Month: August 2019

For those of you who use the MeasureQuick app for system diagnosis and performance testing, you may have noticed the “fan efficacy” results and wondered what it is.

It is simply the CFM output of the system divided by the wattage used by the blower. It is only for the blower motor and has nothing to do with the other components when done properly.

Fan (blower) efficacy is called out in various codes and standards such as California Energy Commissions requirement that all blowers perform at or below a 0.58 fan efficacy. This means a blower that is moving 1000 CFM cannot use more than 580 Watts of power to do so.

CFM

The tricky part is measuring fan efficacy is getting accurate measurements of system CFM and blower amperage. Equipment manufacturer fan charts can be used along with an accurate TESP (total external static pressure) measurement to figure out the CFM when the system is new and clean. When using these charts it’s important that the system is setup and run according to what is shown on the chart, one wrong pin setting or input can lead to vastly different airflow than the chart shows resulting in a fan efficacy that is way off.

Other options like measuring airflow at the return with a hood, anemometer duct traverse or the Trueflow from TEC can be used for measuring system CFM, but all have their own challenges.

Blower Wattage

When measuring blower amperage the panels must be in place which can be difficult to accomplish on some system types making a wireless connected ammeter very handy where the meter can be put in place and the panels put back on for testing.

Traditionally techs calculate wattage by measuring voltage and amperage and multiplying them together. This is actually VA not Wattage becasue it does not account for power factor. The only way to accurately measure wattage is to use a watt or power quality meter like the Redfish IDVM550 which calculates wattage by multiplying the VA by the power factor for the final wattage.

ECM Motors

ECM (electronically commutated motor) motors are more efficient than traditional PSC motors but their efficacy will generally vary based on the static pressure they are subjected to. Becasue most ECM motors are either constant airflow or constant torque rather than constant speed they will increase in wattage as the static pressure increases. This means that the fan efficacy will decrease on these motors as filters and coils become dirtier.

— Bryan

 

 


I’ve been reading a book called “Cool, How Air Conditioning Changed Everything” and it got me interested once again in the history of air conditioning and refrigeration. Like many things the people who are credited with “inventing” are the ones dogged enough to make an idea commercially successful, not the idealists forever tucked away in the lab.

I bought a 1921 version of the periodical “Ice and Refrigeration” and mixed in with the ads for absorption ice machines and “mineral wool” insulation was the advertisement shown above. Willis Carrier understood how to connect ideas and make sense of emerging technology, first to keep paper dry in a factory and later to cool the world with “Manufactured Weather”. Look carefully at the ad, you will notice it mentions many things… but not cooling, the ad is in ICE AND REFRIGERATION and the ad doesn’t mention COOLING.

Many of you know that in 1906 Willis Carrier patented what is now referred to as the “First Air Conditioning System” but do you know what it was that he actually invented?

You may be led to believe that Willis Carrier invented compression refrigeration? Nope, the first commercial attempts at compression refrigeration began in the 1830’s and the patent above actually has no compression refrigeration in it whatsoever. Many will say that he was the first to dehumidify the air, this is also false, there had been compression refrigerated cooling coils in use that dehumidified the air before Willis came along they just didn’t do it on purpose.

What Willis Carrier understood better than anyone else in his day was the RELATIONSHIP between humidity, temperature and saturated air or “dew point” and how to manipulate water temperature, water volume and air volume to produce a CONTROLLED humidity environment first and later a controlled temperature, humidity, and ventilation environment.

The Carrier “Air Washer” was nothing more than water pumped through nozzles that produced a mist of water. The air would blow through the water mist and it would clean the air, drop it to dew point (100% RH) and then continue to sensibly cool the air. Willis worked in northern states with cold groundwater at a time before water use restrictions so the cold water would serve to cool AND dehumidify the air. At the time it seemed like black magic that running air over water could REMOVE water from the air, but so long as the water temperature was below the dew point temperature of the air that is exactly what would happen. All Willis had to do to change the dehumidifier to humidifier was to increase the water temperature or change the dehumidifier to a sensible cooling machine was to use cold water and give the air more dwell time or passes through the water to decrease the sensible temperature.

In the process Carrier and his team made many discoveries about air and in 1911 Carrier presented possibly his greatest work which he called the “psychrometric formulae” which is the founding document on which all of current understanding of psychrometrics is built. Carrier took a VERY SIMPLE idea and pursued it and understood better than the others around him and because of that, we remember him today. He thought about cooling, heating, ventilation, humidity and air cleanliness and combined them together into one machine that controlled it all.

Later on, Carrier would begin actively “cooling” the air with compression refrigeration and replaced water sprays with refrigerant evaporator coils to leverage the latent capacity of refrigerants, but it all started with a mist of water an understanding of dewpoint, some dogged determination and some clever marketing for his “manufactured weather”.

— Bryan

To find the catalog where I found some of this information I created a link to the national archives at hvacrschool.com/willis

 

 

 

Photo Courtesy of Emerson

What is Cascade refrigeration?

Cascade refrigeration is a term you will hear more and more over the coming years, and while some of the systems may be very complex, the concept is actually pretty simple.

Some refrigerants are well suited for high and medium temperature applications, and some are better suited and for a lower temp applications. In a cascade system the high/medium temp refrigerant circuit is used to cool the condenser of the low temp circuit by way of a heat exchanger.

In essence, the condenser for the low temp system is also the evaporator or part of the evaporator of the high/medium temp system.

In the diagram above the medium temp circuit is used in the medium temp cases and is ALSO used in the heat exchanger to condense the refrigerant in the low temp circuit.

There are many reasons for this type of system but one of the big reasons is it is a practical solution for using CO2 (R744) as a low temp refrigerant.

— Bryan

 

Airflow, Airflow, Airflow…. when we setup and commission comfort cooling and heating systems we need to pay more attention to airflow before we worry about the fancy controls or the refrigerant circuit.

So as a thought exercise let’s consider a typical 2-ton, straight cool, TXV, residential system and think through what happens when we alter airflow and what impacts that has on the system.

Rather than talk in terms of advanced psychrometric math we will keep the math to a minimum and focus on “If this than that” relationships between airflow and system function

Mass vs. Volume

First let’s establish that it is the molecules or “stuff” that makes up air that contains and can move heat energy. While we often talk in terms of CFM (Cubic Feet Per Minute) that is a measurement of volume rather than mass. The air conditioner cares about the mass flow of air over the coil not the volume flow which is why more airflow in CFM is required in high altitudes where air density is lower.

In other words…

Mass flow is what matters and when air get’s less dense we need more air volume to move the same amount of heat

So when we speak in terms of CFM/ton (Cubic feet of air per ton of cooling) that is referring to typical air at sea level and needs to be adjusted as air density changes.

400 CFM/Ton 

The 400 CFM/ton design has been used for years and it is an adequate baseline airflow for many types of equipment and in many moderate climate zones. There are several issues with the 400 CFM/ton rule where it needs to be adjusted.

  • Higher altitudes where air is less dense and therefore more air is required to maintain the same mass flow rate over the coil
  • The nominal or listed tonnage on a piece of equipment is often NOT what the equipment produces at current load conditions. A 2-ton system that is designed for AHRI conditions (95° outdoor and 80° indoor return temperature) could easily produce under 20K btu/hr at 73° indoor and 97° outdoor temperatures, so 800 CFM would be well over 400 CFM/ton in that scenario.
  • Areas with higher latent (humidity) load will run lower than 400 CFM/ton on purpose to remove more moisture from the air and areas with arid (dry) climates will often run higher than 400 CFM/ton to remove less or no moisture from the air.

How The Evaporator “Absorbs” Heat

In my refrigeration circuit basics training I call the evaporator coil the “heat absorber” because its end goal is to take heat from where you don’t want it and move it somewhere else.

The heat gained in the evaporator in this scenario comes from the indoor air being moved over the evaporator coil. The air is warmer than the refrigerant so heat leaves the air as it impacts the tubing and fins of the coil because “hot goes to cold”.

The heat is transferred from the air though the walls of the copper tubing and into the refrigerant via conduction while the heat is transferred through the air and refrigerant itself via convection because they are both dynamic (moving) fluids.

The air temperature is decreased because heat is removed from it into the refrigerant. The refrigerant in the evaporator coil is at saturation (boiling) so the coil temperature doesn’t change directly as heat is added to the refrigerant but it does begin to increase indirectly because as the total heat energy in the evaporator increases so does the coil pressure and vice versa. This is similar to the pressure cooker effect where as the water boils in the pressure cooker the pressure increases and so does the boiling temperature of the water.

When the temperature of the coil is below the dew-point of the air moving over it there is also a transfer of latent energy from the air as some of the water vapor in the air condenses to liquid water (condensate) on the evaporator coil. This latent heat transfer does not result in colder air but rather lower moisture content in the air, this heat does impact the evaporator in the same way as sensible heat as it is added to total heat picked up in the evaporator.

Evaporator Coil TD

We use the term “coil TD” a bit differently in different parts of the industry but in air conditioning it is the difference between the air temperature of the return air entering the evaporator coil and the saturated suction temperature often called the “coil temperature”. In typical 400 CFM/ton applications this difference will be around 35° with a higher number meaning a colder coil and a lower number meaning a warmer coil. There are several things that can impact coil TD including refrigerant mass flow rate (how much refrigerant the compressor is moving), metering device performance, return air dew point (moisture content) and most commonly…. airflow.

What Happens When Airflow is Decreased?

In this theoretical system when the airflow is decreased and all else stays the same the following things will occur –

  • Mass airflow will decrease, meaning there are fewer molecules moving across the coil
  • Air velocity will decrease, meaning the air is moving over the fins and tubing more slowly
  • Bypass factor decreases, this means more of the air molecules will be touching the metal as a ratio
  • Air temperature decreases (to a point) due to the air moving more slowly across the coil with less bypass factor
  • Coil temperature decreases because less overall heat is being picked from the air
  • Coil drops further below dewpoint, causing more moisture to be removed from the air increasing dehumidification
  • Suction pressure decreases because less heat energy being picked up means less pressure and as the superheat falls the TXV also futher throttles the flow of refrigerant through the coil
  • Compression ratio increases as the suction pressure drops meaning the compressor moves less refrigerant as the refrigerant density entering the compressor falls
  • Coil TD increases as indicated by the colder coil in relationship to the return air

We all know that if you have far too little airflow a system can freeze up when the coil temperature drops below 32°F. The other consequence of dropping airflow is lower overall sensible capacity and therefore a drop in EER and SEER rating. On the positive side in humid climates, a system with lower airflow will remove more water from the air which can be desirable.

The lesson is, sometimes you need more airflow and sometimes you need less but no matter what, changing airflow changes a lot about how the system operates and should be done carefully and thoughtfully.

— Bryan

 

 

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