Monthly Archives: June 2016
It’s a documented (and unfortunate) fact that a significant percentage of HVAC refrigeration systems that have been serviced, or checked for seasonal operation on a regular basis, are overcharged. The underlying cause behind this problem is the misinterpretation of a suction side pressure reading that appears lower than normal, leading the technician to the conclusion that “adding a little gas” will bring the operation of the equipment to a higher level of performance. However, when refrigerant is added to a system without considering other factors that could affect the low-side pressure reading, the result is a negative effect on the performance and efficiency of the equipment. And, the factor at the top of the list regarding the proper operation of a refrigeration system is its relationship to the volume and velocity of air flow through the indoor coil.
Looking at this idea from a simple perspective, we’ll first consider the general approach of actual coil temperature and how it relates to what your gauges should show when you check a low side pressure. Generally, when considering the fundamental design of a tube and fin coil, it is common to find that the average temperature of the coil is about 5-degrees warmer than the refrigerant in the coil. What this comes down to is this: If a technician performed a simple temperature test of a comfort cooling system indoor coil at an approximate mid-way location with an accurate digital device, and the result was a 50-degree coil, the actual refrigerant temperature should then be 45-degrees. In Figure One, we’re showing the pressures that would result in both an R-410A and R-22 system with the 45-degree temperature we calculated.
From a theoretical point of view, this simple example explains the process of temperature affecting pressure. If the heat load in the building was found to be minimal once an accurate temperature was recorded, a lower suction pressure would be expected. And, a higher-than-normal heat load would result in a higher indoor coil (and subsequently, refrigerant) temperature, which, in the end would result in an increase in suction pressure. The point to keep in mind is that a properly charged refrigeration system in conjunction with correct air flow will allows the equipment to operate at the evaporating and condensing temperatures necessary for the efficient transfer of heat out of the building. The partial temperature-pressure chart in Figure Two explains this point further.
When we apply the 5-degree rule, and consider a 45-degree coil in a situation in which the air flow is correct, we arrive at the conclusion that a suction pressure of 118.1 could be expected for an R-410A system. And, considering R-22 equipment, the suction pressure could be as low as 68.6 PSIG.
What this comes down to is that, when accomplishing PM on a comfort cooling system, we can consider the temperature-pressure information above in conjunction with performing a simplified evaluation of air flow performance of the equipment. (See Figure Three).
With outdoor ambient temperature recorded, liquid line temperature measured, and superheat considered relative to manufacturer’s charging charts, four simple air flow temperature checks can provide valuable information about air flow through the duct system.
With a dry bulb temperature measurement accomplished first at the return air grille, and then finding a significant temperature difference with a second measurement at the point where the air enters the evaporator coil, the indication is return duct system leakage and/or insufficient insulation. A difference in wet bulb readings that ultimately shows a change in specific humidity indicates duct leakage that needs to be corrected. The same principles apply to differences in dry bulb and wet bulb temperature readings found between the leaving point of the evaporator coil and the supply register.
With these fundamental tests accomplished, and conditions corrected when necessary, the groundwork has been laid for further necessary testing to ensure the efficient operation of the equipment.
Learn From Yesterday…..Live For Today…..Look Forward To Tomorrow
When a technician finds that a three-phase motor has failed, replacing the motor is only the beginning of the repair. A critical thing to consider is whether or not the three phases of current applied to the motor are not in a stage of imbalance, which could be the underlying cause of the motor failure, and could shorten the life of the new motor. Consulting NEMA (National Electrical Manufacturers Association) Standard MG1-14.35 offers information on this subject. It reads:
Three phase induction motors are designed and manufactured such that all three phases of the winding are carefully balanced with respect to the number of turns, placement of the winding, and winding resistance. When line voltages applied to a poly-phase induction motor are not exactly the same, unbalanced currents will flow in the stator winding, the magnitude depending upon the amount of unbalance. A small amount of voltage unbalance may increase the current an excessive amount. The effect on the motor can be severe and the motor may overheat to the point of burnout.
From a technician’s perspective, this means that our responsibility is to make that that the voltages applied to each phase are evenly balanced as closely as possible, and the most effective way to determine the condition of the electrical supply is use a professional grade digital meter in order to obtain accurate measurements that will provide us with an average.
On the subject of the effects of imbalance, the NEMA standard also states:
The effect of unbalanced voltages on poly-phase induction motors is equivalent to the introduction of a “negative sequence voltage” having a rotation opposite to that occurring with balanced voltages. This negative sequence voltage produces in the air gap a flux rotating against the rotation of the rotor, tending to produce high currents. A small negative sequence voltage may produce in the winding currents considerably in excess of those present under balanced voltage conditions.
What this means to technicians is that when there is excessive current draw caused by a voltage imbalance, the end result is that the motor will operate at a higher-then-normal temperature, which can lead to premature failure. From a mathematical perspective, the percentage increase in temperature rise will be approximately two times the square of the percentage voltage imbalance. What this boils down is that what may seem like a small problem is actually a big problem. For example, a motor operating with a voltage imbalance of more than 2% will experience a temperature rise that is more than 10% above normal, And, yes, a 10% excess, when it comes to motor operating temperature, is a big deal.
To understand the importance of proper phase-to-phase voltage balance, consider an example….A 220-volt, three-phase motor that is operating with the following voltage measurements.
From L1 to L2: 216 Volts From L2 to L3: 223 Volts From L1 to L3: 225 Volts
With accurate measurements accomplished, the next step is to find the average of the three readings. We do this by adding the three readings together, then dividing the total by 3:
216 + 223 + 225 = 664
And….664 / 3 = 221.33
With an average calculated, we’re then going to apply that number individually via subtraction to each phase voltage reading to find out how much each individual phase is out of balance. One important factor to keep in mind about this step in the process is that we need to wind up with a positive number when we subtract. So, we need to ask the question….Is our calculated average voltage higher than any of the individual voltage readings we found?
The answer to that question is yes. One of our voltage readings (L1 to L2) was 216 volts, and our average is 221.33 volts, so our subtraction for each individual phase-to-phase reading will look like this:
221.33 – 216 = 5.33 Volts
223 – 221.33 = 1.67 Volts
225 – 221.33 = 3.67 Volts
Now, for those who easily wrap their heads around mathematics and formulas, the one that applies in our next step is:
Voltage Imbalance = 100 x Maximum Deviation From Average Voltage Divided By Average Voltage
And, for those who make sense of things from a less formulaic perspective….when you look at our calculations above, the largest imbalance is 5.33 volts, so that’s the number we’ll use in our next step in finding the amount excessive temperature we’re experiencing in our motor windings. We accomplish this by dividing 5.33 by our calculated average, multiplied by 100. We’re doing this so we’ll wind up with a percentage:
5.33 / 221.33 x 100 = 2.4%
So what we’ve determined with our basic arithmetic is that the motor in our example is operating with an imbalance beyond 2%. And, plugging that number into our next step is going to tell us what our excessive temperature rise is (in a percentage) in regard to this motor. To do this, we’ll take our calculated percentage of imbalance (2.4%) and square it:
2.4 x 2.4 = 5.76
Which brings us to our last step in calculating the percentage of excessive temperature in our motor windings. We need to plug what we know to be the maximum percentage of imbalance (as we said, 2%) into the actual imbalance percentage we calculated (2.4% squared….which gave us 5.76%), and we find:
2 x 5.76 = 11.52% Temperature Rise
An important thing for us to understand relative to this subject is that what may seem on the surface to be insignificant, is in reality a critical issue. For example, studies have shown that when a voltage imbalance reaches a level of 3%, it reduces the life of the winding insulation by one-fourth. And, if we found an imbalance situation that resulted in an excessive temperature of 50-degrees, the life of the insulation would be cut in half.
And, on this subject, I’ll leave you with a final thought:
Working from the theory that, as technical professionals, we need to invest time in studying so we can have a complete understanding of a given subject or situation, and then use the tools available to us to make things simple….in this case, not going through all the steps of calculation we just reviewed…. here is a link to a calculation tool that allows you to plug in your voltage readings and find out if the motor you’re evaluating is operating normally.
Learn From Yesterday…..Live For Today…..Look Forward To Tomorrow
If you were to ask someone in their 60’s about their experience of getting into the HVACR business and learning their craft, you would likely get a detailed story about the experience they could only get by leaving their house every day. It could be about their participation in a union apprenticeship program in which they were on the job for a period of time and attending classes a few evenings a week. It could be about their attendance at a year-long (or longer) trade school. Or, it could be about getting hired by an HVACR service company as a ‘step-and-fetchit’, starting out as a helper and working their way up through installation to service.
In either of the above situations, the experience was intense. When you showed up for your first day of work or school, you had very little information about the HVACR industry. Even things like basic terms and what they meant, along with the names of the basic components of a refrigeration system were things that you had no idea about, hearing them for the first time from your instructor or senior technician….trying your best to understand what you were hearing, seeing or doing with no advance information whatsoever before your exposure to this new information…..kind of like a deer caught in the headlights of an 18-wheeler.
My, how things have changed. If you’re interested in getting into the HVACR business today, the information that is available to you before you ever show up in a classroom, workshop, or on the job is staggering….. YouTube videos, blogs, discussion boards, manufacturer’s instruction manuals, etc…it, for better or worse, is out there for you. Of course, you need to know that the information you’re getting is technically and ethically correct and available from someone reputable. That’s a challenge that’s beyond just information overload. But, it’s possible to find good information before you ever show up for your first class or day of work.
It’s often just one click away. For example, here’s a click that will take you to a series of e-books you can open, view, and learn from:
If you know of other good information that’s available with just one click, let me know about it and I’ll add to the list here. My email address is email@example.com .
Learn From Yesterday….Live For Today…..Look Forward To Tomorrow