What is saturation temperature and why does it matter?
One of the many concePTs that HVAC professionals need to understand is the saturation temperature; this includes knowing how to calculate it and how to use a PT (pressure-temperature) chart.
Specifically, HVAC professionals typically apply this knowledge to steam or water and refrigerant. You will use saturation temperature in various calculations and troubleshooting, including when calculating superheat. For those who need a refresher, superheat provides valuable information into the system’s efficiency and whether the liquid may be leaking into areas of the system it should not.
What is saturation temperature?
The saturation temperature is just the official name for the boiling point. The term “saturation” comes from the fact that it is the temperature that a liquid has to be at to boil and enter the vapor phase, based on its saturation pressure.
At constant pressure, if you remove heat and have a vapor at its saturation temperature, it will condense and become a liquid. The opposite is also true. If you have a liquid at its saturation temperature and constant pressure and add heat, it will boil and enter the thermal phase. You can understand this by thinking about your experiences with boiling a pot of water.
How do you calculate the saturation temperature?
You can calculate the saturation temperature with the following steps or use the simpler alternative outlined after.
–Step 1: Measure the system’s temperature in degrees Celsius. Convert it to Kelvin by adding 273 degrees.
–Step 2: Use the Clausius-Clapeyron equation to calculate the saturation pressure. The equation is as follows:
–Step 2a: We will also put those steps into writing. The equation has you solve for the natural logarithm of your saturation pressure but divided by 6.11.
– Step 2b: The other side of the equation is (the latent heat of vaporization divided by the wet air gas constant) times (1/(1/273 – the temperature in Kelvin).
The latent heat of vaporization is 2.453×10^6 J/kg, and the gas constant of wet air is 461 J/kg. Doing the required division in the first step is 5,321.0412.
– Step 2c: Now, you can solve the equation, including the natural log, by putting each side of your equation as a power of e. Remember that your result is the saturation pressure divided by 6.11, so you will have to multiply both sides of the equation by 6.11 to get the saturation pressure.
– Step 3: Use your PT card to find the saturation pressure that you calculated. The card will show you the temperature.
You can also follow these steps and use your chart, a method that is much simpler and relies on tools such as pressure gauges, which you likely have on hand.
– Step 1: Use a tool to measure the pressure at the location in question, such as a gauge.
– Step 2: Use your PT chart to find the saturation temperature.
To make it even easier to calculate saturation temperatures, some scientists are working towards creating new formulas that are simpler yet accurate.
Of course, you can also get accurate saturation temperature figures by using the previous steps, and that method is easier for most HVAC professionals to follow.
Online Calculators
HVAC professionals are far from the only people who occasionally or regularly need to calculate saturation temperature, so there are numerous online calculators to help you. Most of these focus on common substances, like water.
You can encourage your technicians to use these online calculators as a learning tool to check their math when learning how to calculate saturation temperature. In this case, they would calculate the saturation temperature for water in a given real or theoretical instance then use the online calculator to confirm their work. This can give them extra practice before applying the same knowledge to calculating the saturation temperature of refrigerant or other substances they interact with.
Major trends influencing saturation temperature – does saturation increase with temperature?
Yes, when the air temperature increases, a larger amount of water can stay in gaseous form, increasing saturation. When the temperature drops, the water molecules slow down, increasing the chances of them condensing into a liquid.
The following chart shows how this works for water, specifically how an increase in temperature increases the vapor pressure.
Remember the relationship between pressure and saturation – that a higher vapor pressure corresponds to a higher saturation.
The individual pieces of saturation temperature.
With a better understanding of saturation temperature, it is time to look at some related calculations and the saturation temperature of specific substances.
What is the refrigerant saturation temperature?
The refrigerant saturation temperature will depend on the pressure. Based on the comparison, consider that water has a saturation temperature or boiling point of 212 Fahrenheit or 100 Celsius at sea level. When the pressure increases, the saturation temperature will increase as well. If pressure decreases, so will the saturation temperature.
There is a similar pattern for refrigerant’s saturation temperature, but it also depends on the refrigerant itself. One example is that R-22’s boiling point is 45 degrees Fahrenheit when the pressure is 76 psig. Any change in pressure will change the saturation temperature.
What is the saturation temperature of steam?
The following charts show the saturation temperature of steam, along with a few other thermodynamic properties.
The science of saturated steam.
Saturated steam refers to pure steam that comes into contact with the water it came from, which is still in liquid form. Importantly, the saturated steam will be at a temperature that should correspond with water (a liquid instead of gas) at its pressure.
Saturated steam is also known as dry steam, and it happens when the water or substance is at a temperature that allows for either gas or liquid. For saturated steam to appear, this indicates that the rates of condensation and evaporation are the same.
HVAC technicians and saturated steam.
HVAC technicians commonly interact with saturated steam because of the benefits it offers for heating.
- Latent heat transfer delivers quick heating.
- Using water offers a low-cost, clean, and safe heating substance.
- You can use the pressure to adjust the temperature precisely and with control.
- Saturated water has a high coefficient of heat transfer, meaning it does not need to have as large of a surface area to transfer the heat, reducing the amount of equipment needed.
Saturation temperature charts vs. the alternatives – how to use PT charts for success.
When dealing with saturation temperature as an HVAC professional, you will frequently use a PT (pressure and temperature) chart. In the past, these always had to be physical charts that technicians would take with them, but now there are numerous smartphone applications that include charts, saving space.
The following is an example of a PT chart HVAC technicians may use.
It is fairly straightforward to use a PT chart. You look at the saturation pressure for your given refrigerant and then see which saturation temperature it corresponds with. The above is an example of a traditional chart, which shows the temperature and the pressure in psig.
That type of chart is common for single-component refrigerants, as they will have a single boiling point. However, there are also zeotropic blends with multiple components, and these undergo fractionation, where one of its features moves to vapor or liquid quicker than the other.
These PT charts will look like the following:
When using refrigerants with multiple components and reading these charts, you want to pay the most attention to the endpoints of the “glide.” In other words, whichever component will change phases first is the one to pay attention to.
PT charts are a popular tool for a few purposes in HVAC, including:
- Confirming the coil pressure needed to set to get the refrigerant to the right temperature
- Calculating superheating (heating above the temperature at which the refrigerant would be saturated or its saturation temperature, specifically by the evaporator’s outlet)
- Calculating subcooling (cooling below the saturation temperature, specifically by the condenser’s end)
Conclusion: why saturation temperature is important for HVAC professionals.
HVAC professionals will find themselves regularly using saturation temperature and PT charts when setting up systems, troubleshooting problems, and resolving those issues. The saturation temperature is crucial for calculating subcooling and superheating, as it helps technicians determine a system’s problem.
Whether discussing saturation temperatures or service appointments, you need to be in constant communication with your technicians and clients as an HVAC business owner. Podium can help you with both types of communication, with Teamchat helping you communicate with your technicians while Webchat and Inbox let you stay organized and in contact with your clients. With the communications taken care of, you can focus on delivering high-quality HVAC services.
