There have been a few rare but potentially fatal accidents involving exploding compressors due to an effect known as diesel effect. If the gas mixture being compressed contains enough air, the heat of compression can ignite the refrigeration oil in the cylinder, much the same way diesel oil is ignited in a diesel engine. The heat of compression plus the heat of combustion from the refrigeration oil then ignites the refrigerant in the cylinder, creating a dramatic increase in pressure which blows the compressor apart. Note that this is not just a phenomenon limited to flammable refrigerants, but can happen with A1 rated refrigerants such as 134a or 410A. How?
Refrigerants are rated for flammability according to ASTM E681 at a temperature of 60°C. Many refrigerants that will not burn under ASTM E681 conditions will burn at higher pressures and temperatures, including R-22, R-134a and R-410A. It is worth noting that in tests where they were trying to create diesel explosions, the University of Tokyo found no significant difference between the behavior of A1 refrigerants (R-22, R-410A) compared to A2L refrigerants (R1234yf, R32). They also found that compressing refrigerant and air mixtures without refrigeration oil did not create a diesel explosion. The refrigeration oil had to be present.
So how can we avoid compressor diesel effect explosions? Simply put: keep the air out. With no oxygen you cannot have an explosion. Air is never good for any refrigeration system anyway. It should not be news to anyone who has studied refrigeration at all that air does not belong in a refrigeration system. However, you may not realize that leaving air in the system not only hurts system performance and reduces the equipment life, it can create a real hazard to service technicians. Here are a few precautions you can take to avoid the specter of a diesel effect explosion in your refrigeration system.
• Check new installations for leaks using nitrogen and repair any leaks in the system. • Never use compressed air or oxygen for leak testing refrigeration systems. • Thoroughly evacuate the lines and coil of new split system installations AFTER verifying they don’t leak. • Never pump a system down into a vacuum. Reducing the low side pressure to a vacuum can suck in air through leaks or incorrectly positioned service valves. • Never jump out safety controls such a low-pressure switches. Forcing a system to run when it is low on refrigerant creates the possibility of sucking in air through leaks on the low side. • Don’t simply add charge to systems that are low on refrigerant. This is especially true for systems that are significantly low. You should find and repair the leaks.
Instructors worry about not knowing all the answers. I know that I do. Let me put you at ease. You don’t have to know all the answers to be an effective instructor. In fact, I feel that anyone who knows all the answers hasn’t asked enough questions. Our job as instructors is not to be a human search engine, but to teach students how to search for answers on their own. In short, to help students become more proficient at the learning process, specifically applied to our field of HVACR. To be clear, I am not suggesting that instructors do not need to be competent in HVACR. Nor am I saying you shouldn’t want to know as many answers as possible. Just that knowing every answer a student might ask is not necessary. Discomfort with areas where you don’t know the answers can sometimes cause instructors to cling to the specific areas of information they already know and refuse to broaden their scope. This is particularly true when it comes to new technology and industry developments. I believe this is an unconscious effort to “know everything” by limiting the scope of knowledge you expose yourself and your students to. Venturing into areas of new development can be uncomfortable because you don’t have as many answers at the ready. However, it is perfectly OK to tell a student that you don’t know the answer to a question they ask. Help them by directing them to resources where they might find answers. They are going to need research skills when they enter the field. Teaching students to learn on their own is probably the most important thing you can possibly teach them. On those many occasions when you DO know the answer, it can be more helpful to guide them through a search process than to simply hand them the answer. People tend to remember things they discover on their own more than things that people tell them. It takes discipline to do this. Providing the answer immediately basically concludes the interaction with the student. Asking leading questions or discussing relevant informative resources makes the interaction more of a dialogue and requires more student participation. It definitely takes a little longer, but provides a better long-term result. Remember, students learn more by what THEY DO, than by what you do.
When lecturing, you should be more interested in ensuring the students understand what you are saying than covering a specific amount of material. Even if you manage to vocalize every important piece of information about a particular subject, it is largely a wasted effort if the students are not receiving the information. Your job is not to state all relevant facts, but to communicate them to the students. It is easy for us to transmit data faster than the students can absorb it. Remember, you have seen all this information before, probably said it all before, many times. You are not having to mentally connect the facts into a logical framework because you have already done that. However, the students who are hearing it for the first time have to comprehend each statement and then tie the different statements together in some logical manner in order to really understand what they are hearing. Help them comprehend the information by including analogies, similes, and connecting statements. One of the most powerful teaching techniques is to introduce new concepts and ideas using things people already know and understand. For example, “the refrigeration system moves heat from one place to another, much like a sponge can absorb water in one place and then release it in another when you squeeze it.” Like all analogies, it is imperfect, but it starts the process of thinking about absorbing heat in one place and releasing it somewhere else. Once you get that point across you can start talking about what the refrigerant does to absorb heat. Maybe boil some water in a flask. Learning is not just collecting data, it is making mental connections between the data points to develop new concepts. This takes time. If you are talking at gigabit speed while your students are listening on dial-up, most of the information will be lost. I have been guilty of this. I can recall asking students questions at the end of a one hour lecture only to discover that they did not really understand something that I said 15 minutes into my lecture. So although I discharged my duty to cover everything, really, I just wasted everyone’s time – including mine! So when lecturing, take some time along the way to ask a few questions and engage in some dialogue with the students to make sure your message is being received. Remember, the idea is not to demonstrate your knowledge, but to help the students increase theirs.
Undoubtedly, many of you have some time at an amusement park, water park, or other recreational establishment. Probably not the summer of 2020, but hopefully at some point you found time for a planned day of fun. Many parks now have a single price for admission that lets you ride all the rides as many times as you like. After paying the price of admission, most folks try to ride as many rides as possible to get their money’s worth. I can remember planning out my day at Disney World so that I would make the most of my time. I know many of you are smiling because you have done the same thing and planned a manic day at a pricey amusement park so you would get your money’s worth. I read recently that a one day ticket to one park at Disney World is currently $109. But for only $1300 you can get an annual pass. I would have to be riding stuff every day if I paid that. Imagine how you would feel if after purchasing the ticket they announced that they were closing the park early. I bet you would be upset.
So here is my question. Why are we so intent on getting our money’s worth at an amusement park, but beg to be cheated in education? When you pay your tuition for the semester, you are paying to ride all the rides. Every lecture, every lab, every online assignment, and every test you already paid for when you paid tuition. Chances are, your tuition was more than $1300 and it was not for an entire year. If you think tuition and fees are high, why would you not take full advantage of all that you paid for? Try reading the assignments more than once. It does not cost any more, and you get more out of the assignment. When you miss a lecture, lab, or assignment you are cheating yourself. Not only are you not taking advantage of the services you have paid for, but you are also limiting your earning potential later on. My brother Richard has a saying “work hard at school, or work harder all your life.” People with jobs which require less education and skill work harder and longer for far less money. HVACR is a very performance based industry. If you can’t perform, your earning potential suffers. Your diploma may get you in the door, but it won’t keep your job. HVACR is also a very technical field. To excel, you need to understand the systems and how they operate. Sure, without a lot of training you can get a job holding the other end of heavy things or running to get tools for other people. But without training or education, you won’t advance much past that point. And of course, the folks holding the other end of the furnace don’t get paid the big bucks. Attending school is not the only way to learn your trade. There are many ways to educate yourself, but the easiest and fastest is to go to school. Throw yourself into your studies. Attend all the lectures, read all the assignments, and do all the labs. Ride all the rides!
A common problem that many students have across all levels of education is a failure to gain a thorough understanding of the subject matter being taught. I believe that the primary culprit is our over reliance on standardized, multiple-choice tests. Information is presented as a disjointed collection of individual facts to memorize so they can be recalled on a test. Think of these facts as data points. People make poor data storage devices. Computers do a much better job. Now that everyone carries a computer in their pocket that is connected via the internet to supercomputers all over the world, there is very little reason for people to spend much time practicing personal data storage by memorizing and recalling facts. Instead, we should focus on what we are better at: understanding. By studying relationships and processes in addition to data, we gain an understanding of subject matter that is far deeper and more consequential. This level of learning exceeds what is possible by simply storing “facts” in our imperfect personal data storage units.
It takes very little to make our collection of facts useless. A few years ago I was asked to write some technical literature for schools teaching HVAC in Georgia. I readily agreed, after all, I live in Georgia. After agreeing I found out the literature was to be for the Republic of Georgia, the one next to Russia! They don’t measure things in BTUs, CFM, tons of cooling, pounds, Fahrenheit, or any of the other thousand factoids I have rattling around in my head. Things like “400 CFM per ton” instantly became useless. Memorized snippets of code nearly as useless – I had to look up their laws and codes. Most every “fact” that I thought I knew became irrelevant. Fortunately, the principles that make the refrigeration cycle work are still the same. Although pressure is measured in kilopascals, temperature in Celsius, heating and cooling capacity in kilowatts, the processes and relationships are the same no matter which Georgia you are working in. While most of us will not have to worry about working in the “other” Georgia, we will have to adapt to technical advancements and changes which can make our set of “facts” just as useless. Take “400 CFM per ton”. Most new equipment does not come set for 400 CFM per ton out of the box anymore. New refrigerants are going to bring a whole new set of PT charts, so those saturated pressures at 45° and 100° are going to change. It is far easier to adapt to tomorrow’s technology if you truly understand today’s technology. Teach the processes, not an assortment of facts.
“I’m picking up bad vibrations. Bad, bad, bad, bad vibrations.” Oh, maybe that’s not exactly the way the Beach Boys tune goes. Now that you have that earworm, let’s get serious. My friend, John Terry with Canada Blower, has provided this excellent article on blower vibration. Enjoy!
The vibration generated by the fan is one of its most important technical characteristics. Vibration characteristics can be used to judge the quality of the design and manufacture of a product. Increased vibration may indicate improper installation of the fan, deterioration of its technical condition, etc.
For this reason, fan vibration is usually measured during mechanical run tests and during installation prior to commissioning. The vibration data of the fan is also used in the design of the fan support and the connected air ducting.
Fan vibration measurements can be expensive and sometimes far exceed the manufacturing cost of the product itself. Therefore, any restrictions on the values of individual discrete components of vibration or vibration parameters in frequency bands should be introduced only in cases where exceeding these values indicates a fan malfunction. The number of vibration measurement points should also be limited based on the intended use of the measurement results.
Increased vibration of the fan is one of the main causes of premature failure of units, parts, impeller, blades, bearing supports, couplings, destruction of the foundation and the fan itself as a whole.
Reasons for fan vibration are:
shaft or fan wheel imbalance;
misalignment of the drive;
wear or damage to bearings;
defects in the electromagnetic part of the drive or the electric motor.
The vibration level of the fans most accurately reflects the current technical condition of the fan, the quality of its assembly and installation. In other words, by monitoring the vibration level of the fan, it is possible to identify all of the above flaws and take timely measures to eliminate them, ensuring trouble-free operation of the fan.
Large Industrial Fan
A separate agreement may be made between the purchaser and the manufacturer for the installation conditions of the fan so that the factory test of the fan assembly takes into account the planned installation conditions in the field. In the absence of such agreement, restrictions on the type of substrate for factory tests are not established.
The fan manufacturer is responsible for balancing the fans. Balancing is usually carried out on highly sensitive specially designed balancing machines that allow an accurate estimate of the residual unbalance to be obtained.
A fan manufacturer can carry out balancing for several assembled elements at once which in addition to a wheel can include a shaft, a coupling, a pulley, etc.
If the fan is designed to operate from a variable speed drive, higher vibration levels are possible at other speeds due to the inevitable influence of resonances. It should be noted that stalling of the air flow can lead to increased levels of vibration. This is especially noticeable at large angles of the blade opening relative to the inlet air flow.
The vibration state of the fans after their installation is determined taking into account the rigidity of the support. When installed on large concrete foundations the support can usually be considered rigid and when installed on vibration isolators it is flexible. The steel frame on which fans are often installed can be considered either rigid or flexible. If in doubt about the type of fan support, calculations or tests can be performed to determine the first natural frequency of the system. In some cases the fan support should be considered rigid in one direction and flexible in the other.
Vibration Isolators
The vibration of any fan on site depends not only on the quality of its balancing. Installation factors such as the mass and stiffness of the support system will have an impact, for example. Because the support system affects vibration, the fan manufacturer is normally not responsible for the vibration level of the fan at the site of operation.
The vibration of newly commissioned fans must not exceed the commissioning level. As the fan is in use, it should expect an increase in its vibration level due to wear processes and the cumulative effect of influencing factors.
When the vibration reaches the “warning” level it is necessary to investigate the reasons for the increase in vibration and determine measures to reduce it. Fan operation in this state should be under constant supervision and limited by the time required to determine measures to eliminate the causes of increased vibration.
If the vibration level reaches the “stop” level, measures to eliminate the causes of increased vibration must be taken immediately. Otherwise, the fan must be stopped. A delay in bringing the vibration level to an acceptable level can lead to bearing damage, cracks in the rotor and in the welded areas of the fan housing and ultimately destruction of the fan.
When evaluating the vibration state of the fan, the vibration level changes over time should be monitored. A sudden change in vibration level indicates the need for immediate inspection and maintenance of the fan. When monitoring vibration changes, transients such as grease changes or maintenance procedures should not be taken into account.
In addition to blower wheels, fans include other rotating elements that can affect the vibration level of the fan: drive pulleys, belts, couplings, rotors of electric motors or other drive devices. If the order requires a fan without a drive unit it may not be practical for the manufacturer to test the entire assembly to determine vibration levels. In this case, even if the manufacturer balances the fan wheel, there is no assurance that the fan assembly will run smoothly until the fan shaft is connected to the drive and the entire machine is vibration tested when the fan is put into service.
Usually, additional balancing is required after assembly to reduce vibration to an acceptable level. Vibration measurements are recommended for new fans before being put into service. This will allow you to establish a baseline and chart further maintenance actions.
The measuring instruments and balancing machines used must be verified and meet the requirements of the task. The period between verifications is determined by the recommendations of the manufacturer of measuring instruments. The condition of the measuring instruments should ensure their normal operation during the entire testing period.
The correct choice of support or fan base design is essential for smooth, trouble-free operation. A frame structure made of structural steel or a base made of reinforced concrete is used to ensure the alignment of rotating units when installing a fan. Sometimes an attempt to save money during the construction of a support leads to the inability to maintain the required alignment of the machine components. This is especially unacceptable in the case where vibration is sensitive to a change in the degree of alignment. This is particularly the case for machines consisting of separate parts connected together by metal fasteners.
Fan Base
The foundation on which the base is laid can also influence the vibration of the fan and motor. If the natural frequency of vibration of the foundation is close to the speed of the fan or motor the foundation will resonate during the operation of the fan. This can be found by taking vibration measurements at some distance from each other across the entire foundation, the surrounding floor and on the fan supports.
The personnel working with measuring instruments must have sufficient skills and experience to identify possible malfunctions and deterioration in the quality of the measuring instruments. The terms of the contract can stipulate that the customer be provided with a certificate of testing the fan for the quality of balancing.
Excitation of vibration can be caused by the interaction of the fan wheel with stationary structural elements; such as, guide vanes, electric motor or bearing supports, incorrectly selected clearances or improperly designed air inlet and outlet. A characteristic feature of these sources is the occurrence of periodic vibration associated with the wheel rotation frequency against the background of random fluctuations in the interaction of the wheel blades with air. If the above reasons lead to vibration of the blades its nature can be investigated by installing vibration sensors in different places of the structure.
The basic principle of vibration monitoring of machines is to observe the results of correctly planned measurements in order to be able to identify a tendency for an increase in vibration level and consider it from the point of view of possible problems. Control is applicable in those situations where damage develops slowly and the deterioration of the state of the mechanism manifests itself through physical signs available to measurement. .
The vibration of the fans which is a consequence of the development of physical defects can be monitored at certain specified intervals. If an increase in the vibration level is detected, observations should be performed more often so that a detailed analysis of the condition can be done. The reasons for the change in vibration can be detected at the same time. Analyzing the frequency composition of vibration makes it possible to determine the list of necessary measures and plan their implementation long before the damage reaches serious consequences.
ASHRAE has page of downloadable position papers and recommendations for dealing with Covid 19 in HVAC systems and buildings. They updated their Building Readiness guide recommendations in January 2021 for reopening buildings. Some of the key points are
Flush buildings before and after occupied periods with at least 3 air changes of fresh air.
Use minimum MERV 13 filters. Higher MERV filters are preferred if your system can handle them
Introduce outdoor air ventilation continuously during occupied times, even if a small number of people are in the building.
Fan operation and ventilation are desirable, however, avoid airflow patterns that set up strong air currents.
Limit re-entry of contaminated air. For example, make sure your “fresh air” is not actually recirculated from inside exhaust air or pulled from an area where people congregate outside.
You may be familiar with the formula: BTUH = CFM x ΔT x 1.08. This same formula is often rearranged to use for determining airflow by measuring the heat input and temperature rise: CFM=BTUH/( ΔT x 1.08). To get the BTU per hour (BTUH) with electric strips you use the formula BTUH = volts x amps x 3.41 BTUH/watt. Together the formula looks like: CFM = (volt x amps x 3.41)/ ( ΔT x 1.08). The factor 3.41 comes from physics. It is the number of BTUs produced by one watt-hour of electricity. But where does the 1.08 factor come from? The “magic number” 1.08 is convenience factor. It is basically a bunch of math combined into one factor as a short cut.
You may recall that the specific heat formula is used for changing the temperature of something. The specific heat formula is BTU = weight x ΔT x Specific Heat This has one big problem, we don’t measure airflow by weight, but by volume. AHRI Standard air weighs 0.075 pounds per cubic foot. We can convert air volume to air weight by multiplying the air volume by 0.075 lbs/ft3. Another issue is that we tend to measure airflow by the minute and BTUs by the hour. You can fix that by multiplying times 60. Finally, we need the specific heat of air, which is 0.24. When you multiply the air volume by 0.075 to turn CFM into pounds per minute, multiply pounds per minute by 60 to get pounds per hour, and multiply by the specific heat of air 0.24, you end up with 1.08 (60 x .075 x 0.24 = 1.08). The number is not really a constant because the density of the air varies a lot with temperature, which changes the “magic number.” This formula is accurate for dry air at around 70°F, but it is NOT accurate when the air temperature gets very much colder or warmer than 70°F. For example, 1.08 really does not work with flue gas or airflow in freezers because the air density has changed, which changes the convenience factor. At 400°F the air only weighs 0.043 lbs/ft3, changing the convenience factor to 0.62 (60 x 0.043 x 0.24 = 0.62). At 0°F, air weighs 0.086 lbs/ft3, changing the convenience factor to 1.24 (60 x 0.086 x 0.24 = 1.24). Air density also changes with elevation and humidity, although the change due to humidity is small. Even the specific heat of air changes as the air temperature changes, but again, the changes are small. This is all to say that if you are dealing with air around room temperature, feel free to use the 1.08 convenience factor. However, if you are dealing with air at a much different temperature, you should look up the weight of air at the temperature you are working with. The table below lists the weight of a cubic foot of air at different temperatures and provides a reworked convenience factor so that you can perform correct air calculations at temperatures other than 70°F.
Zeotropic refrigerants, or blends, are now the most common refrigerants available. Any refrigerant whose ASHRAE number is 400 something is zeotropic. A whole list of somewhat confusing terms are used to describe the characteristics of these refrigerants. The first issue to tackle is simply what zeotropic means. I find remembering multisyllabic tehno-jargon much easier if I understand the derivation of the word. In ancient Greek zeo means “to boil” while tropo means “to change”. Putting the two together zeotrope means that something changes when it boils. That is why refrigerant mixtures whose percentage composition charges when they boil are referred to as zeotropic. Also in Greek, the prefix a means “not”. It basically inverts the meaning. That is why refrigerant mixtures that do NOT change when they boil are known as azeotropic.
Dew-point, Bubble-point, and Glide
Because zeotropic refrigerants change percentage composition as they change state, the temperatures at which they start to evaporate and condense are different for any given pressure. Dew-point describes the temperature at which the first liquid droplets start to form in a saturated vapor, and bubble-point describes the temperature at which the first bubbles start to appear in a saturated liquid. For a “normal” refrigerant these are the same temperature. For zeotropic refrigerants, at any given pressure the dewpoint is a little higher than the bubble-point. Glide is the difference between the bubble-point temperature and the dew-point temperature at any given pressure. Low glide blends typically have a glide of less than 2°F, while high glide blends have glides as high as 10°F. You may also have seen the term near-azeotropic. Personally, I think near-azeotropic sounds like something coined by the marketing department. It literally means “nearly does not change when it boils”. I believe a more useful description is a low-glide zeotropic mixture.
Zeotropic PT Charts
When calculating evaporator and condenser pressures, superheat, or subcooling I admit I have to think a bit when figuring out if I need to know the dew-point, bubble-point, saturated liquid temperature, or saturated vapor temperature. I find the terminology can be confusing. I would like to offer a way to keep these terms straight. Refrigerant pressure temperature charts typically have one column for pressure and two columns for temperature. The temperature columns will either be the dew-point and bubble-point, or saturated vapor and saturated liquid.
Saturated Vapor and Saturated Liquid
Since superheat involves determining the temperature of a gas in excess of its saturation temperature it makes sense to use the saturated vapor temperature when calculating superheat. Similarly, since subcooling involves determining the temperature of a liquid below its saturation temperature, it makes sense to use the saturated liquid temperature when calculating subcooling.
Dew-point and Bubble-point
The terms here can lead you astray because they describe the beginning of condensation or evaporation. However, they also describe the very last stage of any amount of vapor in a saturated liquid (bubble-point) or liquid in a saturated vapor (dew-point). The key here is to pay attention to the state the bulk of the refrigerant. For example, at bubble-point, most of the refrigerant is a liquid because the first bubbles are just starting to form. So, the bubble-point temperature is used when calculating subcooling. Bubble-point and the saturated liquid temperature describe the same condition. At dew-point, most of the refrigerant is a vapor because the first liquid droplets are just starting to form. So, the dew-point temperature is used when calculating superheat. Dew-point and saturated vapor temperature describe the same condition. I hope this helps you sort out all the techno-babble surrounding 400 series refrigerants.
