# Misting systems for HVAC

Hi Guys,

I'm fairly new here, but I can't solve my problem (or I did but than I found a miracle)

I'm trying to optimize a HVAC system, especially for dry and hot areas. Now I was thinking about the use of a misting system in front of the condensor as done by for instance these guys; https://www.mrmistsystems.com/new/store/document/ac_pre_cooling/Coolnsave_Tulane_Study.pdf

However, I wanted to calculate the actual savings myself, so started to build a model;

Basic working principle HVAC

In the ideal vapor-compression cycle, refrigerant enters the compressor as a saturated vapor. As the refrigerant is compressed, it increases in temperature and pressure. After the compressor the refrigerant passes through the condenser. Heat energy (QH) is exchanged with the surrounding environment causing the refrigerant to cool and become a saturated liquid. Next, the refrigerant passes through the expansion valve causing the temperature and pressure to decreases. Because of the reduction in temperature and pressure, the refrigerant enters the evaporator as a saturated mixture. As the refrigerant passes through the evaporator, it absorbs heat energy (QL) from the environment that it is trying to cool. The refrigerant exits the evaporator as a saturated vapor and returns to the compressor to begin the process all over again.

Model

For the sake of the end product, we are taking the air surrounding the condensor as a starting point for the calculation. Since the effect of pre-cooling has to be shown.

A general rule of thumb is that the saturation temperature of the refrigerant at the condensor pressure should be at least 10-15 degrees above the temperature of the environment. From that criterion, the condensor pressure can be chosen.

The second important input number is the refrigerant mass flow rate of the system. Since, in this case it will not be variable for the different temperatures

Thirdly we have to calculate the heat removal by the evaporator, which can be described as:

Qe = Vair * d-air * Cpair * ΔT incoming air

Where:
Qe = heat removal in evaporator
Vair = volume of the air passing by the evaporator, dependent on refresh rate (m3)
d-air = density of air at room temperature (kg/m3)
Cp air = specific heat of air (kJ/kg*K)
ΔT = temperature between the outside air and the inside air

Based on the heat removal and the refrigerant mass, one can calculate the refrigerant conditions at evaporator outlet;

Qe = mref * (hevap - hconl)

Where
Qe = heat removal in evaporator
mref = mass flow refrigrator
hevap = enthalpy at evaporator outlet
hconl = enthalpy of saturated liquid at condensor outlet, since in the throttle only pressure is reduced; h condensor liquid ≈ h evaporator inlet

hence; hevap = Qe/mref + hconl

Compressor power

W = n / (n-1) .Pevap . Vevap . (Pcon/Pevap)^( (n-1)/n) ) - 1

Where:
W = compressor power
Pevap = evaporator outlet pressure
Vevap = Volume at evaporator outlet
Pcon = condensor inlet pressure
n = polytropic factor, normally between 1.25-1.41

For the throttling valve: % flash = (hf high pressure - hf low pressure) / (hfg low pressure)

Problem

With these formulas I end up with savings of about 12% for a 2 degree (Kelvin) drop of outside temperature, this feels off to me.

What am I doing wrong?