# Can misting systems improve HVAC efficiency?

• jvanniel
In summary, Chris is trying to optimze a HVAC system for dry and hot areas. He was thinking about using a misting system in front of the condensor, but he wanted to calculate the actual savings himself. He started to build a model and based on the air surrounding the condensor, he calculated the refrigerant conditions at evaporator outlet. He found that the system would save around 12% if the outside temperature dropped by 2 degrees Kelvin.
jvanniel
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?

Are you really trying to figure why you should mist the air, or if you should? Hot and dry climates are practically asking for Water Cooled Towers or at least Hybrid Dry Coolers. But if you have to stay dry, most dry condensing units will meet their rated capacities with 115 degree ambient air.

-chris
aec.us.com

Sudden (unexpected) hot whether waves sometimes force to do this on the equpment not designed for high temperatures. But to keep without big reason the condensation pressure lower than designed may turn wrong. (However many would argue it would not.) Besides, for sure, the condenser fins get fouled from the water.

## 1. What is a misting system for HVAC?

A misting system for HVAC is a type of cooling system that uses tiny water droplets to lower the temperature in a room or building. It works by releasing a fine mist into the air, which evaporates and removes heat from the surrounding air, providing a cooling effect.

## 2. How does a misting system for HVAC work?

A misting system for HVAC works by using high-pressure pumps to push water through small nozzles, creating a fine mist. This mist is then released into the air, where it quickly evaporates and reduces the temperature in the surrounding area. The system can be set up to run continuously or on a timer, depending on the needs of the space.

## 3. What are the benefits of using a misting system for HVAC?

There are several benefits to using a misting system for HVAC. Firstly, it can significantly reduce the temperature in a room or building, making it more comfortable for occupants. It also uses less energy than traditional air conditioning systems, making it more environmentally friendly and cost-effective. Additionally, the mist can help to reduce dust and allergens in the air, improving air quality.

## 4. Are there different types of misting systems for HVAC?

Yes, there are several different types of misting systems for HVAC, including high-pressure and low-pressure systems. High-pressure systems use pumps to create a fine mist, while low-pressure systems use gravity to release the mist. There are also different types of nozzles and control options available, allowing for customization based on the specific needs of a space.

## 5. Can a misting system for HVAC be used in all climates?

While misting systems for HVAC are most commonly used in hot and dry climates, they can also be used in more humid climates. However, in humid climates, the mist may not evaporate as quickly, which can reduce the effectiveness of the system. It is important to consider the climate and humidity levels when deciding if a misting system for HVAC is the right choice for a particular space.

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