Estimates of Dry Air Requirements of the P0D. Lets assume a simple model of input dry air (from the membrane filter) with a relative humidity that mixes with water vapor from leaking from the P0D water target bags
Lets assume a simple model of input dry air (from the membrane filter) with a relative
humidity that mixes with water vapor from leaking from the P0D water target bags
and creates a final mixture of output moist air with an increased relative humidity.
The final mixture of output moist air should have a relative humidity that is less than
the saturated dew point of the temperature of the cooling tubes in the P0D to prevent
We use a psychrometric model of humidifier with adiabatic mixing of water inject into
moist air from the 2001 ASHRAE handbook on Fundamentals (HVAC), pg 6.18. In this
model input air is mixed with water and creates moist output air. The temperatures and
humidity ratios (ratio of mass of water vapor to mass of dry air mass) of the airs will
determine the enthalpy which is read off the psychrometric charts. For background
web material see http://en.wikipedia.org/wiki/Psychrometrics. Other useful references
are Schaum’s outline on “Thermodynamics for Engineers”, chapter 12, example 12.12,
Since the processes are isobaric (constant pressure) we can set up energy equations
using enthalpy and mass rate eqns. These eqns are usually sufficient to solve for the
relevant parameters in the problem. The problem is similar to an evaporative water
cooler, but the input and output temperatures are the same.
Psychrometric Chart at sea level pressure. The horizontal axis is air temperature,
the vertical axis is humidity ratio. the black lines are enthalpy, the red lines
are relative humidity, the green lines are temperature, and the purple
lines are humidity ratio (water mass/air mass).
Remarks; I have had two meeting with our thermal engineer, Pat Burns, who also
teaches HVAC and he gave me the ASHRAE standard reference to look up how
to model our dry air questions. I have not yet cross checked the above with Burns.
We use a psychrometric model of of adiabatic mixing of water inject into moist air from
the 2001 ASHRAE handbook on Fundamentals (HVAC), pg 6.18. A schematic diagram
is given below. Dry air enters a volume at an air mass rate of mda, enthalpy h1, and
humidity ratio W1. Water with enthalpy hw is absorbed at a mass rate of mw. The
resulting moist air mass rate is still mda, but with enthalpy h2 and humidity ratio W2.
The energy and mass rate equations are
These two eqns can be readily solved to
eliminate the mass rates.
The interesting number we seek is the
input air mass rate