T. Mine says T17.1, which indicates Tare = 17.1 pounds.

Weigh the tank with a scale that’s accurate in the under-50-pounds range, or else weigh yourself on an accurate scale both with and without the tank in your arms and take the difference in the two readings. At Kele R&D, we tend to prefer the Futek LTH500 super-high-precision load cells for critical pre-ribeye measurements, but we’re willing to compromise at hot dog time.

The difference between measured weight and tare weight is available fuel in pounds. Remember, you only know the tare to three significant digits, so don’t strain your calculator on the math.

So now you know how many pounds of propane are left. How long will it last?

Find the rated burner input for your grill. It should be on the nameplate or in the original owner’s manual. Our grill says 42,000 BTU/hr.

According to the National Fuel Gas Association, propane is good for about 20,000 BTU/lb.

Therefore, my grill will burn (42,000 BTU/hr) / (20,000 BTU/lb) or 2.1 lb/hr at the maximum burner setting. Your mileage will vary, depending on cleanliness, ambient temperature, wind speed, and a number of other conditions. It won’t vary much, though, for identical burners.

Actuator Sizing for Damper Applications

Posted on by Kele Inc

You have selected your damper by size and functional requirements, but now the question, “How much actuator do I need to obtain maximum close-off and to withstand the many cycles of operation?” This is a good question, and yet little information is published on this subject. Hopefully the following guidelines will aid you in your selection process.

Damper torque loading, inch pounds per square foot, used in selecting the correct size actuator should be provided by the damper manufacturer; however, if this information is not available, the following general guidelines can be used.

Damper Type Torque Loading Factor
Opposed blade, without seals, for non-tight close-off applications – 3 in-lb/sq.ft.
Parallel blade, without seals, for non-tight close-off applications – 4 in-lb/sq. ft.
Opposed blade, with seals, for tight close-off applications – 5 in-lb/sq. ft.
Parallel blade, with seals, for tight close-off applications – 7 in-lb/sq. ft.

Where direct mount (rotary motion) actuators are used, the above torque loading will work with most applications under 2” WC static pressure or 1000 FPM face velocity. For higher applications up to 3” WC or 2500 FPM, the torque loading should be increased by a multiplier of 1.5. If your application calls for even higher criteria up to 4” WC or 3000 FPM, use a multiplier of 2.0. This is a good general set of rules to follow. We have found it better to have more torque available because dirt build up and corrosion of the damper over time will put an extra burden on the actuator.

With these things taken into consideration, along with your initial static and velocity parameters, you may find it better to oversize the actuator by using the next larger unit available. This suggestion may offer you some insurance of tight close-off and longer life of the actuator.

Example 1: The damper is an opposed blade type with seals for tight shut off and has an area of 14 sq. ft. The design velocity through the damper is 2200 FPM. From the selection chart, you find the torque loading factor to be 5 in. lb. per sq. ft. The required torque would be calculated as follows:

Since the velocity is 2200 FPM, the torque loading will be increased by a multiplier of 1.5.
5 in.-lb. per sq. ft. x 1.5 = 7.5 in.-lb. per sq. ft. = Actual Torque
14 sq. ft. x 7.5 in.-lb. per sq. ft. = 105 in.-lb. = Required Actuator Torque
Select an actuator that meets or exceeds 105 in.-lb. of torque.
i.e.: Belimo AFB series spring return actuator and AMx series non-spring return are rated at 180 in.-lb.

The above method holds true when selecting direct mount (rotary motion) actuators. However, in choosing an electric (gear train or hydraulic) or pneumatic actuator where linking arms are used, you may find the manufacturer has rated their actuator by the square foot area it can effectively control. This is done, in part, because of the change in torque produced throughout the stroke of the actuator when using crank arms and linking rods. These ratings are typically best case scenarios based on nominal conditions and dampers without seals. To correctly apply these actuators, you must know the conditions under which the actuators will be used. If your application involves pressures or velocities higher than the conditions the actuators were rated for, or involves dampers with seals, you should derate the manufacturer’s square foot area by at least 33% (0.67 multiplier). If your application involves pressure or velocities higher than the conditions the actuators were rated for, and the dampers also have seals, you should derate the manufacturer’s square foot area by at least 50% (0.50 multiplier).

Example 2: An actuator is rated for 24 sq. ft. at 1” W.C. and 2000 FPM. The next larger actuator is rated for 40 sq. ft. under the same conditions. The application is for a 14 sq. ft. low leakage damper with seals, and a velocity of 2200 FPM. Which of these actuators should be used?

Since the application involves both seals and a velocity higher than the actuator was rated for, the actuator square foot rating must be reduced by 50%. Therefore, the 24 sq. ft. actuator should be applied to a damper no larger than 12 sq. ft. (24 sq. ft. X 0.50 = 12 sq. ft.). The actuator rated for 40 sq. ft. would be required in this application.

This method may not be as precise as selecting the direct mount actuators, but is a good rule of thumb to use as a general guideline to select an appropriate actuator. If you are still unsure of your selection, Kele’s sales department will be glad to assist you in making a suitable selection.

Not To Be Used As A Life Safety Device

Posted on by Kele Inc

Reprinted from Spring 2003 Insights

Not to be used as a life safety device – This phrase appears on a number of Kele catalog pages as a warning to customers that the particular product (usually a gas detector) is not to be relied upon to safeguard humans or animals from a particular hazard.  It may, though, be part of a control system that helps prevent the formation of a hazardous environment under normal conditions when the product is properly applied and maintained.  Even then, such devices, in most cases, cannot protect persons or animals that are intimate with the hazard.

Occasionally, a specification may insist upon a device that is listed or certified as a life safety device.  However, there is no such listing or certification available, except by manufacturers of certain firefighting equipment and personal portable gas monitors.  The only nationally recognized code that employs the term “life safety” is the Life Safety Code, and products are not specified in this code.  Its official title is actually “NFPA 101, Safety to Life from Fires in Buildings.”  There is no agency that certifies compliance with this code.

Some common products, which may be required to be applied as life safety devices, are classified into individual functional groups.  Fire alarm equipment, including smoke detectors, is such a group.  Boiler flame safety equipment is another group.  UL, Factory Mutual, CSA, and other certifying agencies will list and label approved equipment for use in these specific categories.  They are not listed and labeled as “life safety devices” but rather as “Fire Protection Signaling Equipment” or “Gas Flame Safety Equipment.”  Again, it must be emphasized that there is no such certification as “Life Safety” unless it is being applied by a manufacturer to firefighting equipment or personal portable protective equipment.  The Life Safety Code does not specify products; therefore, no product can claim compliance with it.

Even a gas sensor that is listed and labeled as a sensor by UL or another agency should never be used as a last line of defense for life.  Considering the following logic will give a better appreciation for the underlying reasons why:

Time

Gas sensors have inherent response times that are significant.  Even the fastest sensors cannot claim better than 30 seconds, and many refrigerant leak sensors may exceed 15 minutes in response to the presence of a gas.  If a multi-zone sensor is considered, response time for any one zone can exceed even the 15-minute level.  Basically, if a person or animal is in the machine room when a catastrophic release of refrigerant occurs, no sensor will save he/she or it from injury or death.

Power 

Gas sensors are powered by electricity, and there is rarely, if ever, a backup power system applied to them.  If the electricity goes out, the sensor cannot function.

Life Scan

The active portions of gas sensors have limited lives.  If not properly maintained, the sensor cannot function.

Location

Even if a gas sensor has an uninterruptible power supply and is properly maintained, it can only measure gas concentration at its specific location.  At the source of the leak, the concentration of the offending gas will be much higher.  A person or animal nearer to the source of the contaminant may be exposed to a lethal dose; however, the level may never rise to the personal exposure limit at the sensor location if the prevailing airflow is toward the source.

Absolute Protection

Absolute protection from inhalation hazards can only be provided by a source of known-to-be-clean air.  Self-contained breathing apparatus (SCBA) and remote air delivered by compressors and tubing are options that can actually safeguard a life from a toxic gas.  In areas where such exposure is likely, other codes require that such clean air sources be available.

To detect combustible or toxic mixtures at very low levels in order to protect lives in confined spaces or other potentially hazardous atmospheres, personal battery-operated monitors that perform well are available.  These can also overcome some of the shortfalls of fixed gas monitoring, as long as they are regularly tested and maintained.

In conclusion, fixed gas detectors may be used as part of a control system that is designed to prevent a hazardous situation from occurring.  These sensors may be used to signal a control system to begin ventilating a potential hazard or one that has already occurred.  However, none of these sensors can be relied upon absolutely to prevent health effects due to inhalation or combustion in a space that is already occupied.  Their reaction time, level of maintenance, source of power, and proximity to the individuals in question cannot be guaranteed.

Hazardous Atmospheres: Intrinsic Safety

Posted on by Kele Inc

Reprinted from Summer 1999 Insights

In the last edition of 20/20 Insights we covered the use of explosion proof construction to prevent a source of ignition from coming in contact with a room full of fuel and air. Strong enclosures with threaded or flanged covers can confine explosive forces within themselves and cool the escaping gases enough to prevent ignition of the surrounding atmosphere.Wouldn’t it be even better to prevent any explosion at all?

Intrinsically safe systems are designed to do just that. Remember that even if the ideally explosive mixture of fuel and air exists, an ignition source of sufficient energy and duration is required to light it up. Intrinsically safe design limits the available release of energy in the hazardous area to a level well below the minimum ignition energy of the worst-case gas mixture. It also maintains this limiting function in the event of two simultaneous worst-case faults. Since this type of protection is very fail-safe and requires no enclosure maintenance, most engineers (and property insurance carriers) consider it to be even safer than explosion proof construction. Surprisingly, it is usually less expensive to boot!

How does it work? First of all, let’s consider a room filled with hydrogen and air in exactly the best proportion for ignition (about 30 percent hydrogen by volume – see Figure 1). This mixture takes an instantaneous release of about 20 mJ in order to start burning, or else its temperature must be raised above the autoignition temperature of hydrogen (932 deg F or 500 deg C). For electrical devices we wish to place in this room, the amount of stored energy and the rate at which it can be released under worst-case conditions must be kept below these critical levels, and the surface temperature (see Table 2 for ratings) must be kept below the autoignition temperature. If these conditions are met (to the satisfaction of UL, FM, CSA, BASEEFA, etc… ), then the device may be listed and labeled as intrinsically safe. This means that, with a properly applied safety barrier, cabling and ground, it cannot start a fire even under the worst-case conditions. There is one subset of such intrinsically safe devices that can be used without a listing or label (although a barrier, proper cabling and ground are still required). These are designated “simple apparatus” and are a group of things that obviously neither produce nor store any energy (RTD’s, thermistors, switches, and a few others). They may be used as if they did have a label, as long as the operating limitations shown in Table 1 are strictly adhered to.

So, if we have an RTD and an intrinsically safe temperature transmitter in a hazardous location, can we wire them up to our controller and power supply in the safe area and turn them on? Not yet! Three more steps are needed first. While the devices in the hazardous area cannot ignite the gas mixture on their own, the controller and power supply in the safe area may each be capable of transmitting enough energy through the wires into the hazardous area to do the job anyway! An intrinsic safety barrier will prevent this, and is required for every intrinsically safe device (except listed self-contained battery powered units). Barriers range from simple to sophisticated, but they all use fail-safe components to limit the voltage and current that can be passed through them even in the event of worst-case wiring errors. For example, suppose someone in the safe area accidentally hooks 240 VAC up to our temperature transmitter signal lines, and one of the lines is frayed enough at the transmitter to cause an arc at that voltage. Without an intrinsic safety barrier, there may not be enough evidence left to figure out what happened. However, a barrier rated for the atmosphere, cabling, and its field device will save the day even under these circumstances. Figure 2 illustrates a simple zener diode barrier circuit. A high voltage at the safe side terminals will cause the zener diode to draw a high current and blow the input fuse. The series resistors limit the current to the hazardous side. Barriers are also rated for how much capacitance and inductance are allowed on the hazardous side.

This leads us into the cabling between the hazardous area device and its barrier. If the cable is very long and has a high capacitance, it could possibly store enough energy to cause an ignition in the event of a wiring fault. It must be checked against the rating of the barrier. If the field device (the transmitter in our example) has a high capacitance, the combined capacitance of the cable and device must be less than the barrier rating. Cable and device inductance are treated in the same manner.

The final factor to consider is grounding. What good is all this built-in electronic safety if a nearby lightning strike raises the ground potential a couple of thousand volts above the potential of the cable shield in our hazardous area? The resulting arc from the shield to ground can be every bit as effective as a butane lighter in touching off an explosion, and we needn’t have bothered with using intrinsically safe products. An intrinsic safety ground, bonded to the earth ground, is the last essential link that makes the system work. Normally provided at the barrier location, it keeps the cable shields at or near the same potential as the earth, even as that value moves around during storms.

Once it is all together, as shown in Figure 3, the benefits of an intrinsically safe system are many. There is no need to power systems down and”safe” the atmosphere for maintenance. Calibrate, adjust, even swap out bad parts with the system turned on and the atmosphere at its worst – a properly designed system cannot release enough energy to do any damage.

 

Hazardous Atmospheres: Explosion Proof

Posted on by Kele Inc

Reprint from Spring 1999 Insights

In the last edition of 20/20 Insights we discussed the elements that must be present in order to produce an explosion. The three legs of the “fire triangle” (fuel, oxygen, and an ignition source) are required to support combustion. In addition, the volume ratio of fuel to air must be within the fuel’s explosive limits, and the ignition source must release sufficient energy to ignite the mixture. Removing any of these three elements will eliminate the explosion hazard.

Perhaps the most common and familiar way to eliminate ignition sources from a hazardous location is through the use of explosion proof construction. An enclosure that is rated explosion proof (NEMA 7, 8, 9, or 10) for a particular hazard group (explained below) is strong enough to withstand the pressure of a worst-case explosion inside itself. Additionally, it is designed to vent the resulting hot gases in such a way that they are cooled below the ignition temperature of a worst-case explosive mixture outside the box.

To contain the pressures of an explosion, these enclosures are made from heavy cast steel or cast aluminum. To cool escaping gases, flanged enclosures have extra-wide flanges that are ground to a smooth finish and tight tolerance – thus yielding a very thin, very long path to the outside as shown in the illustration. Enclosures with threaded covers (and threaded connections to either type of enclosure) produce the same effect by virtue of the long, narrow path through the threads. As the hot gases from an internal explosion pass through these long, narrow channels, they give up heat to the metal and their pressure is reduced. These two effects team up to lower the temperature of the gases to a safe level before they can come in contact with the atmosphere surrounding the enclosure.

With flanged enclosures, it is very important to torque the cover bolts evenly and as close as possible to the recommended value. Also, the flange surfaces must not be scratched or marred in any way. Improper torque or damaged surfaces can allow hot gases to escape and ignite an explosive mixture outside the enclosure. Threaded connections or covers must engage at least five full threads to maintain the integrity of the system.

One additional step is needed to control the spread of hot gases – conduits entering the enclosure must be sealed within the code-required distance (usually 18”) of the box to prevent the buildup of pressure within the raceway system or the leakage of combustion products into the room. If a jacketed cable passes through a conduit seal, the jacket should be removed within the seal so that the sealing compound can completely surround each insulated conductor. An alternative is to seal the cable at the end of the jacket as shown in the illustration.

So where can these types of enclosures be used? As usual in our industry, there are no easy answers! Each explosion proof enclosure will be listed or labeled for use in a particular environment as defined in the National Electrical Code (NEC) or IEC Standards. In turn, the hazardous area itself must be classified according to the same standards. The enclosure must have a listing that meets or exceeds the classification of the area in which it is to be used. In the NEC, considerations are Class, Division, and Group as shown in the table below. IEC standards use different code letters from the NEC but generally follow the same logic.

For gases (the majority of our industry’s hazards), explosion proof enclosures are readily available for Class I, Division 1, Groups C and D. Enclosures rated for Group B (hydrogen) can also be found, but generally only in small sizes since it is so easily ignitable and has high explosive energy. Almost nothing is offered for Group A (acetylene) environments because of its easy ignition and tremendous explosive energy.

Using explosion proof enclosures removes the ignition leg from the “fire triangle” in a potentially hazardous location. Although it is costly and requires care to maintain the system’s integrity, it is an effective method for working with electricity in combustible atmospheres. In a future 20/20 Insights we will discuss intrinsically safe systems, another way to remove potential sources of ignition from hazardous locations.

Hazardous Atmospheres: Introduction

Posted on by Kele Inc

Reprinted from Winter 1998/1999 Insights

Automation dealers are continuing to gain business that was once reserved only for specialty and industrial contractors. It’s a trend that is accelerating very rapidly, and the fastest growth is in the areas of hazardous locations and the monitoring of toxic and combustible gases. Kele is committed to providing the products and technical support needed to assist our customers in these important areas. By way of introduction, this article covers the basics of a hazardous atmosphere and the equipment we use to monitor combustible gases. Future editions of 20/20 Insights will contain information relating to principles and application of intrinsically safe systems, explosion proof systems, and other available means of dealing with electrical equipment in hazardous atmospheres.

The widely known “Fire Triangle” illustration shows the three components required to support combustion. All three must be present, and the methods we use to prevent explosions are designed to eliminate one of the three legs of the triangle. What the triangle doesn’t show, though, is that fuel and oxygen must be mixed in the proper proportion in order to burn. If the fuel is methane (CH4, the major component of natural gas), the concentration in air must be between 5 percent and 15 percent or else the mixture will not ignite. Those of us old enough to have worked with finicky carburetors on gasoline engines are familiar with this principle. If the mixture was too lean (not enough fuel) or too rich (too much fuel), the engine would not start. The same applies to ignition of any combustible gas in air.

 The lowest concentration of a gas in air that will ignite is its lower explosive limit (LEL), and the highest concentration that will ignite is its upper explosive limit (UEL). These values are also sometimes referred to as the lower and upper flammability limits (LFL, UFL). Limits for some common fuels are shown in the table of flammability limits at right. If a system is designed to keep the fuel concentration below the LEL, the fuel leg is effectively removed from the fire triangle. Under certain conditions (in an oil field, for example), it is easier to maintain the concentration above the UEL. In this case, the oxygen leg is eliminated. In either case, combustion cannot happen.

Table of Flammability Limits

Note: Multiply percentages by 10,000 to convert to parts per million (ppm).

It is often advisable to monitor the concentration of fuel in air, in order to take action or sound an alarm if it is moving toward an explosive level. Kele has gas monitors that are ideal for this purpose. Some sensors provide a 4-20 mA signal over the range of 0 to 100 percent of the LEL, with an alarm relay set to energize at 25 percent of the LEL (an industry standard alarm point). For example, if the gas is methane, (5 percent LEL), the sensor will output 4 milliamps at 0 percent methane and 20 milliamps at 5 percent methane. The alarm relay will energize at 1.25 percent methane. With these devices, the alarm relay or an automation system responding to the analog signal can cause electricity (source of ignition) to be shut off in an area if the gas concentration is rising toward the explosive level (LEL). If the sensor itself must remain energized, an explosion proof enclosure is available to prevent it from becoming the source of ignition itself.

The system described here is based on removing the fuel leg from the fire triangle, then having an automatic means of removing the ignition source leg if the fuel begins to return. This is the first of many ways we will discuss to work safely with electricity in hazardous locations.

Measuring Flow in Tight Spots

Posted on by Kele Inc

Often, one of the most challenging aspects of applying a flow-sensing device is the hunt. Tracking down the elusive and mysterious twenty diameters of straight, accessible pipe that the sensor manufacturer demands can be impossible at times. Let’s face it – it isn’t often that the Architect, Engineer, General Contractor, and all the subcontractors conspire to make the automation guy’s job easier, is it?

If the length of straight pipe upstream and downstream of a flow measuring device doesn’t meet the manufacturer’s published specs, then the manufacturer’s guarantee of accuracy no longer applies. But just how bad will the results be? As long as a few common criteria are met, the answer is “not as bad as you’d expect!”

To insure the best accuracy possible, the flow profile of the fluid to be measured must be as uniform as possible across the pipe or duct, and the velocity must be high enough to ensure turbulent flow. With uniform turbulent flow, almost any placement of a differential pressure or turbine device across the pipe or duct diameter will give a good representation of the average fluid velocity. As the flow profile loses uniformity (close to an elbow or tee, for example), error is introduced since the device placement might be in a region with a higher or lower velocity than the actual average. The same effect can occur if the velocity slows enough to create laminar flow. The flow profile illustration to the right indicates these effects.

Robert Benedict’s Fundamentals of Temperature, Pressure, and Flow Measurement cites several studies in the International Journal of Heat and Fluid Flow which demonstrate that about 8 upstream diameters of straight pipe after an elbow are sufficient to produce ±1 percent variance with an orifice meter. Reduction of straight pipe to only 4 diameters yields ±2 percent variance if a constant correction factor of 0.98 is applied to the orifice discharge coefficient. In either case, the downstream straight pipe need only be two to four pipe diameters in length. If the upstream problem is more obstructive than a simple elbow (a valve, perhaps, or a bullhead tee), the lengths of straight pipe needed are closer to 16 diameters upstream for ±1 percent, and 8 diameters for ±2 percent. Table 1 is derived from Benedict’s work, and is valid for any fluid in fully developed turbulent flow. In typical HVAC applications, turbulence is pretty certain. If in doubt, check to see if the flow in question has a product of velocity (V, feet per minute) and the equivalent circular pipe or duct diameter (D, in inches) that meets the following criteria:

For Air, V x D > 650
For Water, V x D > 20
For Steam, V x D > 110

In summary, even if there isn’t enough straight pipe or duct to meet the manufacturer’s requirements, it is often possible to get a “pretty good” reading anyway, and the results will be very repeatable even if they’re off by a few percent.

Table 1: Effects of Upstream Obstructions on Flow Profiles. Add the “Resulting Variance” to the manufacturer’s stated accuracy to get an idea of expected behavior of a flow transmitter when the published straight pipe (or duct) length exceeds those in the table. While we cannot guarantee these figures for every application, they are valid for most HVAC/R flow ranges, and may even be improved upon with the use of straightening vanes or honeycomb flow straighteners.

Table 1: Effects of Upstream Obstructions on Flow Profiles

Lighting Controls Can Brighten Business Picture

Posted on by Kele Inc

Reprinted from January 1992 Insights

Forty to sixty percent of the average commercial utility bill is for lighting.  That bit of gloomy news for building owners is good news for companies that sell and install devices designed to control lighting and lighting costs.  Thanks to the availability of a variety of lighting control options, contractors can take advantage of this potentially lucrative business with a minimum of effort.  Convincing a customer to decide on a new or retrofit control installation is made easier because of attractive paybacks that can be projected.  These payback projections can be enhanced when tied to utility company rebates and investment programs that encourage these installations.  As a result, the customer saves money, relationships are reinforced, and energy waste is curbed as much as possible.

Lighting control methods can be relatively simple to extremely sophisticated – with price tags that escalate with complexity.  Among the more common of these methods in use today are: on-off control, occupancy sensors, and light level control.

On-Off Control

The simplest of all lighting controls is the use of mechanical or electronic time clocks. Their function is to turn lights on and off at pre-set times of the day. Each time clock can control one or more lighting circuits.  The down side to time clock control is lack of flexibility and override capability.

Flexibility can be achieved by interfacing with lighting panels that have either their own time-of-day capability or are controlled from a building automation system. The lighting panel allows multiple lighting circuits to be controlled from one switch or control point on an automation system.  These panels also allow flexibility in the assignment of those circuits to various time schedules.

Most lighting panels or automation systems allow for some sort of override function for after hours use and a “flash system” to notify tenants of an impending “lights out” condition.  Another way of accomplishing the override is with a SENTRY switch that replaces the normal light switch to provide manual override of the time-of-day function in individual zones and which can be “swept” off from the lighting panel or automation system.  A sweep function will reset the lights to an “off” condition periodically during unoccupied hours.  These types of systems may also incorporate a remote dial-in function.

The “time-of-day” capability can be especially effective in retail stores and supermarkets.  The store could automatically be illuminated at 100% during store hours, reduced to 60% during stocking, then further reduced to 40% illumination for janitorial duties.  As a side note, these time-of-day system scan be tied to a security system (like an override) and all the lights can be turned on if the building is burglarized.

Occupancy Sensors

Occupancy sensors provide another method of lighting control.  These devices will automatically turn lights off after a pre-determined amount of time if there is no one in a given area. There are a number of different types of occupancy sensors on the market today.  The most common of these are the infrared and ultrasonic occupancy sensors.  The infrared occupancy sensor detects body heat and the ultrasonic type senses a breakup of the ultrasonic signal due to motion. Some occupancy sensors have a wall switch replacement type of detector for easier installation.  In all cases, care must be taken in selecting the location, sensitivity and length of time for which these sensors are set, based on the type of sensor and where it is to be used.

Light Level Control

Lighting effectiveness is being built into modern office buildings and shopping malls as more windows and skylights are being used than ever before.  To take advantage of this, some lighting control systems allow for a footcandle setpoint and then control the level of lighting required based on a photocell input that senses actual footcandles in a given space.  A number of dimming and ballast control systems are available to achieve this type control.  Two types of sensors, photo resistive or photo diode, are available as inputs.  This method allows the natural lighting in an area to be utilized to its fullest.

Another advantage to this type of control is that various lighting levels can be set based on need.  Hallways and open areas would require less light than stores or office areas.

Parking lot and store sign lights can also be controlled. Typically the store lights would come on earlier than the parking lot lights for their advertising effect. The reverse would happen at closing. The store sign lights would go out first to let people know the store is closed.  The parking lot lights could remain on long enough to allow store employees to close the store or until the following dawn for security reasons.

Future Trends in Lighting

Higher efficiency lights and new types of reflectors that put more light into the space are the latest in attempts to reduce lighting costs.  As control manufacturers study and design new control systems, improved dimming and ballast controls, astronomical time of day control and new strategies in demand control using lighting will be introduced and enhanced.  This new lighting technology is continually creating a wide array of innovative products that offer increased savings for customers and, at the same time, provide contractors potentially greater profits.

Between now and the turn of the century, lighting controls could prove to be one of the brightest spots in your business plan.

 

The Time is Ripe To Do Something About Multi-Zone Air Handlers

Posted on by Kele Inc

Written by Gil Avery, P.E. – Reprint from September 1991 Insights

Multi-zone blow-through air handlers were popular 20 to 40 years ago when energy was cheap.  Fortunately these units are not as popular today.  Because they have so many negative qualities, they are prime for retrofit and offer a real opportunity to save lots of energy while improving the performance of the A.C. System.

A few of the negative features of conventional multi-zone air handlers are:

  • Multi-zone air handlers use a fan that blows into a hot and cold coil plenum.  As a result the fan discharge velocity pressure is lost.  Multi-zone fan systems require 20 to 30% more H.P. than single-zone draw-through air handlers, with the same CFM capacity.
  • The pressure drop through the coil section varies greatly with the position of the zone dampers.  Consider, for example, a multi-zone air handling unit with four equal zones.  Assume the drop through the cooling coil is 1″w.g. when all the dampers are in the full cooling position.  The drop will only be 1/16″w.g. if only one of the four cooling dampers is wide open and the other three are in the full heat position.  Almost 1″ of additional pressure is available to the cooling zone.  Balancing the air flow on multi-zone units is almost impossible.
  • Summer humidity conditions may be unacceptable.  Raw, humid outside air is bypassed around the cooling coil when a zone is not in the full cooling position.
  • The multi-zone unit is a re-heat unit. The zone thermostat is satisfied by mixing heated and cooled air and the re-heating and re-cooling are increased more than normal when the zone dampers are in a mixing mode.  The air flows have increased because of the decrease in the pressure drop across the coils.
  • The heating requirements of multi-zone units with an outside air economizer are high.  Zones requiring heat must use mixed outside and return air at 55°F instead of return air at room temperature.

Converting a constant volume multi-zone air handler to a variable air volume unit will correct most of these issues.

The changes that have to be made to convert to VAV include: 

Adding a variable volume control assembly to each zone of the multi-zone unit.  Each assembly consists of:

  1. Opposed blade zone damper (A) and zone thermostat (J) (Existing thermostat may be reused.)
  2. Modulating zone damper actuator (B)
  3. Reversing relay (C) for zone actuator (B)
  4. Two-position switching relay (D) for zone heating and cooling damper actuator (E)
  5. Adding a static pressure controller (SP) (Connect to the fan discharge plenum.)
  6. Adding a variable frequency drive (Control the drive speed with the static pressure controller. Many contractors just let the fan ride the curve, but the speed controller is generally a good investment, since the power varies as the cube of the flow. The savings add up fast.)

The controls will operate as follows: 

  • If the system is on a heating cycle (damper (F) open) and the space temperature is rising, control damper (A) will modulate closed to shut off the warm air.
  • After damper (A) is fully closed, hot deck damper (F) closes completely and cold deck damper (G) opens fully.
  • On a continued rise in room temperature, damper (A) modulates back open to provide cooling.
  • Cycle reverses on a drop in room temperature.
  • Fan speed controller (H) is modulated by the static pressure sensor (SP) to maintain the proper pressure in the fan discharge plenum.

Some of the features of this retrofit are:

  • Zone air volume and heating and cooling capacity will all be enhanced.  Converting to a VAV system takes full advantage of zone diversity.  The air is squeezed to the zone with the largest load.
  • Zone humidity conditions will be lower in the summer.  All the air passes through the cooling coil and is dehumidified when the zone is on a cooling cycle.
  • All mixing of heated and cooled air is eliminated, which means no more re-heat.
  • Fan H.P. will be reduced from 30-50%
  • Cooling load will not only be reduced by eliminating the re-heat but also by the redirection in fan H.P.

* Available from Kele