Month: February 2017

In this episode Bryan speaks with Jim Bergmann about his path to being a test instruments business owner. we talk about –

– The Challenges in the past with poor quality instruments

– Enthalpy calculations

– The Future of test instruments and readings

– Taking readings with a pitot tube

As always if you have an iPhone subscribe HERE and if you have an Android phone subscribe HERE

Why Defrost?

Let’s start with the basics and move on from there. Defrost is necessary when the coil temperature drops below 32°F. Defrost can be as simple as turning the compressor off for a period of time or as elaborate as reversing the flow of refrigerant for the whole system or for just parts of the system.

 

As we were all taught in school, frost buildup is an insulator and prevents heat transfer, also airflow through a coil is a big factor. If the coil is iced up, the fans can’t move any air and without air movement, the equipment can’t do its job. This applies to all equipment with defrost, really.

Fin spacing

For refrigeration techs, this isn’t surprising, but A/C coils ice over a lot faster than refrigeration coils do. Why? Because the fins on a refrigeration coil are much more widely spaced than those on an A/C coil. So, when an A/C coil starts to get cold and that little bit of frost starts to build on the tube surface and the fin, it affects airflow through the coil much faster than it would if the fins were spaced more widely apart.

Moderate fin spacing medium temp coil with 6 fins per inch

Wide fin spacing on a freezer coil four fins per inch

If we had refrigeration coils with fin spacing like an A/C unit, it would ice up too quickly, and we couldn’t get anything done. The wider fin spacing illustrated shows how refrigeration equipment can run longer between defrost cycles. The evaporator coils are built in such a way to accept a certain amount of frost before the performance starts to degrade.

 

So, how do we get the job of defrosting done?

 

The most basic defrost is one we all probably remember Granny taking everything out of the “icebox,” unplugging it and going after it with a screwdriver, hair dryer or an ice pick. Simple, right?

 

But there has to be a better way, doesn’t there?

 

One of the simplest and most common automatic defrost control strategies is commonly referred to as a “cold control,” more properly called a coil temperature sensing thermostat. You’ll sometimes hear it called a “constant cut in” control.

 

With either a little-coiled bulb on the end of the sensing tube or a tube that kind of embeds in the evaporator, this senses the temperature of the evaporator coil and cycles the compressor based on that. The sequence of events runs like this. Coil temperature rises above cut in which is typically in the upper 30s. I like to see about 37°F at the lowest. This setting is nonadjustable, hence the name “constant cut in.” Control closes bringing the compressor on. As the coil temperature drops, the control eventually reaches its cut out point. I’ve seen this as low as 9°F. The cut out is what you’re adjusting when you adjust the control.

See what’s happening? Every single time it cycles off, the coil temperature has to rise above freezing by enough to ensure a good, complete defrost.

You’ll see this type of control on stuff like prep tables and smaller, under counter type refrigerator units.

 

Simple and easy.

 

A similar method for defrost control uses a pressure control to cycle the compressor. With this type of system, you set the cut in of the control to a saturation pressure equal to the same 37°F to 40°F, remembering this is saturation temp, not air temp, and adjust the cut out to maintain the temperature desired.

 

The big drawbacks of these controls are that they aren’t always predictable in that the defrost happens when the unit cycles rather than at a specific time (or times) every day and that the temperature can fluctuate over a pretty wide range. For some products, especially fresh meat, wide temperature swings are detrimental to product quality.

 

Taking a step up from the idea that every off cycle is a defrost, we’re going to just add a timer to the circuit. Now, we can set that timer up to shut the refrigeration off at regular intervals for a specific period. The interval and duration will be situation dependent as we’ll discuss.

Looking at this mechanical timer, the silver screws in the outer timer ring initiate defrost when they rotate past the pointer at the top left. The defrost ends when the copper colored pointer on the inner ring rotates past the same pointer.

This digital timer has little black bars on the display indicating both time and duration of defrost. In that picture, the time of day isn’t indicated in the photo. In its simplest form, this timer just opens the control circuit to the compressor or the control valve for the set duration of the defrost.

 

So, what’s happening? As far as the system is concerned, the same thing is happening here that was happening before when we used a cold control or a low-pressure switch. We’re shutting the refrigeration off and allowing the frost to melt naturally off of the coil. The biggest difference is that now, with a timer, instead of being subject to the unknown of when the system will cycle off and how long it will take to melt the frost, assuming the time of day is set correctly, you can reliably predict the defrost times. Now, you can say that it defrost at, 6 AM and 6 PM for 45 minutes and the customer can note that and account for it when checking temps on their equipment.

 

Let’s talk for a minute about how long a defrost needs to last… obviously, until the coil is completely clear of frost and ice, but we need to know when that is….

 

In most cases, the manufacturer will give guidelines to set your defrost control system up. It will spell out frequency or interval (time between defrosts) and duration of the defrosts. Because we’re trying to maintain proper product temperatures and we got away from the cold controls and low-pressure controls because they were fluctuating over a wide range of temperatures, we need to look for a way to limit that fluctuation.

 

For years this was only used on defrosts that added heat to the evaporator coil (which we will look at later) but in recent years with more stringent product temperatures requirements and temperature expectations from the customer, combined with government efficiency mandates, trimming even a couple minutes off of a defrost cycles improves both product holding quality and unit efficiency.

 

How does it work? The manufacturer will typically install either a thermostat or a temperature sensor on the coil or in the airstream leaving the coil. After experimentation in their labs, they determine just how warm that spot has to be to ensure the coil is free of frost. So, in the middle of summer in a hot, humid kitchen the defrost runs longer than it does in the middle of winter on an outside access only cooler box. Why?

 

We all learned about sensible and latent heat in school, right? Well, melting frost is just latent heat added to change the state, right? So, since we’ll have more frost on a coil with a higher humidity than on one in a lower humidity environment, the coil with a higher frost buildup is going to take longer to melt off of that coil which means that it will take longer to reach that set temperature.

 

In practice, here’s how that timer handles defrost. Time of day initiates a defrost, so say 6 AM, the timer switches to defrost mode. Internally, that means that the contacts in the timer open to de-energize either the control valve or the compressor. For simple off cycle defrost, the fans continue to run to keep moving air across the coil and accelerate heat transfer. The defrost ends, in the simplest form, when the timer reaches the duration pin, switching the timer contacts back to closed and energizing the load. If we have a termination control, it’s a normally OPEN contact that closes on the rise of temperature. So, when that temperature reaches the termination point determined by the manufacturer, the contact closes energizing a small solenoid in the timer to push the contacts back to normal position regardless of the timer position. In an electronic control, this is just another signal input, either digital (NO\NC contact) or analog (sensor) that tells the software in the controller to switch the relay back to refrigeration. A coil thermostat or sensor might be set as low as 34°F while an air sensing control will typically be set between 48 and 55°F.

 

Electric Defrost

Since some refrigeration equipment runs at temps significantly colder than 32°F sometimes, we’re going to need to add some heat because there simply isn’t enough heat in the refrigerated space to get the frost melted without causing significant damage to the product. The simplest way to add this heat is usually with an electric heater. Let’s take a look at how this adds some complexity to the defrost control system.

 

The basic timer type defrost initiation control doesn’t change. The same type of timer is used and when defrost initiates, the refrigeration circuit de-energizes the same as before. The big difference now is that, at the same time, we’re energizing a heater that is going to add heat to melt frost of the evaporator coil. In the case of most pieces of equipment, we’re also going to de-energize the evaporator fan circuit. This is to keep the heat concentrated where it is needed to do the job in as little time as possible. We also don’t want to blow hot, humid air around the refrigerated space.

 

Defrost termination is really the standard for this type of system. Almost all electric defrost systems will have a type of defrost termination built in. The most common are referred to as a DTFD control (Defrost Termination Fan Delay) or 3 wire control. This dual purpose control handles both termination of defrost obviously and post-defrost fan delay which we’ll get to in a minute or two. The DTFD is normally attached to one side of the evaporator coil in a position that takes the longest to get warm during defrost. This way, the coil gets the best possible defrost.

 

Refrigeration is off; heaters are on, frost is melting away. All is well. Once our DTFD control sees it’s high event temperature, usually about 55°F, it closes the part of the circuit to terminate defrost, same as before. Refrigeration machine starts back up and we’re moving heat again, but wait…. What about the fans? They aren’t running. Quick get a meter and a ladder…

This is the other half of the DTFD control. We’ve terminated defrost (DT) now we have to wait a couple of minutes until the coil temperature drops below freezing. We have to remember that coil was just 55°F and there is some humid air still trapped in that sheet metal box up there. Slam the fans on right now, and you’ll have a wintery Wonderland in your freezer with icicles and snow all over in a week or so. Wait a minute or two and the coil will freeze that last bit of moisture. When the coil temp drops to around 30°F at the control, our fans will restart.

 

Gas defrost, particularly for large refrigeration systems is going to require an entire article in and of itself to cover in any depth. I’m going to try to summarize it in a paragraph or two and give it a more thorough treatment in the future.

 

These, like all other defrosts, operate on a time basis. The systems where this is more common aren’t single system but multiplex systems with multiple evaporator operating on different schedules. When one goes in defrost the rest continue to run in refrigeration.

 

When the timer initiates a defrost, a few things happen all at once. A differential valve de-energizes to create a pressure differential to allow flow in reverse. To create a section of reversed gas flow, two valves actuate. One that stops suction gas flow to the compressor and another that dumps hot discharge gas into that suction line, sending superheated discharge gas out to the evaporator where it rejects its heat to the frost on the lines and is condensed just like in a heat pump. It returns to the system through a check valve piped around the TEV, same as with a heat pump. Without the pressure differential, the hot gas cannot flow properly through the check valve.

 

Once either the time limit is reached, or the termination temperature is reached, all of those valves return to their normal positions, and the refrigeration cycle resumes normally.

 

— Jeremy Smith CM

I’ve heard a lot made of clocking gas meters over the years and honestly, in Florida there isn’t too much call for for heat and even fewer furnaces.

I was pleasantly surprised when I found out how easy it actually is. Here is how you do it, step by step.

#1 – Make sure all gas appliances are off other that the one you are clocking. Even shut off pilot lights or it can mess with your reading.
#2 – Make sure the appliance you are checking is running at high fire (max output)
#3 – Get a stopwatch (your phone has one)
#4 – Watch the smallest unit dial on the gas meter, it will often be 1/2 cubic ft
#5 – Time how long that dial takes to make one full revolution with the stopwatch
#6 – Multiply the dial size by 3600 (3600 is the # of seconds in an hr) so if it’s a 1/2 cu/ft dial it would be 1,800
#7 – divide that # by the # of seconds it took. So lets say it took 22 seconds that would be 1,800 / 22 = 81.82
#8 – Multiply that # by the BTU heat content of 1 Cu/Ft of gas provided by the utility. If it is 1,000 (which is common for NG) the total BTU per hr would be 81,820

The complete formula is Cubic Feet per Hour (CFH) = (3600 x Dial Size) / Time (seconds)

Then to get the ACTUAL device output in BTU’s you would multiply for the AFUE efficiency. In this case if it was an 80% furnace the input is 81,820 btu/hr and the output would be 65,456

Pretty cool huh?

–Bryan

In this video we cover the basics of using the Testo 510i with a pitot tube to do a duct traverse and easily calculate Velocity in FPM and volume in CFM on a small 8″ duct. Using this method is handy because you can use the reliable, accurate and inexpensive 510i to perform the measurement without any other equipment other than tubes and a pitot tube.

As stated in the video, a pitot tube is best (most accurately) used in the following conditions –

  • Medium to High Air Velocities
  • With 4 -8 feet of hose
  • In low turbulence air at least 8.5 diameters downstream of any turns, fittings or diffusers (I was less than this in the video resulting in lower accuracy)
  • In a duct at least 30 times larger than the pitot tube diameter (It was less than this in the video resulting in lower accuracy)

 

For more information see the following links –

Dwyer Guidelines

TruTech Tools Traverse Quick Chart

TruTech Measuring with a pitot tube

Testo 510i specs

Video on the performance of a rectangular time average traverse

You’ve probably heard the famous last words “Dude, watch this” before a concussion, burn, shock, broken bones or some other bodily harm. This phrase has become synonymous with young guys doing something dumb to impress their friends.

Technicians have two common phrases that may not lead to bodily harm (although sometimes it might) and they are –

“That’s Good Enough” and “That’s Normal” 

Pulling a vacuum for 30 minutes without a micron gauge and then “That’s good enough”.

Doing a standing pressure test and the pressure keeps dropping JUST A LITTLE and “That’s normal”.

Running a 0 superheat and “That’s normal” followed by some made up reason about this particular equipment, or load conditions.

I have heard lot’s of made up explanations over the years… some of them out of my own mouth and almost all of them being used as a justification for something being good enough or normal.

 

Don’t misunderstand, normal and good enough are both real concepts, but they need to be backed by deep understanding of the equipment you are working on (have you read the entire installation instructions and / or service manual?) and the readings you are taking (Do you understand what they mean, why you are taking them and how your test instruments / tools work?).

If you can’t follow it up with “It’s normal because…..” or “That’s good enough because….” with a real answer, not a made up reason, then you need to keep working.

This is a journey for all of us, but stop for one second and be honest with yourself. When you get frustrated, short on time or feel in over your head… Do you ever use these phrases? If so, congratulations. You are in an elite group of techs  willing to admit what you don’t know.

Now repeat after me…

“I will no longer make excuses for what I don’t understand, I will stop and work to understand what is actually going on until I have it mastered”

 

— Bryan

 

P.S. – Sorry for the repeat after me thing… It’s a bit too much, but this whole article is nerdy as heck so I figured I would just take it all the way.

Some quick basics –

An Ohmmeter is used to measure the resistance to electrical flow between two points. The Ohmmeter is most commonly used to check continuity. Continuity is not a “measurement” as much as it is a yes / no statement. To say there is continuity is to say that there is a good electrical path between two points. To say there is no continuity means there is not a good electrical path.

In other words, continuity means low or zero ohms and no continuity means very high or infinite ohms. Don’t get the terms zero ohms and infinite ohms confused, they mean opposite things.

 

This type of testing is commonly used to check fuses, Trace wires, check for short and open circuits Etc… Resistance readings are necessary for identifying motor terminals, and checking for a breakdown in insulation. An Ohmmeter continuity can be used to identify normally open, and closed terminals on a relay. Simply place the leads of the meter across the relay points, if there is continuity the relay is normally closed. Now apply power to the magnetic coil of the relay, the contacts that were closed should now open, or vice versa. An Ohmmeter can be used to identify a single wire in a bundle. Go to the opposite end of the wire and expose two separate wires in one sheath. Twist the two wires together and list the colors. Go back to the other end and check for continuity between all wires of that color.

 

Once you find two wires with continuity, you have found the correct wire. If you suspect that a particular wire is shorted to another wire, simply disconnect both wires on each end and check for continuity between the two wires. If continuity is read between the wires you have found a short.

These are only a few examples of ways to utilize an Ohmmeter.  Remember an Ohmmeter should only be used in un-energized circuits, Otherwise the meter could be damaged.

 

— Bryan

 

We’ve all heard about glide, but what is it really and how does it affect our system?

Glide, or temperature glide, is the difference between the bubble point and the dew point of the azeotropic refrigerant mixture.
Well that wasn’t very helpful, was it? All we did was introduced new terms without defining them and further confused the issue.
So, let’s start with zeotrope or zeotropic mixture. A zeotropic mixture is a chemical mixture that never has the same vapor phase and liquid phase composition at the vapor–liquid equilibrium state. Still unhelpful? I thought so, too, so let’s look at what it means to us rather than what the books say.

A zeotrope, as far as we’re concerned, is a refrigerant mixture or blend that boils across a range of temperatures at any given pressure. So, unlike water that boils at a constant temperature of 212°F at atmospheric pressure, a zeotropic mixture will boil between across a range of temperatures at that same single pressure. Using r407a as an example, at atmospheric pressure, the liquid would begin to boil at -49°F and will continue to boil until the last droplet boils away at -37.5°F. I know that it’s kind of weird to think of the process of boiling like that, but that’s what is happening with a zeotrope. Boiling takes place over a range of temperatures.
That temperature range is called the glide.

 

Now that we’ve got a basic concept that we can work from, we can start to understand glide and ultimately get to how it affects a refrigeration system. Let’s start with bubble point. Since we should have a solid understanding of states of matter and the transition between liquid and vapor, let’s assume we have r407a refrigerant in a 100% liquid state at 140 psig. If we start at 66°F, we’ll be just slightly subcooled which is a perfect starting point for this example.. If we start to add heat and raise the temperature of our refrigerant while holding our pressure constant, a single bubble will appear in the refrigerant as it begins to boil. That point is called the bubble point. For our purposes, we can define the bubble point of a zeotropic refrigerant blend is the point at which the first bubble appears.
Still making sense? I hope so.

 

Continuing with our r407a at 140 psig example, we’re going to continue to add heat to the refrigerant with the same constant pressure. The refrigerant continues to boil, but as the mixture of refrigerant changes, the boiling point changes, slowly rising as the liquid boils away. Eventually, we will have added enough heat to reach a point where one last droplet exists, that point is called the dew point. Like we did with bubble point, let’s state an operating definition for bubble point. The dew point is the point at which the last droplet of liquid evaporates. For our example, that temperature is 75.5°F or very near that. Since it’s boiling over a range of temperatures, it is also true that the refrigerant condenses over the same range of temperatures as we remove heat from it. That will happen in reverse of the process I just described.

 

What does this mean for the service guy?

 

Obviously, these different values affect our superheat and subcooling readings. Since the dew point is the point where the last droplet of liquid boils off, we need to know that value to measure and calculate superheat. Similarly, with the bubble point, we need that to calculate subcooling. These are the values found on the PT charts and that are programmed into your digital manifold gauges.
In refrigeration work, evaporator coil temperature can be used for a number of things. Most commonly, we will use it to control fixture temperature and to terminate defrost. It used to be simple to know what our evaporator temperature is. We looked at the gauge and transferred that number to a PT chart. We can no longer look at our evaporator pressure and know what our corresponding evaporator temperature is quite the same way.

 

Let’s look at numbers… say the manufacturer says that you need an 18°F coil temperature. With R22, you simply look at your trusty PT chart, find 40.9# and work from there. Easy enough, right?
Now, let’s look at the same coil with r407a. We have 2 points that are 18°F. The dew point (40#) and the bubble point (52.5#), so which one do we choose?
The correct answer winds up being neither one. Between manufacturer’s recommendations and field experience, I’ve found it best to use something closer to the average of dew and bubble point to find the actual, functional temperature of the evaporator.

 

52.5+40 = 92.5. 92.5/2=46.25

 

Looking at a PT chart, this shows us 13°bubble point and just over a 23° dew point. If you look, 18° will land right about in the middle. This isn’t always a perfect setting, but it’s as good a place to start as you can find. Set the control valve there and fine tune it as needed to get the performance that you need. If we need to use a pressure reading to terminate defrost, we will need to reference bubble point because it is the colder of the two temperatures and will ensure a complete defrost. If we used dew point, the inlet of the evaporator would be several degrees colder than the outlet and frost may still very present.

–Jeremy Smith

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