Enthaply Explained....

Enthalpy Explained....

Enthalpy, Entropy, Pressure-Enthalpy charts, Mollier charts? I am sure there are a few of you (including myself a few years back) heard of these words, but had no real clue to their meaning, reason for understanding them, and what would we really need to understand about these strange terminology, that will allow the technician to perform his or her job any better?

I for one, in 2008, heard these terms for the very first time, and i had no clue what they meant. My problem was, and still is today, that should i come across a topic or subject, that i have very little knowledge about, i immediately get to work researching, speaking to industry related friends, engineers, just about everything to fully understand and comprehend these new found terms, and its meanings, and exactly what its reason for existence is. Most importantly, what i am missing, and how can i be the best i can be if i do not know this..... Yes, i create my own problems, i know, however it allows me to share with all of you, who hopefully will start to follow me in my blogging journey.

Lest start with Entropy...

Entropy measures the energy dispersion in a system divided by temperature. this ratio represents the tendency of energy to spread out, to diffuse, to become less concentrated in one physical location or one energetic state. That spreading out is often done by molecules because molecules above absolute zero always have energy inside of them. That's why they are incessantly speeding through space and hitting each other and rotating and vibrating in a gas or liquid. Entropy is measured in KJ/Kg/℃.

Entropy = Change in heat/temperature
              = △ q / t

Energy is the capacity of a system to do work where "system" refers to any physical system, not just a refrigeration system.


Enthalpy

Enthalpy is the total amount of heat in one Kg of a substance. its units are therefore kj/Kg. The imperial part is BTU/Lb.

Mollier Charts

Mollier charts are used in designing and analyzing performance of vapour compression refrigeration systems. Each refrigerant has its own chart which is a graph of the Enthalpy of a refrigerant during various pressures and physical states. Mollier charts are also called Pressure-Enthalpy diagrams. Pressure is shown on the vertical axis, enthalpy is on the horizontal axis.

Below is an example of a Pressure - Enthalpy chart or Mollier Chart

R410a Pressure - Enthalpy Chart

Now, i will illustrate and explain the several other parameters which can be shown on a mollier chart including temperature, volume, saturation, quality and entropy.




























Now that i have illustrated exactly how the Pressure - Enthalpy chart looks, like, it is clear that without this understanding, we can not fully commission, re-commission a refrigeration system and be completely satisfied with its performance, after all, what are we referencing our final commissioning data to? We need the correct refrigerant chart in order to plot our recorded and measured data and perform the necessary calculations and identify whether the evaporator has the designed cooling effect, whether the heat of compression and the evaporator enthalpy equals the heat rejected by the condenser. We can also calculate the refrigerant mass flow rate, and calculate the capacity of the system.

These logs, and data should be placed in the refrigeration plant room for future reference.

I trust this blog was informative and created a new found knowledge to any one who ever questioned these terminologies. I really do hope that these blogs of mine do assist those technicians who really want to be the best they can be in this trade.

Please comment on my page www.cyrilmacquetmaurel.blogspot.com.

Superheat Explained

Checking and setting superheat at commissioning is important to ensure the efficient running and long life of the equipment. This is critically important with split systems to ensure that the refrigerant volumes are correct particularly where additional refrigerant charge is required.

Whilst modern air conditioning equipment makes checking Superheat as simple as plugging a "Service Interface”  or reading the built in display on PLC control board, there is still plenty of equipment in the field with older style control systems that require the manual superheat testing process outlined below.

So what exactly is superheat?

Superheat is an indication of the amount of extra heat (or superheating) the refrigerant vapour has received after it has passed through its latent phase in the heat transfer coil.

Why is correct superheat so important?

The compressors lifespan!! If the superheat is set right then there is no reason why the compressor won’t run for twenty or more years. If it’s not set right and the compressor is flooding or starving excessively, then expect as little as a year or two before failure. The compressor needs the correct amount of returning refrigerant to keep its motor windings at the correct temperature and most importantly its oil at an optimum.

High Superheat: (starving)

In this scenario, the compressor will get too hot causing its sump oil to overheat, slowly break down and lose its viscosity (lubricating effect). If you don’t have the correct lubricating effect, the compressor bearing will wear and eventually seize.

Low Superheat (flooding)

In this scenario, there is too much refrigerant coming back. Some say that these new scroll compressors can withstand liquid flooding, which they can, but it’s the oil that can’t.

What happens is the sump oil becomes diluted with the refrigerant and loses its viscosity (lubricating effect). If you don’t have the correct lubricating effect, the compressor bearing will wear and eventually seize.

Points to note:

1.    Bearing failure often gets reported as a winding failure because the resulting locked rotors high current draw can cause the winding to short out. Its not until the compressor is cut open for inspection that you realize the seized bearing caused the winding to blow.

2.    Superheat should be checked after the compressor has run continually for at least ten minutes to allow the system refrigerant and oil to settle.

3.    The indoor units return air temperature and airflow should be as close to normal operating conditions as possible as well as the outdoor ambient.

Checking Superheat

It is preferred to set superheat in cooling cycle when the indoor return air temperature is just above set point e.g 23ºC plus or minus a couple of degrees. The target superheat temperature is generally 3ºC to 5ºC (4ºC to 6ºC for EEV’s)

1.    Fit your LP gauge to the common suction line port situated between the compressor and the reversing valve.

2.    Convert this Low Pressure to temperature using a pressure/temperature conversion chart or the relevant refrigerant temperature scale on your manifold gauge.

3.    Measure the temperature of the suction line as it enters the condensing unit with an “accurate” digital thermometer.

4.    Subtract the converted pressures temperature away from the pipe surface temperature and that’s the superheat.

Example 1: If the R410A units low pressure was 755kpa and the suction pipe temperature was 12ºC then: 755kpa converted to temperature is 2ºC so we then subtract 2 from the 12ºC pipe temperature which results in a superheat of 10ºC. Indicating in this case it’s a little high and requires lowering.

Example 2: If the R22 units low pressure was 380kpa and the suction pipe temperature was 4ºC then: 380kpa converted to temperature is -1ºC so the difference between this -1 and the 4ºC pipe temperature is a superheat of 5ºC Indicating in this case it is OK.

Adjusting superheat

TX Valve

Wind the valve stem IN to increase or wind it OUT to decrease, then wait approximately ten minutes for it to settle before rechecking.

Accurators

Add refrigerant to decrease Superheat or Remove refrigerant to increase Superheat, then wait approximately ten minutes for the system to settle before rechecking.

EEV

Electronic expansion valves can only be adjusted by changing their internal program parameters which are usually factory set and chances are the problem is lack of system refrigerant or a fault elsewhere.

There are many influences associated with superheat like evaporator airflow, evaporator return & supply air temperatures and condensing temperature. So if you’re unsure, you need to contact the relevant supplier for technical information.

Compressor Discharge Pipe Temperature (hot gas)

The compressor discharge pipe temperature is in direct relationship to the superheat. A ball park figure is that this temperature on a scroll compressor is usually 60ºC to mid 70’s when the superheat is acceptable and the operating conditions are normal.

As a crude rule of thumb, you will find that if the discharge pipe is cool enough to hold your hand on then you can almost guarantee that the refrigerant is flooding back (0ºC superheat). If it’s too hot to touch, then its superheat is probably too high (starving).

You should be able to touch the compressor discharge pipe for about half a second before the heat is unbearable. Next time you set the superheat, measure the discharge temperature and after a few settings you should see a pattern emerge