Tag: enthalpy

In HVAC/R we are in the business of moving BTUs of heat and we move these BTUs on the back of pounds of refrigerant. The more pounds we move the more BTUs we move.

In a single stage HVAC/R compressor, the compression chamber maintains the same volume no matter the compression ratio. What changes is the # of pounds of refrigerant being moved with every stroke(reciprocating), oscillation (scroll), or rotation (screw, rotary) of the compressor. If the compressor is functioning properly the higher the compression ratio the fewer pounds of refrigerant is being moved and the lower the compression ratio the more pounds are moved.

In A/C and refrigeration the compression ratio is simply the absolute discharge pressure leaving the compressor divided by the absolute suction pressure entering the compressor.

Absolute pressure is just gauge pressure + atmospheric pressure. In general, we would just add the atmospheric pressure at sea level (14.7 psi)(1.01 bar) to both the suction and discharge pressure and then divide the discharge pressure by the suction. For example, a common compression ratio on an R22 system might look like-

240 PSIG Discharge + 14.7 PSIA = 254.7
75 PSIG Suction + 14.7 = 89.7 PSIA
254.7 PSIA Discharge ÷ 89.7 PSIA Suction = 2.84:1 Compression Ratio

The compression ratio will change as the evaporator load and the condensing temperature change but in general, under near design conditions, you will see the following compression ratios on properly functioning equipment depending on the efficiency and conditions of the exact system.

In air conditioning applications compression ratios of 2.3:1 to 3.5:1 are common with ratios below 3:1 and above 2:1 as the standard for modern high-efficiency Air conditioning equipment.

In a 404a medium temp refrigeration (cooler) 3.0:1 – 5.5:1  is a common ratio range

In a typical 404a 0°F to -10°F(0°K to -5.5°K) freezer application 6.0:1 – 13.0:1 is a common ratio range

As equipment gets more and more efficient, manufacturers are designing systems to have lower and lower compression ratios by using larger coils and smaller compressors.

Why does the compression ratio number matter? 

When the compressor itself is functioning properly the lower the compression ratio the more efficient and cool the compressor will operate, so the goal of the manufacturer’s engineer, system designer, service technician and installer should be to maintain the lowest possible compression ratio while still moving the necessary pounds of refrigerant to accomplish the delivered BTU capacity required.

The compression ratio can also be used as a diagnostic tool to analyze whether or not the compressor is providing the proper compression. Very low compression ratios coupled with low amperage and low capacity are often an indication of mechanical compressor issues.

Compression ratio higher than designed = Compressor overheating, oil breakdown, high power consumption, low capacity 

Compression ratio lower than designed = Can be an indication of mechanical failure and poor compression

Understanding compression is critical to understanding the refrigeration process. Don’t be tempted to skip past this because it is a really important concept.

Look at the pressure enthalpy diagram above. Top to bottom (vertical) is the refrigerant pressure scale, high pressure is higher on the chart. Horizontal (left to right) is the heat content scale, the further right the more heat contained in the refrigerant (heat, not necessarily temperature).

Start at point #2 on the chart at the bottom right. This is where the suction gas enters the compressor. As it is compressed it goes to point #3 which is up because it is being compressed (increased in pressure) and toward the right because of the heat of compression (heat energy added in the compression process itself) as well as the heat added when the refrigerant cooled the compressor motor windings.

Once the refrigerant enters the discharge line at point #3 it travels into the condenser and is desuperheated (sensible heat removed). This discharge superheat is equal to the suction superheat + the heat of compression + the heat removed from the motor windings. Once all of the discharge superheat (sensible heat) is removed in the first part of the condenser coil it hits point #4 and begins to condense.

Point #4 is a critical part of the compression ratio equation because the compressor is forced to produce a pressure high enough that the condensing temperature will be above the temperature of the air the condenser is rejecting its heat to. In other words, in a typical straight cool, air cooled air conditioning system the condensing temperature must be higher than the outdoor temperature for the heat to move out of the refrigerant and into the air going over the condenser.

If the outdoor air temperature is high or if the condenser coils are dirty, blades are improperly set or the condenser coils are undersized point #2 (condensing temperature) will be higher on the chart and therefore will put more heat strain on the compressor and will result in lower compressor efficiency and capacity.

As the refrigerant is changed from a liquid vapor mix to fully liquid in the condenser it travels from right back left between points #4 and #5 as heat is removed from the refrigerant into the outside air (on an air cooled system). Once it gets to #5 is is fully liquid and at point #6 it is subcooled below saturation but ABOVE outdoor ambient air temperature. The metering device then creates a pressure drop that is displayed between points #6 and #7. The further the drop, the colder the evaporator coil will be. The design coil temperature is dictated by the requirements of the space being cooled as well as the load on the coil but the LOWER the pressure and temperature of the evaporator the less dense the vapor will be at point #2 when it re-enters the compressor and the higher the compression ratio will need to be to pump it back up to point #3 and #4,

This shows us that the greater the vertical distance between points #2 and #4 the higher the compression ratio, which means that both low suction pressure and/or high head pressure result in higher compression ratios, poor compressor cooling, lower efficiency and lower capacity.

In some cases, there isn’t much that can be done about high compression ratios. When a customer sets their A/C down to 69°F(20.55°C) on a 100°(37.77°C) day they will simply have high compression ratios. When a low temp freezer is functioning on on a very hot day it will run high compression ratios.

But in many cases, you can reduce compression ratios by –

  • Keeping set temperatures at or above design temperatures for the equipment. Don’t be tempted to set that -10°F freezer to -20°F(-5.5°K to -11°K)or use that cooler as a freezer
  • Keep condenser coils clean and unrestricted
  • Maintain proper evaporator airflow
  • Install condensers in shaded and well-ventilated areas

Keep an eye on your compression ratios and you may be able to save a compressor from an untimely death.

— Bryan

 

 

 

 

 

Both wet bulb temperature and air enthalpy are extremely useful to understand when calculating actual system capacity as well as human comfort. Dry bulb temperature is a reading of the average molecular velocity of dry air, but it does not take into account the actual heat content of the air, or the evaporative cooling effect of the air.

Like we mentioned in the last tip, when air is at 100% relative humidity the dry bulb, wet bulb and dew point temperatures are all the same. This is because at 100% relative humidity the air is completely saturated with moisture and can have no evaporative effect.

When air is less than 100% RH it will provide an evaporative cool effect and wet bulb temperature is a measurement of that effect. In fact, wet bulb temperature is the temperature a damp thermometer bulb will read when exposed to a 900 FPM (Feet per minute) air stream. If you have ever seen someone using a sling psychrometer, that is exactly what is happening (Hopefully you have a wrist that is well calibrated to 900 FPM). The lower the wet bulb in comparison with the dry bulb (This differential is called wet bulb depression) the lower the relative humidity and the greater the the evaporative cooling effect.

Enthalpy is the total heat content of the air and is represented in BTUs per lb of air. By converting lbs of air to cfm we can calculate the amount of heat in an air mass as well as the change in the enthalpy across a coil to calculate the heat moving capacity of a coil, BTU losses / gains over a length of duct and much more.

You will notice that wet bulb and enthalpy are slanted lines, descending from left to right and they are equivalent. This means that a particular wet bulb temperature is also equal to a particular enthalpy (At 14.7 PSIA at least). In the chart above you can see that a 62.8 degree wet bulb mass of air contains approximately 28.4 btus per lb. The tricky part is reading at this extreme level of resolution, because 28.4 vs. 28.6 can make a significant difference when it is multiplied out over a large air mass. This demonstrates why VERY accurate tools and careful calculations are required for enthalpy calculations in HVAC/R.

— Bryan

For a full WB ot Enthalpy calculator go HERE and look for the enthalpy chart

Just so you don’t get bored and quit reading let’s go straight to the point.

When the blower runs for more than a few minutes after the system has cycled off in cool mode the air may continue to be “cooler” (lower sensible temperature) coming out of the supply but the heat content of the air will remain unchanged. 

The only reason I say “may” be cooler instead of “will” be cooler is that we are assuming there is moisture on the coil and/or in the pan and the indoor RH is less than 100%.

Translation: When you run the blower once the system has gone off in cool mode you will continue to cool for a while, but that extended cooling comes from the evaporation of water out off of the coil and out of the pan. This results in sensible cooling and greater sensible efficiency but also increased indoor humidity.

Translation of the translation: It may feel cooler but there ain’t any less heat in the air by the time you figger for humidity.

Translation of the translation translation: If you live in a humid place run shorter off cycle run times and think twice before running the fan in the “on” position. If you are in a dry place then let it blow until your heart is content.

Whenever cooling occurs by direct evaporation of a substance into an airstream (think a swamp cooler) it occurs at no net decrease to the heat content in the air. The heat is just going from sensible (what you can measure with a thermometer) to latent resulting in higher relative humidity air.

If you go below this line it is going to get nerdy… BEWARE


Now let’s talk about why, but first some terms.

Heat = Molecular energy or total molecular movement within a substance
Temperature = Molecular velocity, the speed that the molecules are moving
Adiabatic Process = A change in temperature without a change in heat content

Think of  adiabatic process like this – You have a whole room full of ping pong balls bouncing around in a zero gravity room. The balls are molecules, their total motion is the amount of heat and the speed they move is temperature. If you were to change the size of the room by bringing in one of the walls the balls the balls would bounce faster because the available space was decreased so the “temperature” would increase but the number of balls and the total motion would remain constant (this is what happens to refrigerant in a compressor by the way). If you were to move a wall outward and increase the size of the room the speed of the of the molecules would decrease, resulting in less speed and lower “temperature”. All the while the number of balls and the total motion remained constant (which is what occurs at the outlet of the metering device). In both of these examples temperature (Sensible heat) changes but the total heat content does not change, these are both examples of adiabatic process due to compression and expansion of contained molecules.

Adiabatic process can also occur in uncontained systems like open air streams, and evaporation of water is one such example.

Evaporation of water is a process where heat is absorbed into water molecules as they evaporate from liquid water and become entrained in the air as a vapor displacing some of the nitrogen and oxygen in the air. When that heat is absorbed from the air into the water it results in lower sensible temperature, but the water is still CONTAINED IN THE AIR. This means that while the air may be cooler it still has all the heat contained in it in the form of water vapor.

Now for the real shock..

Water vapor is NOT more dense than dry air at the same temperature it is actually less dense / lighter than dry air, however is does contain more heat (enthalpy for you nerds like me). This means that when you run the blower after a cooling cycle the moisture on the coil and in the pan are evaporated back into the space and depending on the RH of the air it will lead to sensible cooling but latent gains. This means cooler but higher RH and this is due to the higher heat content of higher RH air at the same temperature..

Once again, depending on where you live this may be a positive or a negative.

In Arizona or Colorado? Run that blower after the cooling cycle.

Florida? May wanna shut it off right after the cycle or maybe 90 seconds at most and leaving the fan in the “on” position will likely result in a small increase of indoor RH.

— Bryan

 

 

P.S. – I also did a Facebook Live Video about it today

also… Here are some great videos on the subject by Jim Bergmann

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