psychrometric chart high temperature carrier

This may seem counter intuitive however the ideal gas law states that a fixed volume of air at a certain pressure has a fixed number of molecules and each of those molecules has a particular mass. However density is useful when measuring the flowrate of air through pressure drop devices (such as orifice plates).Ĭonsidering the formula for air density above, moist air is less dense than dry air. Specific volume is particularly useful when undertaking calculations where there is a change in moisture content during a process (humidification and dehumidification), as the amount of dry air will remain constant while the amount of water vapour changes. The difference will rise as moisture content increases and in conditions that are likely to prevail in heating, ventilation and air conditioning applications there is likely to be maximum of about 4 per cent difference at high temperatures and high moisture contents between the simple reciprocal value of specific volume and the true density.

m -3Īnd to see what the practical difference is between the inverse of specific volume and density (at this particular condition) consider that the reciprocal of the specific volume calculated above is 1.192 and this compares with a density of 1.200 – a difference of less than 1 per cent.m -3Īs an example consider air at 20.0 deg C dry bulb and 13.9 deg C wet bulb at an atmospheric pressure of 101.325 kPa = 101325Pa (ie standard atmospheric pressure).Ī value of p s’ is required and so using….Rearranging this and putting in values for R a and R v gives

So as m a/V=p a/R aT and m v/V=p v/R vT the overall density of the single unit volume of moist air will be (p a/R aT + p v/R vT). To evaluate the mass of 1 m 3 of moist air the masses of the dry air and the water vapour are added together. m -3), is the mass of moist air that occupies a volume of 1 m 3.Where V is the volume occupied by the moist air, m a is the mass of the dry air (that is combined with the mass of water vapour, m v, to give kg moist air), p a is dry air partial pressure, p v is water vapour partial pressure and p at is atmospheric pressure.ĭensity, ρ (kg V = V/m a = R aT/p a = R aT /(p at – p v) kg -1 da), is the volume of 1 kg dry air and its associated water vapour, so using the gas law pV = mRT,.Many texts simply relate one as the inverse of the other – however there is a convention in the HVAC&R world that, for humid air, this is not the case. The final volume of the psychrometric story?Īn element of the standard psychrometric chart that causes widespread uncertainty is the specific volume and more particularly its relationship with density. Note that the saturation vapour pressure is not dependent upon the atmospheric pressure. Log p s = 30.5905 – 8.2 log (θ + 273.16) + 2.4804 x 10–3 (θ + 273.16) – Īnd this is ideally suited to use in a spreadsheet or web tool (and is valid for temperatures above 0 deg C). Where p s’ is the saturation vapour pressure at the wet-bulb temperature that can be determined from tables or from the rather lengthy but suitably accurate empirical equation that is used to determine saturation vapour pressure, p s (kPa)

To establish the vapour pressure in air, p v(kPa) the relationship based on Sprung’s original experiments with in the late 19th century known as the psychrometric equation may be used So a key requirement of psychrometric analysis is the ability to establish the vapour pressure that in turn can be used to calculate the air moisture content. Arguably the one process that determines whether a system is air conditioning, as opposed to simply ventilation, is the control of air moisture content.

Psychrometric chart high temperature carrier