Blower External Static vs CFM

  

It is possible to use a magnehelic gauge or a digital manometer and two static pressure probes to determine the amount of airflow a blower is moving. You can read the static pressure difference across the blower and compare it to the manufacturer’s data to determine the blower CFM. It does make a difference what type of blower you have and what type of motor the blower uses. A typical residential blower uses a forward curved centrifugal blower and a PSC motor. The airflow these blowers deliver decreases as the static pressure the blower is working against increases. You need the manufacturer’s data to compare the measured static pressure across the blower to the fan performance table or curve. Here is a table from a unit with an air handler with a PSC blower motor. Note that on high speed at a static pressure difference across the unit of 0.1” wc the airflow is 1150 CFM, while at 0.7” wc across the unit the airflow drops to 775 CFM.

PSC Blower	External Static
Motor Speed	0.1” wc	0.2” wc	0.3” wc	0.4” wc	0.5” wc	0.6” wc	0.7” wc
High	1,150	1,095	1,045	1,025	950	865	775
Medium	890	855	835	775	715	665	605
Low	640	605	565	530	485	440	360

With ECM motors, the airflow varies very little as the static pressure across the blower changes. That is the point of an ECM blower. It recognizes the amount of static pressure it is working against and adjusts the blower motor RPM and power output to keep the same programmed airflow – up to a point. ECM motors do have a programmed speed limit. When they hit their speed limit, they shut off. A key point here is that although the ECM motor can compensate for extra restriction, it does this by using more electricity – which can turn an energy efficient blower into an energy hog. It is far more cost effective to remove the restriction than to pay for enough electricity to shove the air through. At any rate, checking the static pressure across an ECM blower is done primarily to make sure it is operating within its design parameters and in an efficient manner. It does not tell you how much air the blower is moving. A table from an air handler similar to the one above, but with an ECM blower motor is listed below. Note that there is very little change in the CFM as the static pressure across the blower increases. The CFM delivered is determined by the program: A, B, C, D.

ECM Blower	External Static
Motor Program	0.1” wc	0.2” wc	0.3” wc	0.4” wc	0.5” wc	0.6” wc	0.7” wc
A	630	625	625	620	610	605	600
B	895	885	875	865	845	825	815
C	1030	1020	1005	995	970	945	935
D	1185	1175	1160	1145	1120	1090	1080

X13 motors are also electronically commutated, but they are programmed for a specific torque or power output, not a specific airflow. The airflow across an X13 motor does drop off as the static pressure across it increases, but not as dramatically as a PSC motor. They are considerably more efficient than a PSC motor and considerably cheaper than a fully programmable ECM. You can determine an airflow from the static pressure across the blower and the manufacturer’s specifications. Below is data from an air handler with an X13 blower motor. Note that its performance compared to external static pressure is somewhere between  a PSC motor and an ECM blower.  

X-13 Blower	External Static
Motor Program	0.1” wc	0.2” wc	0.3” wc	0.4” wc	0.5” wc	0.6” wc	0.7” wc
Tap 1	960	905	890	810	760	720	665
Tap 2	1,060	1,010	995	920	875	835	790
Tap 3	1,150	1,100	1,090	1,025	990	945	905
Tap 4	855	810	775	755	750	720	690
Tap 5	1,470	1,440	1,425	1,405	1,375	1,260	1,315

Hand Cranked Generator

I have had several people ask about building a generator students can turn by hand. I am putting a short explanation and several pictures on my blog. We built one using an ECM motor minus the module.

It puts out 3 phase AC. It is regulated only by the speed of the person turning it: you would not want to run your computer on it. To get enough speed we used a 12 inch pulley for the students to turn and a 1 inch pulley on the motor shaft: This produces 12 spins of the generator for every one revolution of the hand crank. I ordered the bearings, pulleys, belt, and shaft from Grainger.

We used heavy slotted shelving material to make the frame. This also came from Grainger - but it was left over from other projects.

We brazed a large hex nut onto a hub that mounts on the shaft and we turn it with a large socket on the end of a speed wrench.

 It has been working for years - just needs a little tightening and adjusting from time to time. Students can fairly easily get to 120 volts - until I turn on a light.

 


One change I would make - we mounted the bearings to a couple of pieces of slotted angle and it is too flexible - hard to align the bearings so they don't bind. Plus, the bolts holding the slotted metal are also the ones that have to be loosened and tightened to adjust the belt. I would replace that with a solid block of metal and then mount that block to the frame.

 It took 2 of us about 4 hours to make.

Evap Temp Effect on Compressor Capacity

I really appreciate the detailed data that some manufacturers have available to the public on the internet. For example, Bristol Compressors has detailed performance specifications for all of their compressors online for anyone to see. Not only do they have the specs for standard rating conditions, but you can also enter in your own. This allows you to see the effect that changing system conditions has on the compressor capacity, energy use, and EER. For example you can see what happens if the evaporator temperature drops from 45°F to 40°F. At first, that does not seem like a very big deal. But in the case of a Bristol H81J223ABC, that change drops the capacity from 21,800 Btuh to 18,800 Btuh and drops the EER from 9.8 to 8.9. Now ask yourself this question: “How easy would it be to have an evaporator temperature 5°F too cold because of a dirty air filter, dirty coil, restrictive duct, slight undercharge, partially clogged filter drier, incorrect piston (orifice), incorrectly adjusted TEV – and I am sure you can think of a few more.  In fact there are so many ways to be just a little off it seems likely to occur. Unfortunately, the effect on system performance is not little. The capacity drops by 14% (18,800/21,800 = 0.86 or 86%) and the EER drops by 9% (8.9 / 9.8 = 0.91 or 91%).  For a more dramatic drop in capacity, look at what happens to a commercial refrigeration compressor when the evaporator temperature drops from -10°F to -20°F. The Bristol L61J113ABC produces 7000 Btuh at its -10° rating point. At -20°F this drops to 3800 Btuh. At only 10°F colder, the compressor capacity drops to 54% of its rating! That extra 10°F colder costs dearly in electricity. With some creativity and exploration you can use comparisons of different operating points to illustrate many issues in HVACR systems.

You can find this interactive compressor playground at www.bristolcompressors.com Engineering Support > Compressor Search or just CLICK HERE  Enter whatever details you want to start the search. Once on the page for a particular compressor, click “Specific Point” towards the top right of the screen to enter your own operating conditions. Have fun!

The Younger Generation

I often hear people talk about the problems of the “younger generation.” In my case, people younger than me comprise a pretty large swath of society, but usually folks are talking about teens and young adults under 30. It is easy to see that there are young people with problems. In fact, the news sometimes seems like a constant stream of depressing facts. However, you must remember that these things are news because they are NOT the norm. The truth is there are many fine young people going about their business in spite of hardships and challenges and doing quite well. For every story you read about a high profile athlete doing something despicable, remember there are thousands of high school and college athletes – most who are getting an education and working hard at their athletic pursuits. In fact, the NCAA says there are 430,000 student athletes currently in colleges affiliated with the NCAA. A little closer to home for those of us in trades education, more than 300,000 students join Skills USA annually. There are 5900 contestants taking part each year at the national competition. Then there are thousands more students who participate in state and local competitions. When you add up the students participating in bands, orchestras, choruses, theatre, and a host of other extracurricular activities the numbers run into the millions. That is millions of young people who not only do what is expected – but go beyond to build skill in an area that of special interest to them. Because I have a son in high school, I have had the opportunity to meet a many of these young people personally, and I am encouraged by what I see. So if you have had an overdose of news or too much “reality” TV, check out the Skills USA magazine Champions for some encouraging news about impressive young people excelling.

Essential Skills

When many employers are asked what they feel is most lacking from their recently hired employees, the most common answers fall into what are generally considered soft skills. “Soft skills” refer to how you interact with other people while “hard skills,” are the nuts and bolts of actually performing your job. Together with work ethics, I refer to these soft skills as essential skills because they are essential to most types of work. Nowhere is this more true than in an industry that deals with the public on a daily basis, such as HVACR. I have known technicians who lost jobs because of their lack of these essential skills. You can readily see the effect of essential skills in fast food restaurants. While most servers now say something to the effect of “thank you, have a nice day”, that is not always the meaning that is conveyed. The tone of voice and flat delivery sometimes convey the message “god I hate this job, please go away.” Since I have no desire to torture anyone by purchasing something from them, I find somewhere else to go. Why should they care? If enough people are made uncomfortable by the greeting, the company can lose enough business that employees will lose their jobs. In HVACR, customers need to trust you with much larger decisions than what to have for lunch. They want someone who is professional in appearance, attitude, and behavior. They really want to trust you because they want their problem solved. But if you convey the message that their troubles are a nuisance to you, they will feel uneasy asking for or accepting advice from you. As a service tech you might feel that you work on machines, and you do; but you work FOR the customer. The condensing unit is not going to write you a check.  So make sure and make time to listen to and communicate with the person writing the check. After all, getting paid is a pretty essential skill.