Nigel, as requested, Psychrometrics a beginners guide.

This is a psychrometric chart, Nigel Smith dared me to talk about them. You dont come across them in renewables. They leave this stuff for the big boys in Air conditioning. I'm teaching this at the moment at Eastleigh College so why not blog about it here too. It saves me writing up the notes.

When you are doing heating maths life is simple, all you need to know is the specific heat capacity of water, or that if you add 4180 Joules of heat to 1 kg of water it goes up in temperature 1 degree C. Then you bung this into this formula

Q heat = M (mass) x C (specific heat capacity x Delta T (change in temperature). and you've got your heat output. Heating is simple this is one of only 2 formulas you need to be a guru.

In Air conditioning there is a problem, the air. The same formula is true

Q heat = M (mass) x C (specific heat capacity x Delta T (change in temperature).

but the problem is C changes with pressure, humidity, moisture content etc. And mass of air also depends on pressure, humidity, temperature and moisture content. So we use the Psychometric chart instead of the formula.

First we need to measure the room temperature and humidity and plot it on the graph. Nice and simple to do with a thermometer and a hygrometer. You plot these two bits of info on the chart and where they cross is your room point. see below.

Next we need to work out how many kgs of air go through our unit per second. The reason we have to do this is you have to do your calcs by weight not by volume. But why?

Think about a fan, its like a compressor, it sucks in 1000 litres of air, it slightly pressurises it, so less than 1000 litres of air leaves the unit, However if it sucks in 1 kg of air its going to spit out 1 kg of air. (ok I now you have to allow for any moisture that falls out but lets ignore that for now).

We can look up the flow rate through the unit in 1 minute in m^3 (from manufacturers data) lets say our unit does 25m^3 per minute.

To do our maths we need to convert this from m^3/ sec into kgs / sec

The chart shows Specific volume which is how many m^3 (or 1000 litres) does 1 kg of the air take up.

What we really need though is the Density, which is just how much does 1 m^3 (or 1000 litres) of the air weigh in kgs?

since

1 / density is specific volume

and

1 / specific volume is density.

its not very hard to do.

Now we know that 1 m^3 of air weighs 0.8kg then we also know that that's 25 m^3 * 0.8 kg = 20kg / minute.

or 0.33kg/ second

last bit: the energy, air has energy, as you cool it you remove energy and as you heat it you add energy. So you plot the air before it goes through your air-conditioning unit and the air after it leaves the air-conditioning unit on the graph. Using a long ruler you read off the amount of heat before and the amount of heat after,

The heat is given on the Ziggy zaggy scale on the side and bottom of the graph. Minus the small number from the big number and that the amount of heat you will remove from every kg of the air that goes through the unit.

lets say the air has 100kJ/ kg in the room and 50kJ/kg as it leaves the unit then we remove 50kJ of heat for every kg of air that goes through the unit.

from above we know that or 0.33kg / second of air goes through the unit.

or 0.33kg / second x 50 kJ / kg is 16.5 kilo Watts or kW

Simple huh? Its like heating but harder. I have really missed air conditioning.