Friday, February 2, 2018

Understanding Loads in Series

We don’t often see loads wired in series with each other in HVACR, but you do occasionally run across a circuit which has two loads wired in series. Loads in series spit the voltage based on their individual resistance compared to the overall resistance. So if each load is about the same resistance, they will split the voltage in half. However, if one load has a much higher resistance than the other, the loads with the higher resistance will get the most voltage.


This diagram is a partial diagram of a residential refrigerator-freezer. It shows the defrost control, freezer fan, and defrost heat circuits. During normal operation the freezer fan is in series with the defrost heater, as shown in diagram 1. The heater has a resistance of around 28 ohms while the fan has a resistance of around 280 ohms. But since the fan is an inductive load, its total impedance (effective AC resistance) is much higher – more like 2800 ohms. The current for the circuit is very small 120 volts / 2828 = 0.04 amps. Voltage across each device is calculated by multiplying the current times the resistance. The heater voltage drop is 28 x .04 = 1 volt. The remaining 119 volts are all used by the fan. Essentially, the defrost heater is acting like a wire.


When the defrost control switch moves to the defrost position, the fan shuts off and the defrost heater energizes. The fan shuts off because now the same leg of power is applied to both sides of the fan. With no voltage difference from one side to the other, no current flows through the fan. The heater now has its own source of power ad receives a full 120 volts. The heater will stay energized until the defrost termination thermostat opens, breaking the circuit.

When the defrost control takes the freezer out of defrost (diagram 1) the fan will remain off until the coil is cold enough to close the defrost termination thermostat.

Monday, January 8, 2018

Unlocking the Secret Refrigerant Numbering Code

I have wondered why the new HFO refrigerant numbers look like an internet password. In short, the numbering system describes the chemical makeup of the refrigerant. But that is also true of the much simpler numbers, such as HCFC 22. So why does HFO 1233zd(E) look like a secret code? Mainly because the chemical is a bit more complicated. HFC 22 is a methane based molecule, with only one carbon. All that is needed to describe it is a way to determine how many fluorine, chlorine, and hydrogen atoms surround the single carbon atom. There is really only one way to put the molecule together.

HFO refrigerants are decidedly more complicated. They are built around a propene molecule. Propene has three carbon atoms surrounded by hydrogen atoms. Propene is similar to propane, except propane has all single bonds between its atoms while propene has a double bond between two of the carbons atoms.

You can think of each carbon atom as having four Velcro hooks. Molecules like propane only use 1 hook for each bond. This allows each carbon to bond to the most possible other atoms. Molecules constructed this way are referred to as saturated.  Two of the carbon atoms in a propene molecule use two Velcro straps to bond to each other, which reduces the number of other atoms the carbon molecules can bond with. Molecules built this way are referred to as unsaturated.

To unlock the secret code which describes fluorinated hydrocarbon refrigerants, just add 90 to the number, leaving off the leetrs for now. For example, 1233 + 90 = 1323. Working backwards from the right, the first number describes the number of fluorine atoms. In this case it is 3. The second number from the right describes the number of hydrogen atoms. In this case 2. The third number from the right describes the number of carbon atoms. In this case 3. The fourth number from the right lists the number of double bonds in the molecule. In this case 1. Notice the number of chlorine atoms was not addressed. The number of chlorine atoms is found by subtracting the fluorine and hydrogen atoms from the number of bonds. A propene molecule has 6 bonds. 6- 3 -2 = 1. There is one chlorine atom.

So what are the letters at the end of 1233zd(E)? The short answer is that all the letters following the number describe the particular molecular arrangement. We know that 1233zd(E) contains 3 carbons, 3 fluorines, 1 chlorine, and 2 hydrogens. However, even if you know exactly which atoms there are, you must also describe where they are attached because there are now many places to put them.

Each different arrangement of the same atoms produces different properties, so it is important to specify which arrangement the refrigerant is using. These different arrangements are called isomers. The two lower case letters after the number describe the specific arrangement (isomer). But note that this refrigerant number has yet another upper case letter after the two lower case letters. Some isomers have the same arrangement, but differ in spatial orientation. The upper case letter identifies which spatial orientation.

This is about as deep as I feel I should go in a blog post (maybe even a bit too deep). If you want more detail, it is all explained in the ASHRAE Standard 34-2016.

Thursday, December 28, 2017

Glove Cut Resistance Ratings

Did you know that gloves have ratings? When looking for a glove to protect your hands from cuts and scrapes you should get a pair that matches the required duty. There are actually two different glove ratings for cut resistance: ANSI/ISEA 105 and EN 388. In the United States we use ANSI/ISEA 105 which had a significant update in 2016.  There are nine levels, A1 – A9, with A9 being the most cut resistant. The gloves are tested by placing a fixed amount of pressure on a blade while moving it across the glove for a distance of 20 millimeters (roughly ¾ of an inch). The tool used is called a tomodynamometer which moves a razor blade slowly across the material being tested at a specified pressure for a specific length. The glove cut resistance levels are established based on the amount of pressure required to cut through the material. The Table below shows the nine levels in the ANSI system. You can see that the minimum recommended cut level for HVAC work is A4.


ANSI/ISEA 105 2016 Glove Cut Rating
Grade
Pressure Required to Cut Through
Recommended Use
A1
200-499 grams
General Purpose Material Handling
A2
500-999 grams
Packaging, paper handling
A3
1000 – 1499 grams
Handling construction materials
A4
1500 – 2199 grams
HVAC, duct work
A5
2200 – 2999 grams
HVAC, metal fabrication, metal stamping
A6
3000 – 3999 grams
HVAC, metal fabrication, metal stamping
A7
4000-4999 grams
HVAC, metal fabrication, glass manufacturing
A8
5000 – 5999 grams
HVAC, metal fabrication, glass manufacturing
A9
6000 or more grams
HVAC, metal fabrication, metal recycling


There is no requirement for glove manufacturers in the United States to test and label their gloves, so many gloves are sold without the cut rating. However, better manufacturers test and label their gloves. Look for gloves that have an ANSI/ISEA rating of at least A4 to protect your hands. To learn more, check out this page from Superior Glove Company.

Glove Rating Systems Explained

Wednesday, November 22, 2017

Replacing Wires Inside a Unit

Occasionally it is necessary to replace original factory wiring inside a unit. Sometimes critters have nibbled on them, sometimes the weather has degraded them, and sometimes the overheating or failure of a connected component has made the wire stiff and brittle. Whatever the reason, it is important to note that all wiring must meet the original manufacturer factory specification. Of course you should replace the original wire with the same material and gauge, but there is more to the specification than just the actual wire.

The wire insulation rating is just as important. For example, a wire from an NM-B cable should never be used to replace power wring inside a unit even though it might be the same material (copper) and gauge. The wiring inside most equipment is rated as machine tool wiring (MTW).  MTW insulation is a thermoplastic that is rated for up to 600 volts and is moisture, heat, and oil resistant. The insulation on NM-B cable is not moisture, heat, or oil resistant.

The wire insulation rating can be found printed or embossed on the wire. The figure below shows the marking.
Note that the wire gauge can be seen in the yellow circle and the insulation type, MTW or THHN, can be seen in the green circles. You should check to make sure any wiring you plan to use inside a unit meets the manufacturer’s original specification. This specification can often be found in the wiring diagram notes. A common note is that any replacement wiring must be rated for a temperature of 105°C. The insulation of NM-B cable is only rated for temperatures up to 60°C, and so should not be used inside the equipment cabinet.

Tuesday, November 14, 2017

Zero is NOT Nothing!

Many times people have remarked that even though they recovered a system to 0 psig or lower, when they opened the system, there still appeared to be some refrigerant. Often the presence of some refrigerant was most noticeable when they started brazing on a system that supposedly has nothing in it, and noxious green flames come out of the joint. This is because zero is not nothing. And no, that is not my “casual” grammar coming out. Don’t assume that because you have recovered a system down to 0 psig, or even into a vacuum, there is no refrigerant remaining. In fact, there can be quite bit of refrigerant in the compressor oil even under a vacuum. Since the refrigerant oil and the refrigerant are miscible, refrigerant dissolved in the refrigerant oil leaves the oil very slowly. The attached video shows refrigerant boiling out of oil removed from a compressor that was removed from a system which was recovered down to 28” of vacuum. The oil continued to boil for hours after being removed.



 This helps explain why system pressure can rise in a system which is left under a vacuum. This also explains why you should make sure the compressor oil sump heater(crankcase heater) is on before recovering refrigerant. If you are planning on recovering refrigerant from a system that has an operable compressor, run the system until the compressor is warm before beginning recovery. The compressor can draw out the refrigerant from the oil faster than your recovery machine. A heat gun applied to the bottom of the compressor can also help a great deal. While you are heating places that trap refrigerant, go ahead and heat the bottom of accumulators, receivers, and filter driers as well. A little time spent warming these areas trap oil and refrigerant will save time during recovery and evacuation. Remember, zero is not nothing!

Thursday, November 2, 2017

New HCFO Refrigerants

We might be seeing refrigerants containing chlorine again sometime in the near future. Some of the new olefin based refrigerants contain chlorine but still manage to have almost no ozone depletion potential. These chemicals are hydrochlorofluoro-olefins, or HCFOs. Why are chemical manufacturers taking a new look at chemicals containing chlorine? In short, to produce chemicals that have a low global warming potential, are non-flammable, and work at pressures common to “normal” refrigeration systems. Hold on - what about ozone depletion? Turns out, eliminating chlorine is not the only way to make a refrigerant that will not deplete the ozone. Another way it to make a compound with a very short atmospheric life. The atmospheric life of HCFOs is measured in days instead of years. HCFO refrigerants break apart quickly in the atmosphere, before they are able to reach the stratosphere. This same characteristic also helps reduce the global warming potential of HCFO refrigerants. For example, HCFO-1224yd(Z) has an atmospheric life of 21 days and HCFO 1233zd(E) has an atmospheric life of 26 days. They also have nearly 0 ozone depletion potential; for example HCFO 1233zd(E) has an ODP of 0.00012, which is commonly reported as 0.  Both a GWP less than 1. In addition, both have an A1 safety rating and can be used with both POE or napthenic mineral oil. HCFO 1233zd(E) from Honeywell and HCFO 1224yd(Z) from AGC Asahi Glass are currently the only two HCFO refrigerants I know about. They are both for low pressure centrifugal chillers. One more characteristic of these new HCFO refrigerants - they are very expensive. Like, if you have to ask how much you can't afford it expensive. 

Wednesday, September 27, 2017

Bearing Failure Leads to Cooked Winding

I ran across a failed capacitor start motor on an air compressor recently. It is obvious that the start winding got barbecued (see picture).




A student asked me how I knew right off that it was the start winding.  Notice that the winding which is burned has smaller gauge wire than the winding that appears OK. The start winding in single phase motors is constructed of smaller gauge wire than the run winding and has fewer turns. It is tempting to call this an electrical failure after seeing the cooked winding. However, most motor failures can be traced back to bearing failures.

Disassembling the motor we saw that the rotor had been dragging – a sure sign of bearing failure. (see picture)

The lead end bearing was to blame in this case. You can see that the dragging all took place on the lead end of the motor. Taking a close look at the stator you can see where the rotor has been rubbing the stator. (see picture)

This can cause two types of winding failures: one where the rotor knocks some of the metal layers into the slots where the windings are, and another where the rotor stays magnetically locked down at startup, which is what I believe happened here. Even though the rotor turns easily by hand and there is no play that can be casually observed by hand, it is obvious the bearing was allowing the rotor to touch the stator. If this happens on startup, the reaction will be like two magnets with opposite polarity pulling together. The motor will lock down, draw high current, and heat up.