What is Vacuum Hose Conductance?

You may have heard or read the term “conductance” in reference to vacuum hoses and fittings. According to VAC AERO, conductance is volumetric flow rate divided by pressure drop, expressed as liters per second. Simplified, the conductance of a vacuum hose means its ability to allow gas to flow through it. The really important point is that no matter how big your vacuum pump is, it cannot move gas through a hose any faster than that hose’s conductance. Vac Aero

Hose conductance is not fixed, but varies with the type of gas, pressure, temperature, geometry of the passageway, hose diameter, and hose length. Duniway Stockroom Corp offers this formula: Conductance = 75 x Diameter³/Length. Note this formula calculates the conductance through a smooth tube in liters per second for dry air at 75°F and very low vacuums (under 50 microns). It won’t really calculate the conductance of a hose removing water vapor or refrigerant at atmospheric pressure. However, we can use it to get some general idea of the effect of diameter and length on hose conductance. Duniway

For example, the conductance of a ¼” diameter, 60” hose would be 75 x 0.25³ /60 = 75 x .015625/60 = 0.0195 liters per second. Turning that into cubic feet per minute (CFM) we get 0.0414 CFM. Suppose we shorten the hose to 36 inches. Now the conductance is 75 x 0.15625/36 = 0.03255 liters per second. That translates to 0.069 CFM. More than a 50% increase just by switching from a 60” hose to a 36” hose. What about changing the diameter? Using a 3/8” hose that is 60 inches long, the conductance becomes 75 x 0.375³/60 = 75 x 0.0527/60 = 0.066 liters per second. Translated into CFM, that is 0.14 CFM. We get over three times the conductance by increasing the diameter to 3/8”. Using similar calculations for ½” and ¾” we get 0.33 CFM and 1.12 CFM respectfully. So comparing different diameter hoses using this formula we see that a 3/8” hose has over three times the conductance of a ¼” hose, a ½” hose has more than twice the conductance of a 3/8” hose, and a ¾” hose has more than three times the conductance of a ½” hose. All together, a ¾” hose has 27 times the conductance of a ¼” hose. Large diameter hoses really do make a difference in the time it takes t pull a vacuum. However, there are other restrictions that must be addressed before the hose size matters: the Schrader valve cores. We’ll talk about them next time.

Wiring Communicating Controls

In some ways, communicating controls are easier to wire than conventional 24 volt control systems. There are fewer wires, and generally speaking, everything connects letter to letter. Still, it seems technicians keep figuring out ways to incorrectly wire four wires.

Wire Type

Early communicating systems used shielded cables that looked like computer cables – because that is what they were. You did not connect bare wires to anything, you plugged in the connector to the socket on the control board. Besides being somewhat expensive, these proved to be less robust than was needed for equipment installed outside. Most communicating control systems for traditional split systems and packaged units work fine with traditional thermostat wire: 18 gauge. In fact, the connections are designed expecting that type of wire.  Do not use anything smaller than 18 gauge. The control wire is generally not shielded. The control wire should not be run parallel to power wire to avoid interference. If you have to cross a power wire, it is best to do it at right angles. As with any installation, the control wire and power wire should not be run in the same conduit. It is fine to tape the wire to the line-set.

Mini-Splits

Nearly all mini-splits and multi-splits use communicating controls. Their wiring breaks all the rules discussed above. Typically, they use 14-4 cables. These cables contain four #14 gauge, stranded wires. Two wires in the cable are for powering the indoor unit and two provide communication. Sometimes this cable is shielded. If you do use shielded cable it is important to only ground one end, not both. I prefer grounding the end at the outdoor unit because you are closest to the power supply and the equipment ground. The connectors on mini-split and multi-split units are made to use stranded wire. Although there are only four connections, it is important that the same wire used to connect to each outdoor terminal connects to a similarly labeled indoor terminal. Sounds simple, but it is amazingly easy to cross up even just four wires. After connecting the outdoor wires, take a picture with your phone so you can verify that you are connecting them to the correct terminals inside.

Multi-Splits

Most multi-split units have terminal connections for each head at the outdoor unit. A 14-4 wire runs from the outdoor unit to each head. A crucial detail is to insure that the wires for each head correspond to the correct set of refrigerant lines. Although this seems simple, it is very easy to screw up. One way to avoid getting wires and lines crossed is to tape the 14-4 cable for each head to the line-set for each head.

Read and Follow Instructions

When it comes right down to it, most installation issues could be avoided by actually reading and following the instructions. Most manufacturers provide specific instructions for wiring their equipment, even if it is as simple as connecting four labeled terminals to four other similarly labeled terminals. There is not really an industry standard for the terminal labels. However, that is not important if you follow the manufacturer’s instructions. Most of the time, whatever label is used outside it also used inside.  

Low Global Warming Potential Refrigerants

You probably have heard that the most popular HFC refrigerants being widely used today are global warming gasses. In fact, some popular HFC refrigerants have higher GWPs than the CFCs and HCFCs they replaced. A refrigerant’s Global warming potential (GWP) compares it to CO², the global warming gas produced by burning hydrocarbons. A GWP of 1 indicates that a gas has the same effect on global warming as CO². The retired popular air conditioning refrigerant, HCFC 22, has a GWP of  1760. HFC 410A that is now widely used in air conditioning applications has a GWP of 1924. It is actually worse! Meanwhile HFC 404A, popular in refrigeration applications, has a GWP of 3943. HFC 134a is popular in domestic refrigerators, commercial refrigeration, and car air conditioning has a GWP of 1300. These high GWP numbers have made HFC refrigerants the target of regulatory efforts to limit their use and replace them with more environmentally friendly refrigerants. Europe has moved aggressively, passing their F-Gas regulations. The ultimate objective of the F-Gas Regulations is to cut the availability of HFCs by 79% between 2015 and 2030. There will also be a servicing ban on HFCs with a GWP >2500 for certain sectors. Here is a link to a quick overview of the F-Gas regulations byMitsubishi.

While the US has not moved nearly as aggressively, there have been attempts by the EPA to regulate refrigerants based on their GWP. Worldwide regulatory restrictions on current HFC refrigerants has spurred development of lower GWP refrigerants. Manufacturers in the HVACR industry have been actively developing lower GWP alternative refrigerants.

HYDROCARBONS

Propane (R290), Isobutane (R600a), and R441A all have very low GWPs of (3, 3,0). They are all non-ozone depleting and non-toxic. Their limitation is their flammability – they are all highly flammable. In the US they are approved only for systems with a charge of 150 grams (5 ounces) or less. In Europe hydrocarbon refrigerants have been used in refrigerators and freezers for years. These refrigerants are now common in residential refrigerator and small commercial refrigeration units in the US. While highly flammable refrigerants are likely to remain a factor in small commercial refrigeration systems, it is unlikely that these refrigerants will be used in larger systems in the US due to our aversion for being sued and the large number of lawyers in the US.

CO² R744

It is interesting that the main global warming culprit, CO², is also a refrigerant with a very low GWP of 1. It does not deplete the ozone, it is non-toxic, non-flammable, and cheap. What’s not to like? Unfortunately, CO² has a critical temperature of 88°F. It cannot condense above 88°F. This means that CO² systems are not “normal” systems. CO² systems must either be transcritical or cascade systems. Transcritical systems operate at very high pressures of 1200 – 1500 psig on the high side. Cascade systems use the evaporator of one system to cool the condenser of another system. Either way, CO² systems are more complicated and expensive than traditional system. One place that CO² has taken root is in large scale commercial refrigeration rack systems. Complexity in large rack refrigeration systems is normal and the extra cost of the transcritcial components is offset by the savings in refrigerant cost. However, in smaller scale systems the cost of a CO² system is prohibitive. For a quick explanation of a transcritical system check out https://www.achrnews.com/articles/94092-co2-as-refrigerant-the-transcritical-cycle

AMMONIA R717

Ammonia refrigeration has been around since the earliest days of refrigeration. Ammonia has always been used in large scale food commercial refrigeration and freezing for food processing because of its efficiency and low cost. Unfortunately, ammonia (R-717) has many application challenges. It is toxic, somewhat flammable, and cannot be used with some metals, such as brass or copper. It will continue to be a mainstay of commercial food processing, but I doubt you will see it expand into other market segments.

LOWER GWP HFCs

There are some HFC refrigerants that have a GWP in the hundreds instead of the thousands. While these refrigerants are probably not long-term solutions, they can provide a way to drastically reduce the GWP footprint of a system without a drastic change in technology or design.

R 32

HFC R-32 has been adopted by many manufacturers in air conditioning systems sold outside of the United States. R-32 is an HFC with a lower GWP of 667. That is still not really low compared to CO2 (GWP 1) or ammonia (GWP 0), but it is considerably lower than R404A, R410A, or R134a. HFC32 has the advantage of being a relatively “normal” refrigerant, making designing systems to use it less challenging than say, CO². However, R-32 is flammable. While not as flammable as propane, it does burn. That precludes its use in most applications in the US, at least right now. The building and safety codes in the US do not allow a flammable refrigerant in systems where the air in the building flows directly over the evaporator. These codes make no distinction between A2L and A3 refrigerants. To them, flammable is flammable.  Manufacturers and code officials in the US are working to determine what new requirements an A2L refrigerant system should have to make it safe for use. The one place you will find R32 in the US is in window air conditioners. The EPA allows use of R32 in limited quantities in window units. Here is a link for more information on R32. 

R466A (Solstice N41)

Honeywell has developed an A1 rated, non-flammable HFC based refrigerant with a GWP of 733. Like R-32, R-466A provides a refrigerant with a much lower GWP than HFC refrigerants currently in use, but not really low. Its big advantage over R32 is that it is non-flammable. R466A achieves this by using a mix of 49% R32, 11.5% R125, and 39.5% R1311. R32 and R125 are the two components found in R410A. R1311 has been previously used as a fire suppressant. This blend performs similarly to R410A, making adoption relatively easy.  Here is a link to more information on R466A. 

HFOs

Hydrofluoroolefins (HFOs) are a special type of HFC. They have at least one carbon double bond, making them less chemically stable than a “normal” HFC which has all single bonds. Because they are less chemically stable, they do not persist in the atmosphere for long, and this reduces their global warming potential. For example, HFO1233zd has a GWP of 0. HFO1233zd is a low pressure refrigerant for chiller applications. It has an A1 safety rating and does not deplete the ozone. HFO1234yf has a GWP less than 1. It has an A2L safety rating – meaning that it is somewhat flammable. HFO1234yf is used in auto air conditioning systems. It has been what most auto manufacturers now use instead of HFC134a. Here is a link to more information on HFOs.

R1234ze

R1233zd

R1234yf 

Lower GWP refrigerants are the future of HVACR. Some old and some new. Understanding how to safely work with these lower GWP refrigerants will be an important part of all technician’s knowledge set going forward.

Refrigerant Flammability Safety Rating

This is a re-post of an article I posted earlier. Flammable refrigerants are now really a fact of life, and so it is important that technicians understand the different classifications regarding refrigerant flammability. In particular, it is helpful to understand the difference between class 3 highly flammable refrigerants and class 2L, lower flammability refrigerants. 

I confess that I have always thought of flammability as an either or question: it either burns or it doesn’t. So the concept of different levels of flammability was a hard one for me to grasp. I wondered: what is the difference between 3,2, and 2L refrigerant designations? What follows is a somewhat lengthy discussion of what I learned.

First off, I found that it is not all that simple. There are several flammability characteristics that can be compared: lower flammability limit, upper flammability limit, auto ignition temperature, minimum ignition energy, heat of combustion, and flame velocity. The table at the bottom of the article shows these different specifications for a small selection of flammable refrigerants. Note that pressure and temperature also play a part. For the ASHRAE safety tests, a temperature of 140°F at atmospheric pressure is specified. You get different results when applying higher pressures and temperatures.

The original three classifications (1,2,3) were determined by the lower flammability limit and the heat of combustion. Later, ASHRAE added a 2L category for refrigerants with burning velocities less than 10 centimeters per second. The table below summarizes the different flammability classifications.

Classification	Lower Flammability Limit % by volume	Heat of Combustion	Burning Velocity
1	Does not support combustion at atmospheric pressure
2L	Greater than 3.5%	Less than 19 kj/g	10 cm/s or less
2	Greater than 3.5%	Less than 19 kj/g	Greater than 10 cm/s
3	3.5% or less	19 kj/g or more	NA

Lower flammability limit (LFL) is the minimum percentage required in air to be combustible. For example propane (R290) has an LFL of 2.1% by volume while ammonia (R717) has an LFL of 15%. Notice that propane only requires 2.1% while ammonia requires 15%. So that is one difference – the amount that must build up before it can burn.

The upper flammability limit (UFL) describes the maximum concentration which will still burn. If the concentration of flammable vapors exceeds the UFL, it will not ignite. It is more difficult to draw a straight line comparison using the UFL. However, you can say that refrigerants whose LFL and UFL are closer together are generally a bit safer simply because the conditions for a flammable mixture are less likely to occur.

The auto ignition temperature is the temperature which the flammable mixture will ignite. With the exception of 1234yf, the lower flammability refrigerants have higher auto ignition temperatures than the more flammable refrigerants.

The minimum ignition energy is a bit different than the auto ignition temperature. It is the amount of energy that must be used to ignite a flammable mixture, measured in megajoules. Note that in this case R1234yf stands out because the minimum ignition energy is so high compared to the other refrigerants. Also note that the class 2L refrigerants all have minimum ignition energy ratings in the hundreds of megajoules or higher while propane’s minimum ignition energy is a very small 0.25 megajoules. Basically, this means it takes a lot more energy to ignite the 2L refrigerants than a highly flammable refrigerant such as propane. Again, this means that the chance of having the right condition for combustion is much lower for class 2L refrigerants.

The heat of combustion is a measure of the amount of heat created when the refrigerant burns. Note that the class 2L and class 2 refrigerants have a heat of combustion in the single digits per gram while propane jumps to 46 kilojoules per gram. This means that the heat produced by combustion of a class 2L or class 2 refrigerant is far less than a class 3 refrigerant. Indeed, it would be possible for a class 2L refrigerant to burn and not ignite other nearby flammable materials.

Burning velocity is the characteristic which distinguishes 2 and 2L refrigerants. It is the speed with which the flame advances. Note that the 2L class refrigerants have a burning velocity in the single digits while 152a, a class 2 refrigerant, has a burning velocity of 23 cm/sec. Propane’s burning velocity is twice that of 152a. The take home point here is that the flames from higher flammability refrigerants spread faster.

So wrapping it up, my general impression is that lower flammability refrigerants are less likely to burn in the first place and when they do burn, the flames are not as hot and do not spread as quickly as a high flammability refrigerant such as propane.

Refrigerant	R1234yf	R32	717 Ammonia	152a	290 Propane
Safety Group	A2L	A2L	B2L	A2	A3
Lower Flammability LImit	6.5%	14.4%	15%	3.9%	2.1%
Upper Flammability Limit	12.3%	33.3%	28%	16.9%	10%
Auto Ignition Temperature	405°C	648°C	651°C	440°C	455°C
Minimum Ignition Energy	5,000 – 10,000 mJ	30 – 100 mJ	100 – 300 mJ	0.38 mJ	0.25 mJ
Heat of Combustion	9.5 kJ/g	9 kJ/g	22.5 kJ/g	6.3 kJ/g	46.3 kj/g
Burning Velocity	1.5 cm/sec	6.7 cm/sec	7.2 cm/sec	23 cm/sec	46 cm/sec