# What are psychrometric charts?

The page titled: psychrometric charts guidelines is the first of a series of documents in the field of HVAC Designs.

At first, when you look at psychrometric charts it looks very confusing and it contains a lot of information. Simply, it is a graph you can use as a tool to help you size the cooling/heating coil capacity.

The red curves in the graph represent the **relative humidity** of the air. It is the ratio between the actual water vapor pressure of air and the maximum possible water vapor pressure at a constant temperature. Additionally, the horizontal lines represent the **humidity ratio**. In other words, It is the ratio between the mass of water vapor and the mass of dry air.

Another property is the **dry bulb temperature** on the X Axis which represents the actual air temperature. On the other hand, **enthalpy** is the heat energy in the moist air. **Enthalpy** has a unit of kj/kg. **Enthalpy** is represented by the diagonal lines in the graph.

## Total cooling or heating coil capacity

For instance, in order to calculate the total load for your air conditioning unit, you need 2 pieces of information. First, you need the mass flow rate of air entering the unit. Additionally, you need the initial enthalpy value of the air and the desired value at the outlet of the unit.

**Unit Load = Mass flow rate X ( Enthalpy In – Enthalpy Out).**

In the next document, we will go through some examples in details. Consequently, we will learn how to calculate the **total cooling coil load**. Additionally, we will discuss the **sensible heat factor** and the **coil bypass factor**.