Tag: efficiency

This article was written by Gary McCreadie from “HVAC know it all”. You can learn more about Gary and his tips and growing community on Facebook and on LinkedIn


What is an economizer?  Simply put, it is a mechanical device that is designed to reduce the consumption of energy, whether it be fuel, electricity, or other. According to Wikipedia, the first economizer was patented by Edward Green in 1845.  It was used to increase the efficiency of stationary steam boilers.

This article will revolve around air-side economizers.  You will typically see them as an accessory built into rooftop units used for the purpose of “free cooling”.  Free cooling is a funny term because it’s not actually “free”, the fan motor and economizer controls must be powered in order to operate, which consumes energy.  The term merely demonstrates the fact that less power consumption is taking place due to the fact we are utilizing outdoor air to cool a space rather than the use of a compressor or compressors.  Economizers also offer the added feature of providing fresh air to the building and its occupants.  A carbon dioxide sensor can be integrated into the setup.  As CO2 levels increase within the building, the outdoor air dampers are commanded to open, filling the space with fresh air.  As CO2 levels drop off, the dampers return to their minimum position.
The Guts of an Economizer
The economizer set up employs several parts in order to operate correctly.
1) A set of outdoor air dampers that are directly linked to the return air dampers are used to control air flow.  They move together as one, as the outdoor air dampers begin to open, the return air dampers begin to close and vice versa.
2) An outdoor air sensor.  This sensor is responsible for determining if the outdoor air is acceptable for free cooling.  In most cases, there will be an option between a sensible temperature sensor or an enthalpy sensor.
Sensible Temp Sensor – Measures dry bulb temperature of the air
Enthalpy Sensor –  Measures heat content within the air measured in btu/lb.  This sensor takes dry bulb temperature and wet bulb temperature into account for total heat content.
3) An indoor air sensor, this sensor reads sensible temperature and is responsible for maintaining mixed or discharge air temperature.  The damper assembly will modulate according to feed back from this sensor to maintain a pre-determined mixed or discharge air set point.  On newer economizer controls, like the Honeywell Jade for example, you are able to set the mixed or discharge air temperature as desired.
4) The damper actuator, which receives a signal from the economizer control board and moves to the assigned position to maintain the mixed air or discharge air set point.
5) When using free cooling you must remember that you are introducing fresh air, this added air into the space can cause positive pressure issues within a building.  To eleviate this problem economizers in most cases will have a built-in barometric relief damper or power exhaust system.
6) The control board is the heart and soul of the operation.  The control board receives sensor input signals, internally calculates the next step and relays the output signals to the damper actuator and power exhaust motor if utilized.
Order of operation
To keep it simple, the following example will be based on a single stage cooling rooftop unit complete with an economizer package.
On a call for cooling from the thermostat or BAS (building automation system), the Y1 terminal will be powered.  In most cases, the signal will first move through the rooftop control board and over to the econmizer control.  At that point, the econmizer control will then decide whether to proceed with free cooling or mechanical cooling based upon the outdoor air conditions either using sensible temperature of the air or the heat content of the air measured in enthalpy.  If the outdoor air is not suitable for free cooling, the control signal will be then relayed back to the main control board of the rooftop and initiate mechanical cooling (compressor operation).  If the outdoor air is suitable for free cooling, the outdoor air dampers will modulate from their minimum position (damper minimum position is set up during commissioning to maintain constant fresh air to the building and occupants) to maintain the mixed air or discharge air set point until the space temperature is reached.  Once the thermostat or BAS has been satisfied, the call for cooling will cease.
Most air side economizers in general, work as explained above.  It is best to contact the manufacturer of the equipment you are working on for technical advice or when issues pertaining to that system arise.
— Gary McCreadie – HVACKNOWITALL.com

There was a story that came out recently based on an ASHRAE study performed by David Yuill from University of Nebraska that appeared to indicate that cleaning condenser coils makes no difference on system performance and efficiency.

Those of us who have worked in the field know that coil cleaning matters because most of us have had a system that wasn’t working well, or possibly even cutting out on high head pressure. We cleaned the coil and the system started working properly… over and over again

But as an exercise… a thought experiment… let’s work through this and see some possible reasons why this conclusion may have been reached.

The job of an air cooled condensing coil is to reject heat from the refrigerant to the air. The rate at which it does this is a function of contact time, temperature differential, the thermal conductivity of the material through which heat is being transferred and turbulence of both fluids (refrigerant on the inside of the tubing and the air on the outside).

You may have noticed that modern condenser coils are larger than they used to be, the reason for this is simple, the larger the surface area of the coil, the more heat can be transferred from the refrigerant to the air resulting in a lower required condensing temperature and lower head pressure. In other words, by increasing the contact time we don’t need as great of temperature difference between the the refrigerant in the tubing and air passing over it to accomplish the same amount of heat transfer.

Engineers have also learned that by changing the design of coils we can get greater contact surface area with less refrigerant with coils such as micro-channel or they can get greater internal turbulence by adding grooves or rifling in the tubes of better external turbulence by adding little kinks to the fins of the coil. They do all of this to attempt and move heat from the refrigerant in the most efficient way possible and I applaud them for their efforts.

So how could a “dirtier” coil ever be more efficient? it is at least theoretically possible that certain types of surface fouling might act to create more air turbulence and actually increase heat transfer… and if you tested 100 systems in field conditions you may find a few that exhibit this undesigned behavior depending on the type of coil and the type of soil.

In the field we know this isn’t normal…

How many of us who do small kitchen refrigeration have gone out to a freezer not keeping temp or an ice machine making ice like it once did, only to clean the condenser and everything starts working properly again?

In our minds we imagine that the dirt or grease is acting like an insulating “blanket” preventing heat transfer, and that is certainly one factor, but it isn’t the only thing going on.

Condenser Fan Efficacy 

Condenser fans are prop fans, more technically known as “axial” fans as opposed to blower wheels which are known as radial or centrifugal. Axial fans are good at moving a lot of air against very low pressure, but as soon as the pressure starts to build their performance drops off REALLY quick. We have all walked up to a condenser fan where the air was just sort of beating out of the side instead of really pushing out the top like it’s supposed to. Once you clean the coil it starts moving air again and you can really tell the difference.

So much of the decreased heat transfer comes from the fact that dirt blocks the airflow causing less air to move over the coils which drives up the condensing temperature and head pressure.

Compression Ratio 

As the head pressure and condensing temperature increase the compression ratio increases (absolute head divided by absolute suction) which causes the amount of refrigerant the compressor moves to decrease resulting in both higher compressor amperage and lower system capacity. This effect is greater with TXV/EEV systems because the valve will tend to throttle down as the head pressure increases to maintain superheat further increasing the compression ratio.

Evaporator Temperature

On fixed metering device systems higher head pressure will also drive up suction pressure which will tend to keep the compression ratio slightly lower but will result in higher coil temperature and poor latent (humidity) control.

So to put my money where my mouth is we picked a nice dirty coil and ran a full, white paper style test. For the sake of complete disclosure we used the fan curve charts to come up with evaporator air flow, which is fine because it was a before / after test. I used MeasureQuick for the calculations and my phone was giving me trouble and kept losing my manually entered data so I realized later that in my AFTER report (that some of you may have seen in my group) the airflow was set to 750 and before was set to 700 so I went back in and changed the math so everything was apples to apples. Either way… the results are pretty self evident. You will notice that the “official” results below are slightly different than those in the screenshots at the top, and that math change is the reason.

Equipment Cleaned

2-ton 1999 Trane R22 10 SEER “Spine Fin” Heat Pump Split system with a direct return operating and 0.4” WC total external static pressure on a PSC blower and a fixed piston type metering device.

Test Process

I Allowed the system to run 20 minutes continuously and took detailed measurements sufficient to compare wattage, total BTU/H removal and therefore the EER of the system using wireless connected digital instruments and the MeasureQuick app.

We cleaned the condenser coil only while performing this test no other cleaning or servicing and making no adjustments to refrigerant charge.

We then allowed the system to run continuously for another 20 minutes to ensure the coil is completely dry while confirming by measuring condenser air dew point entering and leaving. Retake the same measurements and compare the results.

Cleaning Method

CoilJet using Refrigeration Technologies Viper cleaner and then rinse working inside out

Results

The before results showed clearly that the head pressure and liquid line temperature were both high with a low subcooling and superheat. The measured system performance was poor even though the evaporator coil, air filter and blower wheel were quite clean considering the age of the system.

After cleaning the head pressure and suction pressure dropped, the subcooling and superheat increased and the compressor amperage dropped. It became clear after the cleaning that the system was slightly low on refrigerant because it maintained a stable 31° superheat.

The system performed significantly better in terms of decreased wattage and increased BTU removal after the cleaning.

BeforeAfter
Suction Pressure / Evaporator Temp75.967.9
Liquid Pressure / Condensing Temp278.6216.5
Outdoor Air DB 89.091.0
Superheat1.331.5
Subcooling3.011.9
Airflow CFM750*750
Condenser Voltage245244
Condenser Amperage 11.410.3
Total BTU Capacity19,37220,992
Total Wattage 2,6442,367
EER7.328.86

After this test was complete we added 9 oz of R-22 to achieve the factory required superheat. Following the adjustment the EER and total system capacity improved even further.

This illustrates that cleaning this condenser indisputably improved –

System Capacity
Performance
Compressor Longevity

 

I wrote to David asking him to come on the podcast and explain his findings a few moths ago and he responded to that via email with this –

“At some point I’d like to set everybody straight in one fell swoop, and maybe your HVACrSchool is the venue for that, but I haven’t decided yet.”

I don’t think David’s research is “wrong”, I’m sure they got the results they said they got, the issue must be a disconnect in the way the tests were performed and the way many systems perform in the field. I do think the conclusion the article came to was incorrect

no… I KNOW IT IS INCORRECT

The point of the study was all about heat transfer and in real life if we control for ambient conditions all we would need to do it measure head pressure, clean the coil, let it dry and measure head pressure again. If it goes down then more heat transfer is occurring (again, controlling for changes in ambient conditions and indoor load).

For fun, I would encourage you to try the same tests and let me know your findings. I used MeasureQuick, a Redfish meter and Fieldpiece Joblink probes to collect the data but you could do it with any accurate modern digital instruments. Just make 100% sure the coil is dry after cleaning or you will get false measurements. If you find a system that doesn’t improve, or gets worse it would be great to know the “why” behind that example by reviewing the application and data.

If you want to come to your own conclusions as to why the research came to the findings it did… the test apparatus is shown below.

This is the peer reviewed article

Image shown under Creative Commons from –

Mehdi Mehrabi & David Yuill (2019) Fouling and Its Effects on Air-cooled Condensers in Split System Air Conditioners (RP-1705), Science and Technology for the Built Environment, 25:6, 784-793, DOI: 10.1080/23744731.2019.1605197

— Bryan

P.S. – Out full report on coil cleaning will be available on Speedclean.com in a few weeks so keep your eyes peeled.

 

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