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Impacts of Decreasing Evaporator Air Flow
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.
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.