Psychrometric Chart is a useful tool in HVAC. It shows the properties of air and the relationship between temperature and humidity. Hence, it’s important for HVAC engineers to understand how to read and use the chart.
Ultimately, we want to use the Psychrometric Chart to design and troubleshoot HVAC systems. However, in order to use the chart properly, we must first understand how to read the chart.
The Psychrometric Chart has been made available by many HVAC organizations. To standardize it, I’ll be using the Psychrometric Chart by ASHRAE in SI unit for this guide. You can download it from my google drive.
How to Read the Psychrometric Chart?
The Psychrometric Chart has 4 important elements to master. They are the dry bulb temperature, wet bulb temperature, relative humidity and saturation line.
Dry Bulb Temperature
Dry bulb temperature is the temperature of the air as we know. It can be measured using most of the temperature measurement instruments. For instance, a sling psychrometer. The dry bulb temperature is shown in the Psychrometric Chart as the horizontal x-axis (vertical lines).
Wet Bulb Temperature
Wet bulb temperature is the temperature of the air at maximum humidity. It is measured by wrapping wet cotton around the bulb of a thermometer. The wet bulb temperature is shown diagonally in the Psychrometric Chart.
Most of the time, the wet bulb temperature is lower than the dry bulb temperature because the temperature is further brought down through evaporative cooling. Hence, the wet bulb temperature is also known as the lowest possible temperature can be achieved through evaporative cooling.
Relative Humidity and Saturation Line
Relative humidity shows the current percentage of moisture contain in the air compared to the maximum moisture of the air at a given condition. Meanwhile, the saturation line represents 100% relative humidity.
By intersecting the dry bulb and wet bulb temperature at the relative humidity line, we can determine the dew point of the air by dragging the line horizontally to hit the saturation line.
As shown in the above chart, when the dry bulb and wet bulb temperatures are 24°C and 18°C respectively, at 55% relative humidity, the dew point of the air is 15°C. It is the temperature at which the air starts to condensate into liquid water.
Moisture: How Relative Humidity is Derived?
Moisture shown in the Psychrometric Chart is a sub-element of relative humidity. It shows the amount of moisture in the air in grams. At a given dry bulb and wet bulb temperatures, the ratio of the moisture contain in the air is the relative humidity.
The chart below shows how relative humidity is derived from the ratio of moisture:
From the above chart, given that the dry bulb temperature is 24°C and the wet bulb temperature is 18°C, at around 55% relative humidity, the moisture contain is 10.25 grams.
If we continue the trend and reach 100% relative humidity, the maximum moisture can be held by the air at 24°C dry bulb and 18°C wet bulb is 18.25 grams.
Coincidentally, if we divide 10.25 grams by 18.25 grams, the result in percentage is 56% which represents the relative humidity (allow some tolerances when using the chart manually).
Relative humidity is dependent on the dry bulb and wet bulb temperature. It changes according to the condition of the air. For instance, at 30°C and 14°C dry bulb temperatures, the maximum moisture that can be held by the air is 27 gram and 10 gram respectively.
So, the same air will have a lower relative humidity when the dry bulb temperature is increased and a higher relative humidity when the dry bulb temperature is decreased.
In other words, air can hold on to more moisture when its temperature is higher. Hence, we feel drier during hot days and stickier during cold days.
How to Use Psychrometric Chart?
The Psychrometric Chart can be used to design and troubleshoot HVAC systems. Nowadays, manufacturers incorporate data from the chart into their equipment selection software. In the field, design and commissioning engineers use the chart to troubleshoot humidity problems.
Determine the Normal Off Coil Temperature
Most of the time, high humidity problems are caused by high off coil temperature or supply air temperature. When the off coil temperature is too high, condensation couldn’t take place to extract moisture from the air.
Usually, air conditioners are set to achieve a room temperature of 23°C at 55% relative humidity for optimal human comfort. Supposedly, the off coil temperature should be about 10°C below the room temperature which is around 13°C in this case.
If you look carefully, when we pull the dry bulb temperature at 23°C to hit 55% relative humidity (RH), the line can be dragged horizontally to hit the saturation line (red dotted line) before bouncing back down to the dry bulb temperature at 13°C.
The 13°C dry bulb temperature is also known as the dew point temperature where air condenses into liquid water. So, if we want the room condition to be at 23°C and 55% RH, the off coil temperature should be at 13°C.
With the information on the off coil temperature, you can confirm that the air conditioner is properly sized and the room will achieve a temperature of 23°C at 55% RH.
Determine the Maximum Off Coil Temperature to Prevent Mold Growth
If the off coil temperature rises above 13°C, the dehumidification capability of the air conditioner is reduced and high humidity problems such as mold and fungus growth may start to occur.
This can be explained by the following chart:
As shown in the above chart, if we reverse the process and start at 16°C dry bulb and follow the line vertically to hit the saturation line, we can pull the line horizontally and hit 65% relative humidity at 23°C dry bulb temperature.
It means that if the off coil temperature is 16°C and the room temperature is set at 23°C, the resulting humidity level is expected to be at 65% RH.
In most cases, mold and fungus start to grow at 65% relative humidity. Hence, when we design the HVAC system, we must bring the relative humidity down below 65%.
Therefore, we can say that the maximum off coil temperature of the air conditioner is 16°C when maintaining a room at 23°C. Beyond that, the relative humidity is too high. Even at 65% RH, the humidity level is still considered high.
However, if the room temperature is increased, the relative humidity can be brought down to 55% RH even if the off coil temperature is 16°C. For instance, room temperature at 26°C. Obviously, it is not the solution to high humidity problems.
Determine the Maximum Off Coil Temperature Based on Room Temperature
In addition, we also can determine the maximum off coil temperature at any given room temperature to maintain a good humidity level.
As you can see, if we want to keep the humidity level at below 65%, the off coil temperature must not exceed 17°C and 18°C when the room temperature is set at 24°C and 25°C respectively.
Nonetheless, properly designed air conditioners usually have an off coil temperature of around 13°C. The highest off coil temperature that I’ve seen in a working HVAC system is around 16°C.
So, when you have high humidity problems, you can quickly check the off coil temperature to make sure that it is not beyond the maximum limit. Otherwise, the air conditioner may be undersized or there are other issues that are causing high off coil temperature.
Nowadays, the Psychrometric Chart is available in many different forms including software and online calculators. Hence, fewer and fewer engineers know how to read and use it the manual way.
For more about the application of Psychrometric Chart, I highly recommend you continue reading my post How to Use a Psychrometric Chart?
The working principle of heat pipes in pre-cool AHUs can also be demonstrated and explained using the Psychrometric Chart. However, I’ve already covered it in my blog post What is PAU in HVAC? (PAU for ACMV). Feel free to continue reading.
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