Measuring airflow is easy… measuring airflow accurately is quite a bit more difficult. In many cases when we as technicians measure airflow we are trying to get to the almighty CFM (Cubic Feet per Minute) volume measurement. You can take CFM readings fairly easily with a hood like the Testo 420 shown above, but even a hood has some limitations when the goal is to measure total system CFM vs. register / grille CFM.
In this series of videos Bill Spohn from Trutech tools demonstrates all of the tools you can use to measure airflow from hot wire and rotating vane anemometers, to flow hoods, to smart grids and pitot tubes, all the way down to using a GARBAGE BAG.
I had the privilege of seeing this presentation in person (I am the one behind the camera) and I wanted to share it with you. It is well worth your time.
My technician (and brother in law) Bert made a good point the day (It's hard for me to admit it, but it's true). When he needs to open the refrigerant circuit to make a repair regardless of whether he is recovering or pumping down, he pulls out his nitrogen tank and his regulator (We like the VN500 shown above).
Once the refrigerant has been fully pumped down or recovered, instead of opening the system to the atmosphere and exposing it to air and moisture, he simply puts it on “BRZ” mode and introduces a very low flow of nitrogen. Now when he cuts into the system to replace a line drier, or a coil, or a compressor, or an accumulator (you get the idea) the system will stay dry and it will be less likely that anything undesirable enters the system. You simply connect the regulator to your center hose and direct the flow to the high side, low side or both depending on what part of the system you have open.
Once the system is all dry fit into place you are then ready to flow nitrogen while brazing, pressure test and even triple evacuate if nitrogen is needed for that.
The biggest hurdle to getting techs to flow nitrogen while brazing is getting the nitrogen tank off the truck. If you get in the habit of connecting nitrogen before you ever cut or open the lines it even further reduces the chance that you “forget” and increases the chances that your system is clean and dry.
This article is written by Christopher Stephens of JVS Refrigeration in California with just a few additions by me (Bryan) in italics. Thanks, Chris!
Reach in refrigerators are an interesting side of our industry, often looked at as frustrating and troublesome. Often working in kitchens or convenience stores the refrigerators are never located in a convenient place to work on them, and that tends to lead to frustration on the technician’s part. Please understand my article pertains to medium temperature refrigerators. I also advise you to use manufacturers OEM parts when possible as the unit was designed to work with them. One of the more misunderstood and misdiagnosed parts is the temperature controller.
Keep in mind that some refrigeration temperature controls sense the evaporator coil temperature (not the desired box temperature) some use intake air sensors and some use supply air sensors. The medium being sensed (Coil, return air (intake) or supply air (discharge) will greatly impact how the controls function and what impacts them.
Personally, I break temperature controllers down into five different types, please understand that these are generic descriptions and you should always lean on the manufacturer if possible to understand their control strategies.
Standard Pressure Control – these work on the principle that at any given pressure saturated refrigerant is a constant temperature. This style of control is not used very much anymore as a means of temperature control because it is not very precise and to an untrained technician, it can be hard to set the temperature correctly.To use this control strategy, you need to understand what evaporator T.D. (Temperature differential) your reach-in was designed with, you will need a temperature pressure chart, you will need an accurate set of refrigeration gauges, and an accurate thermometer. With all these tools you can take your desired box temperature and find it on your pressure chart read across the pressure chart and find the corresponding pressure for your desired box temperature and that will be your cut-in pressure to set your control at. Now we need to find the cut-out setting typically we want the system to have about a 5-8 degree differential between the cut out and cut in to reduce system short cycling this will likely be about 8 degrees colder than the cut-in temperature, so take your desired box temperature subtract your differential of 5-8 then subtract your designed evaporator T.D. (specific to the equipment but likely 20 -30 degrees for reach ins) and find that number on the temperature pressure chart than read across the pressure chart and find the corresponding pressure and that will be your cut out setting. Understand that pressure controls are never exact, so you will need to adjust accordingly in the field.
Constant Cut in Control (electromechanical) – These are one of the most common temperature control’s that you will find in reach in refrigerators because they are the most economical for the manufacturers as they have an off cycle defrost built into them. They work by inserting the sensing bulb into the evaporator coil and they have a set temperature that they turn back on (cut in) at no matter how cold you turn the dial. They work very similar to the pressure control as they are designed with the evaporator T.D. (Temperature Differential) in mind, but instead of using pressure they sense the evaporator temp on the surface of the coil, they do have a knob to adjust the cut-out temperature, but you have no control over the cut in temperature that is why they are called constant cut in. By design they also have a built-in defrost as the cut in temperature is usually 37 to 41 degrees (for a cooler/refrigerator) depending on the manufacturer. They rely heavily on proper superheat and proper refrigerant charge. If the charge is incorrect or the superheat is not correct the coil could get too cold and the control could prematurely shut off. This could lead a technician to diagnose a bad control if they did not understand how they work. If you come upon a reach in that is short cycling and shutting off too soon, make sure to check the charge and measure the evaporator superheat before you diagnose a bad control.
Constant cut in (Digital) – These work the same as the electromechanical control, but they typically have two probes one to be located in the coil and one to be located in the return air stream. They tend to have more features that are available, such as an added defrost cycle based off time (every four hours, every six hours, etc.…..) while still using box temperature as a fail-safe. For example, say the control has a defrost every four hours if the coil temperature comes above a pre-determined temp say 40 degrees the control will terminate the defrost. The controls can also shut off the evaporator fan motors during the off cycle to save energy and reduce warm is intrusion into the unit. These types of controls are on many HC (Hydrocarbon) units being built today.
Universal electromechanical – these typically have one sensing bulb that you mount in the return air stream and they turn on and off via the temp setting.
Universal digital – These are usually aftermarket controls and can have several different control strategies and can usually be customized to do anything, from heating to cooling to defrost depending on the manufacturer.
Something to understand is that reach in refrigerators is usually designed to perform in a certain environment and if something changes such as the ambient temp in that environment, or if doors are left open. The box will not perform correctly, I suggest you take a step back before you start throwing parts at a reach in and evaluate the environment you may find your problem there!
Microchannel is a coil type used in many evaporator and condenser coils and can easily be identified by its flat tubes and fins between them that appear as waves between the tubes. The technology was developed for use in the automotive industry and is used for radiators and automotive A/C condenser coils.
These coils are made of all aluminum and are used because of their superior heat transfer due to increased surface contact between the refrigerant and the metal as well as the lighter weight and smaller refrigerant charge.
These coils have come under a lot of criticism by technicians due to an undisputed high failure/leak rate of the condenser coils in some systems. Some have felt these failures occur to inherent issues with the design while others have stated that the leaks were due to specific manufacturing issues on a few coils and that these issues are largely in the past. No matter how you feel it's likely that microchannel coils are here to stay due to the increased heat transfer efficiency and decreased weight. Here are a few things you need to know when installing and servicing microchannel coils.
Don't Pump it Down
MicroChannel condensing units are not sent with the full system charge and must have the charge added to manufacturers specs even with a short (say 15′) line set with more charge carefully added for line length.
You cannot, and must not attempt to pump down a system with a microchannel condenser or you risk causing a catastrophic failure of the coil.
Instead, you must recover the charge when making a refrigerant circuit repair and then carefully weigh the proper charge in after the repair is made.
Use Proper Brazing / Evac / and Refrigerant Practices
It's right in the name “micro” channel. The flat tubes have tiny refrigerant channels in them and they are susceptible to blockage by any solid contaminants in the system. Make sure to flow nitrogen while brazing, install a new liquid line drier after making a refrigerant circuit repair and pull a proper vacuum (as always). You also need to take extra care to keep shavings out of the system when cutting and reaming and keep tubing ends and hoses away from dirt and debris. For example, if you replace a compressor, anything allowed to get in your pipework will hit the condenser coil before it ever reaches the liquid filter/drier and has the opportunity to clog part of the microchannel coil.
Most manufacturers advise against using any cleaner on microchannel coils to avoid damage. Either use a garden hose, low pressure “fan” pattern pressure washer less than 100psi or a cleaner that is approved for use with microchannel and work carefully. The refrigerant channels go all the way to the edge of the coil and can be easily damaged if impacted.
The Charge is CRITICAL
When charging microchannel you will want to follow manufacturers specs and weigh the charge in whenever possible. If you see low suction doesn't just start dumping in charge until you are certain it is a charge issue and not an airflow issue or a restriction. Subcooling on microchannel systems tends to be more erratic due to the lower volume of the condenser coil.
Many manufacturers will swear that microchannel is just as resilient as tube and fin coils, based on my personal experience I would suggest taking greater care to protect microchannel. It may make sense to keep microchannel away from areas of the lawn that will have equipment going near the unit and possibly shooting debris into the surface.
When a microchannel condenser leaks it is often fairly evident by the oil stain that appears on the surface. Because of the channels, these leaks can be quite small, so if you see the telltale oil spot it is best to investigate.
I confess I have never attempted a microchannel repair myself, but there are many who claim to do it regularly. Here is a video showing it being done.
So take extra care when installing and servicing microchannel systems when cleaning, charging and repairing.
One of my techs (Jim Walch) mentioned to me that another common “double trap” style issue that comes up often is techs and installers running a drain too far into a condensate pump.
When you run the system drain too deep into the pump reservoir the water level can rise high enough to cover the drain end. This can create the same type of “airlock” you get with a double trap.
When draining into a condensate pump only extend the drain tube 1″ or so into the reservoir of the pump to prevent the water from rising and covering the end. Also, make sure to wire up the overflow switch on the pump in series with your system condensate switch so that whether the pump itself fails or the system drain pan has a blockage it will shut the system off.
Simple stuff, but it can prevent thousands of dollars in damage.
While there are various line lift, sizing and trapping guidelines depending on oil and refrigerant type there is one guideline we can all understand easily and remember to apply and this is to slope suction lines towards condensing units / compressors.
Keeping the suction line sloped toward the compressor, especially with long, overhead lines helps to return the oil back which assists with capacity and lubrication resulting in fewer issues.
A good general rule is to slope the suction line 1/2″ per 10′ of horizontal run.