Wednesday, April 27, 2016

Pressure Transducers

On my last post I discussed common refrigeration pressure switches. As the name implies, these are switches which are opened and closed by pressure changes. They can make or break circuits, but they cannot indicate pressure. Pressure transducers are often used for electronic controls because they can actually indicate system pressures.

The word “transduce” means to change from one form to another. A pressure transducer turns pressure changes into analog electrical signal changes. This is most often a change in a DC voltage, typically 0 – 5 volts DC. This changing voltage can then be interpreted as a pressure by the electronic control to which it is connected.

The most common pressure transducers used in HVAC use a small stainless-steel diaphragm with strain gauges bonded to it. A change in pressure causes the diaphragm to bend, which causes the strain gauges to change resistance. These transducers have three leads: two are wired to DC+ and DC- and the third carries the signal. Pressure transducers ohm out like a potentiometer. On diagrams this looks like a potentiometer with a pressure bellows connected to the wiper arm. The resistance between the two leads that connect to DC voltage should stay the same regardless of the pressure. The third lead changes resistance relative to the two other leads as the pressure changes.




When the two other leads are connected to 5 volts DC, the signal connection will vary between 0 and 5 volts DC depending on the pressure. The control then interprets this voltage and controls the system based on the board’s program. If you want to check the transducer signal, read the voltage between the signal lead and DC- and then compare this voltage to a chart published by the manufacturer. Here are a couple of links to more information on presure transducers

Omega Transducers

Emerson Climate Technologies

 

Tuesday, April 19, 2016

Pressure Switch Lingo

Pressure Range
The range of a pressure switch is the  minimum to maximum pressure at which the switch can be set. Because refrigeration systems have two basic pressure areas, high side and low side, pressure switches are often described as either high or low pressure switches, based on which side of the refrigeration system the switch is designed to operate. However, this is a bit of an oversimplification. It is possible for two pressure switches with identical ranges to behave very differently if they have different switching actions.

Switching Action
The switching action  describes what occurs when pressure rises above the switch set-point. Basically, only two things can happen: the switch either closes or it opens. So, pressure switches are classified as either close on rise, or open on rise. For refrigeration system safety applications, close on rise switches are used to protect against low system pressure, and open on rise switches are used to protect against high system pressure. But there are many other applications for pressure switches. For example, a close on rise pressure switch can be used for condenser fan cycling to maintain head pressure. The condenser fan is energized when the condenser pressure rises to the switch cut-in point. Similarly, an open on rise pressure switch can be used to control the harvest cycles on an ice machine. When the evaporator pressure drops to the switch cut-in point, the harvest cycle is initiated. The safest way to describe pressure switches is by both their switching action and their pressure range.

Cut-in, Cut-out, and Differential
It is important to understand that the switch contacts cannot both open and close at exactly the same pressure – there has to be a difference between the pressure when the switch closes and the pressure when the switch opens. This difference is called differential. Three terms are used when describing pressure switch settings: cut-in, cut-out, and differential.  Cut-in is the pressure when the switch closes, cut-out is the pressure when the switch opens, and differential is the difference between the two.





















High Event and Low Event
Sometimes the terms high event, low event, and differential are used. In the case of a close on rise pressure switch, the high event would be the cut-in and the low event would be the cut-out. In the case of an open on rise switch, the high event would be the cut-out and the low event would be the cut-in.

What Difference Does it Make?
There are both physical and practical reasons for having a differential. Physically, pressure switches are mechanical devices which use levers that are controlled by springs and pressure bellows. There must be some mechanical motion to open or close the electrical switch contained within the pressure switch. Since this motion is created by a change in pressure, the opening and closing point cannot be the same point. Practically, you really would not want the control system to respond so quickly. For example, once the compressor stops running, the high side pressure almost immediately drops a little, even on systems with hard shut-off expansion valves. If a small drop caused the high pressure switch to close again, the compressor would quickly cycle on and off repeatedly, which would cause more damage than the high pressure alone. I hope this helps explain why there are multiple settings on a pressure switch and what they all mean.



Monday, April 11, 2016

Using Manual D Speed Sheet

The free ACCA Manual D speed-sheet makes properly sizing residential duct work really easy. However, you need to complete three important steps BEFORE you are ready to use the Manual D speed-sheet. You must first do a Manual J room by room load study, select the specific equipment you will be installing, and draw out your duct system in stick form. The speed-sheet will help you size your ducts according to the required heat load in each, the unit output, the unit airflow, and the external static pressure requirements of the unit. There are three basic steps: determining the total effective length of the duct system, determining the design friction rate, and finally sizing the duct.

Effective Length
Manual D looks for the worst case duct run and bases the design friction rate on that longest run. The idea is that if the blower can move the air through the longest run, it can easily push the air through the other ducts. The assumption is made that each run will have a balancing damper, and that the balancing dampers will be used to balance the system airflow once the system is installed. The speed sheet gives you four columns to use for determining the longest effective length. You don’t have to use all of them if the worst case run is obvious. Note that you are NOT entering data for every run, just looking for the longest run.

There are three rows labeled Trunk. They are there for systems which have multiple branching trunks. Most systems will only use one.  Enter the length of the trunk duct from the plenum to the branch takeoff in one of the Trunk rows. Then enter the length of the branch run beside Runout Length. The second set of rows on this tab are for entering the equivalent length of all the fittings. They are arranged in groups of fittings with similar functions. Click on a group to go to the tab showing the different fittings. Choose a fitting that best matches the fittings you will use. You will need to remember it, or jot it down. Click return to return to the Effective Length tab. You will probably NOT have a fitting for every group. Just leave spaces blank which do not apply to your system. Repeat the process for the return. Note that the group numbers change bit because the equivalent length of return air fittings varies from supply air fittings.

Friction Rate
The second tab is for determining the design friction rate. You need to know the specific unit for this part because you will be entering the unit airflow and external static pressure. The idea is pretty simple. All things that the air moves across cause a pressure drop. You list the pressure drop in wc for all the air components. This is totaled and subtracted from the system external static pressure, and what remains is available static to be used for moving air through the ducts.

Since friction charts are based on 100 feet, the friction rate, or wc friction drop per 100 feet, needs to be determined. For example, if your available static was 0.12 and your total effective length is 200 feet, the design friction rate would be 0.06. A duct which would cause a pressure drop of 0.06 per 100 feet would create a total pressure drop of 0.12 by the time the air traveled 200 feet. The speedsheet does this for you based on the total effective length calculated on the Effective Length tab.

Duct Sizing
You need to have a Manual J calculation of the heating and cooling loads for each room before using the final tab, Duct Sizing, Simply list the room name, heating BTUs and cooling BTUs and the speed sheet calculates the duct size based on the friction rate and CFM from the Friction Rate tab. Note that it re-sizes the ducts every time you enter more data – so don’t be alarmed if it tells you the first room you enter requires a 16 inch run. The sizes are not accurate until you have all the room information in.

Trunk sizing is as easy as clicking a box for each branch duct that t trunk feeds. Note there are probably more rows for trunk ducts than you will need.  Returns are also sized the same way. You click the box of each supply run which you believe will be served by that return. This is obviously not an exact science. However,  it is important that each supply run is selected in a return. If you have return trunks, you size them by selecting the return branches which feed into the return trunk.

Creative Application of Manual D Speed Sheet
You can use the Manual D Speed Sheet as a teaching/learning tool by varying some of the entries. For example, play with different equivalent length fittings to see the effect between best case and worst case fittings. Try different external static pressures and airflows to see the effect on duct sizing. Thi is a great way to see the effect different design decisions can have on the end result.

Monday, March 28, 2016

Planning for Success

If you are waiting for success to just drop in your lap, you may be waiting for a long time. Successful people have to work at being successful. Even after achieving a degree of success, you have to keep working to maintain your current level. However, hard work is not the only pre-requisite to success. Planning makes your efforts more productive by focusing your attention on the details and directing your energy to items which you might otherwise overlook. When teaching HVACR, lab work can be chaotic. Planning helps bring the chaos under control. I will be giving a Webinar on " Tips for Running an Effective Air Conditioning Lab" Wednesday, March 30 at 3:00 PM. The webinar is free. To register, got to the following link

https://attendee.gotowebinar.com/register/8836657677340179713


Friday, March 18, 2016

March Madness is Here!

I am caught up in my own version of March Madness! No, not the basketball championship brackets, but the annual educational seminars and meetings that come every year in the spring. I will be speaking at HVAC Excellence National HVACR Educators and Trainers Conference in Las Vegas on Tuesday, March 22 on “Developing and Evaluating Performance Exams.” If that is not your idea of a good time, then check out some of the other 50 sessions! There will be hundreds of HVACR instructors, and 70 exhibitors showing the latest and greatest at the expo. Youi can find me at the Pearson booth where I will be showing off the brand new third edition of “Fundamentals of HVACR.”  This event is always energizing and inspiring. But wait, there’s more! I will be giving a webinar through Person on “Tips for Running an Effective Air Conditioning Lab” at 3:00 Eastern time on March 30. Please join me to learn how you can plan for success.

Thursday, March 10, 2016

Variable Capacity Comfort Systems

Every major manufacturer is now offering some form of capacity modulation on their top line residential HVAC equipment. For several years we have had two stage compressors in air conditioners and heat pumps and two stage gas valves on gas furnaces. Now most manufacturers are offering variable capacity air conditioners, heat pumps, and furnaces. You might wonder why having the ability to produce less heating or cooling is a good thing? Variable capacity equipment is better than fixed capacity equipment for both comfort and energy efficiency,

First, think about the comfort issue. A fixed capacity system is only the right match for your house at one operating condition. Most of the time it is larger than you need. Fixed capacity systems have to shut on and off because they typically produce more capacity than is needed at most operating conditions. This means that they will be over-conditioning, shutting down and waiting for the temperature to fluctuate, and then coming back on. A variable capacity system can operate close to the actual capacity needed to heat or cool your home even at different conditions. This means you will not be over-conditioning, shutting off for a while, and then over-conditioning again. The temperature can remain more constant.

 Energy efficiency is the second big plus you get from a modulating system. The least efficient operation for any system is when it starts. For air conditioners and heat pumps, it takes a few minutes for the refrigerant pressures to stabilize and start producing at rated capacity. However, your system uses the most energy at initial startup. This means that at the very moment the system is using the most energy it is producing the least capacity. Obviously this represents inefficient operation. If a system only runs a few minutes before shutting off, it is always operating at its most inefficient state. Furnaces have similar issues. At startup you are not delivering any heat into the house, but the furnace is using fuel. Again, the most inefficient operating condition. A furnace that cycles on and off every few minutes is operating at its leas efficient condition. Variable capacity systems increase efficiency by reducing these cycling losses. The system operates for longer periods, but uses less energy because it is only using as much energy as is needed for the current condition.

In cooling, longer operation often gives better humidity control in areas where humidity control is as important as temperature control. If an over-sized cooling system cycles rapidly, it does not have time to remove a lot of moisture. The homeowner feels uncomfortable even though the temperature is satisfied. So what do they do? They crank the temperature down until they do feel comfortable. Net result, they over-cool the house to feel comfortable because the system is not removing moisture adequately. Modulating systems can solve this problem by varying both the refrigeration capacity and the airflow capacity – keeping a cold coil even at reduced capacity operation. They operate longer because of the reduced capacity, allowing them to remove more moisture.  Variable capacity systems gaining in popularity because they keep you more comfortable while using less energy.

Friday, March 4, 2016

Affordable Wireless Probes

I just got my first opportunity to play with my new Testo smart probes. The 549i measures pressure and the 515i measures temperature. They are wireless Bluetooth devices which rely on an app that you load on your smart phone or tablet. They are part of a complete lineup of wireless Bluetooth devices that Testo is introducing. I was interested in them because of the low price point. The 549i and 515i are each around $50.  To read both system pressures, the suction line temperature, and the liquid line temperature simultaneously costs around $200.

The App
The Testo Smart Probes app interfaces with all of the smart probes. My first impression is generally positive. Like Testo’s Digital Manifold Gauges, the software does have a small learning curve. However, once you get past the initial setup and learning where all the settings are, you can easily check system pressures, superheat, and subcooling simultaneously. You can choose from a menu of applications which range from a basic list of each probe’s output to a software application designed to make a particular job easier. The software will also do data logging. Each application allows viewing the information as a list, trending (a graph), or a table. With the graph or table view, every time the probe updates its reading, that new reading is plotted on the graph or added to the table. You can export these to pdf, excel or jpg.

Accuracy
The accuracy is reasonable. For improved accuracy, the 115i temperature probe uses an NTC sensor rather than a thermocouple. The specification is plus or minus 1.3° C ( 2.3° F). Its resolution is 0.1, meaning it can display tenths of a degree.  Its range is -40°C to 150°C ( -40°F to 302°F). The 549i pressure probe has an accuracy of plus or minus 0.3 bar (4 psi). The resolution is 0.1, meaning it can display tenths of a pound.  The range is -1 to 60 bar (-14 to 870 psi).

In the Shop
The software and Bluetooth worked well with both my i-phone and my Android tablet. The range seemed fine, I connected to the system and walked around the shop. The app did drop some of the probes occasionally, but they always reappeared in a few seconds. This happened even if I was right next to the probes – so I don’t think it had to do with the Bluetooth range. There is also a latency in the readings of a few seconds. When I disconnected the pressure probes the pressure still showed on the screen for a few seconds.

Batteries
What remains to be seen is how long the batteries last. Each probe uses 3 AAA batteries. It could get a little expensive if I have to replace batteries a bunch. Also, I know neither my phone nor tablet will go all day, so to use these in the field a lot I would need to figure out a convenient way to recharge them.

Wrap up

The exciting part is that these probes bring the wireless world to you at a very affordable price. Just a good quality thermocouple pipe clamp that plugs into your multimeter costs $50. The 115i gives you more functionality and convenience for about the same price. The 549i lets you check system pressures without filling a manifold or hoses up with refrigerant. And both allow you to export the data, so you can provide your company and customers with verification of the system’s performance. Here is  link to a web page about the Testo Smart Probes. https://www.testo.com/en/home/products/smart_probes/smart_probes_heating_1.jsp