What is a Control Voltage Transformer?

Posted on January 17, 2011 by Jonathan B. Levine

Every machine with controls almost with 100% certainty has a control voltage transformer. Simply put, a control voltage transformer is an electrical system component designed to provide a reduced control voltage for the energizing of electromagnetic coils in motor starters, contactors, relays, and timers. These simple and straight forward items are the primary loads supplied by the control voltage transformer.
Control voltage transformers are also known as machine tool transformers, control circuit transformers, and industrial control transformers. They primarily differ from a power transformer in that they are designed to handle large inrush currents. When an electromagnetic control device is energized, the current it draws is often times a full order of magnitude higher than the holding current for the device. Making matters worse is the tendency of the secondary voltage to sag when a high load is placed upon them. Control voltage transformers are engineered to account for these large inrush currents and still maintain voltages at sufficient magnitudes to seal electromagnetic coils.
The two predominant voltages for the secondary side of control voltage transformers in HVAC equipment is 24 volts AC and 120 volts AC. 24 volts is predominant in light commercial equipment of 25 tons and less. 120 volts is common on large tonnage equipment and chillers. Air handlers probably have the most variation in control voltages, but 24 volt controls are gaining popularity with the advent of DDC controls over the last two decades.
Control voltage transformers are typically powered by the main unit connection in rooftop units and by a separate power feed in air handlers, chillers, and large specialty equipment. There is often an option for the control power to be factory connected to the line voltage. If this isn’t an option it is a relatively simple modification to make in a modification facility. Proper fusing of the primary side of the transformer is the critical step in designing this modification.

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Transformers – Multi-tap or Not?

Posted on January 12, 2011 by Jonathan B. Levine

In the mass production world, the use of a multitap transformer means added cost and even a relatively low priced transformer may be a few extra dollars per unit if you choose multi-tap over a dedicated purpose transformer.  In the realm of custom HVAC the rules are slightly different, but you may be choosing to save a few dollars in your design at the increased price of flexibility.  A flexibility you may not realize that you need until it is too late.

In a custom piece of equipment, the job site voltage is typically known in advance so the need to use a multi-tap transformer is avoided.  However a hidden danger is often lurking at the job site awaiting your machine’s arrival.  In the United States there are three popular three phase voltages available; 208, 230, and 460.  The hidden danger lies in the similarity of 208 volt and 230 volt systems.  Making this worse is that most motors designed to work at 230 volts will also work just fine at 208 volts.  They just draw a little more current.  So most people never stop to consider that there is indeed a difference in the two lower nominal voltages.

With transformers this is a significant problem.  A typical control voltage transformer that is setup to accept 230 volts on the primary side will often times be configured to output 120 volts on the secondary side.  If this same transformer is hooked up to a 208 volt primary, the secondary output will drop to nearly 105 volts.  This is significantly lower than the nominal 115 volts that a control circuit is designed to operate.  This lower voltage can result in chattering contactor and erratic operation from controllers that are hovering on the edge of a brown out condition.

By now you are probably wondering how this relates to your decisions to use multi-tap or not.  Here it is.  If you decided to save a few dollars on your control voltage transformer and install a dedicated 230:120 step down transformer and you ship your machine to the job site.  There is a strong possibility that the field agent failed to notice that the job site is actually 208 volts.  The only course of action at this point is to replace the transformer.  On the other hand if you had installed a multi-tap transformer you would be in the position of just having to have a service technician move a couple of wires and the output of your control voltage transformer would again be exactly where it needed to be.

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Custom vs. Customization Part II

Posted on November 8, 2010 by Jonathan B. Levine

In the last post we discussed what facets of a project defined it as being a custom unit. Today we are going to present what elements of a project make it a customization project. When we set out to customize a piece of HVAC equipment we are beginning with a standard off the shelf piece of equipment. This could be something as simple as a blower coil, or something as complex and expensive as a centrifugal chiller. When we seek to customize a particular piece of equipment we are looking only to change a small part of the item to meet the customer’s particular needs. This could be as mundane as resheaving the fan to meet a non-catalogued airflow; or it could be as rigorous as replacing all the electrical equipment in a rooftop unit with explosion proof equivalents so the unit can be safely installed in a classified location per the NEC.
If we want to continue the car analogy from the last post, a customized car might have a high performance transmission replacing the stock transmission, or we might add a nitrous oxide injection system to it, but the car started off as a 1970 Chevelle, and at one point it was driven off the dealers lot looking like a few million other 1970 Chevelles.
Something that has to be seriously considered and evaluated in the custom/customization business is when it makes sense to build a custom unit and wan it makes sense to customize a unit. At some point it actually becomes cheaper to start from scratch and design the unit from the ground up. It’s very hard to put an exact number on the quantity of modifications that have to happen to justify this, because the count depends upon the nature of each modification or customization. When the total cost of the modifications start equaling two times the cost of the base unit, it’s probably time to start thinking about building a unit from the ground up.
Agency listing or certification is another area that may drive you decision one way or another. A listing on a unit built from scratch can be a long time consuming process. It certainly isn’t impossible, but it can add a significant amount of time to your project. However when you start with a listed base unit it is relatively reasonable to contract UL, ETL, or any other independent testing laboratory to come and evaluate the changes you have made. If you built to an established code, and documented your changes well, it will typically take less than a day to realist a unit that has undergone modifications.

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Custom vs. Customization Part I

Posted on November 5, 2010 by Jonathan B. Levine

We specialize in the customization of standard HVAC equipment. There are times when we build custom HVAC equipment, and there are time we need to complete limited production runs of our customized equipment or our built from the ground up custom equipment. It can be a challenge to move from one project type to the next. If you don’t keep the differences is in mind as you sell, design, and build a given project you are all but doomed to failure; if not in execution at the very least financially. To begin we should probably begin by defining the differences in the three types of projects, only then can you begin to address some of the special needs of dealing with each of them.
Custom Unit
A custom unit is built from the ground up out of component parts. I’m not implying that the fan, compressor, or coil is fabricated. The major components are selected from catalogs and joined together in the right order to make a functioning piece of HVAC equipment. If this were an automobile it would be the same as buying a Chevy engine, a Ford transmission, hand making the frame and body panels, and eventually ending up with a vehicle that did exactly what you wanted. The time, labor, and material cost to complete such an endeavor would be astronomical to the price of just buying a new car. But you wouldn’t have to make any sacrifices on getting exactly what you want.
With a custom unit you could in theory build a unit with an unusual aspect ratio. If the unit had to fit into a conventional elevator, you could build the unit in modules where each piece of it would fit into the elevator and up to the penthouse. This might make sense financially if the cost to block off Whacker Drive while a crane parked in the middle of it was far greater that the price of building the custom unit. To continue the car analogy, if you had a garage that was only 6 feet wide, you might decide to build a car that would fit widthwise into your abnormally narrow garage.
Maybe the decision to build a custom unit has nothing to do with a physical constraint, but it is based upon a performance criteria. A typical air handling unit will have a minimum leakage rate. This might be five percent or it might be three percent. If you had an application where you could only tolerate one tenth of a percent it might make sense to build an air handler from scratch. Mercedes has an option similar to this on their engines. The standard engine is built on the assembly line. If you want a few extra certified horsepower out of the engine, you can get an engine that is hand built and assembled by one craftsman. The engine is identical in design to the assembly line version, there is just a lot more care and less slop in the final product.
That pretty well wraps up the thoughts behind a custom unit. Our next entry will define a customized unit and present some subtle differences between designing and managing the two projects.

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Nominal Airflow in Packaged Rooftop Units

Posted on November 3, 2010 by Jonathan B. Levine

The HVAC industry is rife with rules of thumb.  Just in case you just joined our little group of engineers and were formally the lead designer for cores of nuclear reactors, most HVAC calculations don’t have to be exact.  There isn’t enough time in a typical project to warrant the design to 6 decimal places.  I always chuckle when I see a schedule that lists an airflow with a number like 3456 cfm.  There is no test and balance technician on the planet that will measure the delivered airflow to that level of accuracy, let alone a velometer, flow hood, or flow meter that can measure with that level of precision.  That said I feel that that level of accuracy in precision in our delivered product lead to a great number of the rules of thumb we deal with.

One of the first rules of thumb that most young HVAC design engineers, technicians, and designers learn is that of nominal airflow.  The nominal airflow for a typical packaged rooftop unit is 400 cfm per ton of cooling capacity.  For example a 4 ton rooftop would nominally deliver 1600 cfm and a 25 ton rooftop unit would deliver 10,000 cfm nominally.  It really is that simple.

Another concept related to nominal airflow is cataloged airflow.  In the manufacturer’s catalog or technical guide is a list of approved airflows for a specific size unit.  These upper and lower bounds of allowable airflows represent the spectrum of airflows that a specific unit will properly operate across.  This doesn’t mean that the equipment will not properly operate outside this range, it just means that there might be problem if airflow is permitted continuously outside this range for extended periods.

If a particular piece of equipment is expected to operate at higher than catalogued airflows the following problems should be considered and evaluated:  Excessive pressure drop through filters; filters collapsing and being drawn out of their rack; excessive moisture carryover from cooling coils; poor latent cooling performance; high suction temperatures resulting in premature compressor failure; and potentially several others.  If the piece of equipment operates at too low of an air flow, then the following risks need to be considered:  frosting of the evaporator coil; low suction pressure; refrigerant slugging at the compressor; excessive temperature rise across the heat exchanger; and potentially several others.

As a designer of customized equipment it is important for you to remember that when you operate a piece of equipment’s airflow outside the nominal band listed in the manufacturer’s catalog you assume all the risk and your design must account for this risk to protect the machine, building, and its occupants.

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Energy Recovery Wheel Vocabulary

Posted on November 1, 2010 by Jonathan B. Levine

The rotating wheel heat exchanger, or energy recovery wheel, is comprised of a rotating disk filled with an air permeable material resulting in an extremely large surface area. The surface area is the material is utilized for the sensible energy transfer. As the wheel rotates between the ventilation and exhaust air streams it picks up heat energy and releases it into the colder air stream. The energy transfer is a simple as the difference in temperatures between the opposing air streams which is also called the thermal gradient. Typical media used consists of polymer(plastic) and aluminum.

The Enthalpy Exchange is accomplished through the use of desiccants embeded in the surface of the wheel media. Desiccants transfer moisture through the process of adsorption which is primarily caused by the difference in the partial pressure of humidity between the supply and exhaust air-streams. The most common desiccants in use consist of a silica gel compound and molecular sieves compounds that bind at the molecular level with the water molecule.

A typical wheel has two air streams.  The first is the fresh air stream.  The second is the exhaust air stream.  The fresh air stream can be divided into two smaller components; the first is the outdoor air component.  This is the fresh unconditioned air entering the energy recovery wheel.  It will eventually be delivered to the space.  The second component of the fresh air stream is the supply air component  The supply air component of the fresh air stream is the air leaving the wheel.  It has been conditioned i.e. either warmed up or cooled down and is closer in energy to the temperature of the space than unconditioned outdoor air.

The second airstream can also be divided into two subsections.  The first of these two subsections is the return air.  This is the air coming back from the space.  It is eventually destined for the outdoors once the energy present in it has been recovered.  The second of these subsections is the exhaust air.  This is the air that was return air before it passed through the energy recovery wheel.  All available energy has been extracted from it and it is now a waste product.

Another concept important to understand about energy recovery wheels is concurrent flow vs. counter flow.  Concurrent flow occurs when the two airflows are physically moving in the same direction.  The entering side of the wheel is the same for both air paths.  Counter flow is the opposite of this.  It is more efficient and leads to better energy recovery efficiencies.

Some energy recovery wheels will have a device known as a purge plate.  I will cover its exact operation in a dedicated blog entry, but for now understand that the purge plate is used to help prevent cross contamination between the two air streams.

As stated earlier, energy wheels rotate to exchange energy between the two streams.  This rotation can either be at a constant speed or at a variable speed.  If the wheel is rotating at a constant speed, the wheel leaving temperature is determined by the difference in temperature and humidity of the two air streams.  If the wheel will rotate at a variable speed the temperature of the leaving supply air can actually be modulated up and down, by spinning the wheel faster and slower through the use of a variable frequency drive.

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Smoke Detectors in RTUs and AHUs

Posted on October 29, 2010 by Jonathan B. Levine

Smoke detectors are often time required to be factory installed in air handling equipment.  Knowing when to install them, what type to install, and how to properly install them can save you time and money on a relatively low price item.

There are two basic types of smoke detectors: photoelectric and ionization.  The technology differences between the two of them are small but significant.  Photoelectric smoke detectors use a beam of light and a light detector to initiate the alarm.  The obvious conclusion to jump to upon hearing this is to assume it works just like a door alarm when you walk into a store.  The beam is shining on the detector and as the smoke fills up the detector it blocks the beam and the alarm goes off.  Unfortunately that isn’t the way they work.  To block the beam enough to set off the alarm there would need to be a lot of smoke in the detection chamber.  There is a better way.  The beam of light isn’t shining on the detector; in fact there is no light at all shining on the detector.  When the tiny particles of smoke enter the detection chamber, they will begin to scatter the light.  This scattered light will bounce around and some of it will hit the detector.  When this light hits the detector the alarm is initiated.

The ionization detector works on a different principle.  They use ionization energy and the detection of the minute current flow this produces to initiate the alarm.  In the ionization smoke detector there is a small source of alpha particles.  These alpha particles are a type of radioactivity.  They are the largest type of radioactive particles, and when they collide with an oxygen or nitrogen molecule in the ionization chamber they knock an electron out of the molecule.  The free electron now moves toward the positive plate in the ionization chamber.  The old molecule is now a positive ion and it is in turn attracted to the negative plate of the ionization chamber.  This migration of charges sets up a current through the ionization chamber.  The electronics of the detector measure this current and initiate an alarm should the current fall below a preset threshold.  The presence of smoke in the chamber attracts the positive ions and blocks the passage of the electrons keeping them from setting up a current.  Pretty simple eh?

Within the fire protection and prevention industry, it is recognized that neither sensor type, photoelectric nor ionization, is universally better at detecting all types of fires.  There is a loose consensus that a specific type of detector will work better in certain situations, but no authority will come out and declare these situations as gospel truth.  However it is believed that ionization sensors may respond slightly faster to flaming fires, whereas photoelectric sensors may respond slightly faster to smouldering fires.

That said, the NIST did observe that ionization detectors had a propensity to alarm from some of the aerosols that are produced early in the cooking process long before any visible smoke is produced.  If you wish to subscribe to this logic and observation, you may want to consider always installing photoelectric detectors in air handling equipment that serves kitchen and food production areas.

Finally I’d like to address the importance of installing an adequately sized aspiration tube that draws the air being sampled across the entire unit.  Quite often in an air handler there is a stratification of air streams and if there is smoke in the return air stream it might not mix with the outdoor air stream and not be detected if the detector only samples air on the outdoor air stratification layer.

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Equivalent Pipe Lengths

Posted on October 27, 2010 by Jonathan B. Levine

When designing the piping in a small system or machine, the pipe fittings are often what drive the pressure drop due to the fittings.  Understanding how to handle the pressure drop through these fittings is an important step in your journey to design maturity.

Flow through straight pipes has a pressure drop associated with it.  Flow through an abrupt opening or abrupt restriction has more pressure drop than flow through a gradual opening or restriction.  Turning a corner though a close radius 90 has more pressure drop than turning a corner through a long radius 90.  Turning a corner through the branch port of a tee has more than both of them.

A way of characterizing these pressure drop differences is something called equivalent length.  There are standard tables of equivalent length for all standard fittings.  All that you need to do to estimate pressure drop in a piping design is tally all the fittings sorted by type.  Each type of fitting has an equivalent length.  Multiply the number of that type of fitting times that fittings equivalent length and add all of those numbers to the length of straight pipe in you system.  It really is that simple.  Here is an example:

Assume that you have a system with 34 feet of 1″ pipe.  There are 6 90′s in the system as well.   Each 90 contributes 2 equivalent feet of pipe, so you can consider the total length of pipe in the system to be 34 + 6*2 or 46 feet of 1″ pipe.

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Pitot Tubes

Posted on October 25, 2010 by Jonathan B. Levine

The pitot tube was invented by the French engineer Henri Pitot in the early 18th century and was modified to its modern form in the mid-19th century by French scientist Henry Darcy. It is widely used to measure air velocities and pressures in HVAC applications.

There are main types of pitot tubes used in HVAC applications; static pressure pitot tubes, total pressure pitot tubes, and velocity pressure pitot tubes.  Each has its use in a different application.  The velocity pressure tube is a combination of the first two and requires two connections.  Today we will demonstrate the differences between each of these three types.

Static Pressure Pitot Tubes

Static Pressure Pitot Tube

The static pressure only pitot tube is used to measure static pressure in a duct or air stream.  Note the small ports drilled into the side of the tube and the closed end.  In a typical application the tube is pointed directly into the air stream.  When the tube is pointed directly into the air stream there is no component of the velocity pressure entering the static ports the the tube will only measure static pressure.  To implement this in a live application the tube should be inserted into the airstream and connected to a manometer.  The tube should then be rotated until the manometer indicates a minimum.  It is at this point that the pitot tube is theoretically reading only static pressure.  Lock the tube into position and proceed with calibration of any electronic sensors and transducers.

Total Pressure Pitot Tubes

Total Pressure Pitot Tube

The total pressure pitot tube is used to measure total pressure, a combination of static and velocity pressure in the duct or airstream.  When the tube is pointed directly into the airstream it will obviously measure velocity pressure.  In addition you need to recall that static pressure is also acting in this direction and is indicated on the gauge as well.  To install in a live system you should insert the tube and connect the tube to a manometer.  The tube  is then rotated until the manometer reads a maximum value.  It is at this point, the tube is capturing 100% of the velocity pressure plus the static pressure.  It should be noted that if the tube were rotated until a minimum value were indicated, it would be measuring static pressure only.

Velocity Pressure Pitot Tubes

Velocity Pressure Pitot Tube

The velocity pressure pitot tube differs from the static and total pressure pitot tube in that it requires two connections.  In reality, this tube design is just a combination of the two previous tubes in one engineered package.  One of the ports reads static pressure and the other port reads total pressure.  If you recall from our last discussion on static pressure in duct work, the velocity pressure is derived by subtracting these two values.  To install one of these insert the tube into the air stream and connect a manometer to just the total pressure side of the pitot tube.  Rotate the tube until the manometer is reading a maximum value.  At this point lock the tube in place and reconnect the tube so that the manometer is reading the differential across the pitot tube.  This is the velocity pressure.  The physical arrangement of the tube and the connections handles the subtraction of the two physical measurements.

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Static Pressure Basics

Posted on October 22, 2010 by Jonathan B. Levine

Understanding static pressure in air handling equipment is essential to a good design.  Static pressure considerations are also one of the fundamentals that are the most expensive to fix in the field if a unit was designed under poor assumptions or if an error was made during the design process.  Today we are going to cover some of the fundamentals of static pressure in HVAC equipment.

Static pressure, usually expressed in inches water column(iwc), is the pressure exerted on a surface at rest with respect to the air moving in duct, but not the pressure due to the motion or velocity of the air, and is also known as resistance, friction, friction loss, or pressure loss. Read that last sentence again.  Static pressure is the force exerted on the duct not due to the pressure from the moving air.  Another type of pressure in a duct is known as velocity pressure.  Velocity pressure is the force that the moving air exerts on a surface in the direction of the moving air.  The final type of pressure in a HVAC system is total pressure.  Total pressure is the algebraic sum of velocity pressure and static pressure.  Expressed mathematically

P_{total}=P_{velocity} + P_{static}

What is important to understand is that static pressure is exerted equally in all directions and that velocity pressure is exerted only in the direction of airflow.  This makes it difficult to directly measure velocity pressure in a duct.  Simply put, because static pressure is also pushing in the direction of airflow, you can never measure just velocity pressure. Practically, velocity pressure is calculated by measuring pressure perpendicular to the airflow(Static Pressure) and also measuring pressure parallel to the airflow(Total Pressure).  Once you have these two values you can just subtract static pressure from the total pressure and derive velocity pressure.

P_{velocity}=P_{total} + P_{static}

Static pressure is always measured relative to another pressure.  Typically it is measured with respect to atmospheric pressure, but it can be measured between any two points in the system.  Furthermore, because of the additive relationship between pressure measurements, you could take a pressure reading at two points in a system with respect to atmospheric and then subtract those two values and you will find that the derived value is equal to the pressure if it were measures strictly between the two points.  Mathematically

P_{1 to 2}=(P_{1}-P_{atm})-(P_{2}-P_{atm})

P_{1 to 2}=P_{1}-P_{atm}-P_{2}+P_{atm}

P_{1 to 2}=P_{1}-P_{2}

The final topic I want to touch on in this introduction to static pressure is Total Static Pressure(TSP), External Static Pressure(ESP), and Unit Static Pressure(USP).  The first thing to note is that total static pressure is different than total pressure.  Total Static Pressure is defined as the sum of External Static Pressure and Unit Static Pressure.

TSP=ESP + USP

External static pressure is the static pressure in the supply and return duct work that a fan would typically need to work against.  Unit static pressure, also known as Internal Static Pressure, is the pressure drop across filters, coils, and twists and turns inside the air handler.  As a custom unit designer it is your responsibility to calculate and measure the static pressure drop through your unit.  The external static pressure is usually a given to you.  The ESP is set by the building design engineer as they layout their ductwork, diffusers, and terminal devices that your custom unit would need to serve.

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