HVAC Psychrometric Buddy

Quickly calculate thermodynamic properties of moist air at temperatures commonly used in HVAC Systems. Given a dry bulb temperature, pressure, and any one of four properties it calculates the remaining properties. Tabs allow entry of two sets of properties which can then be combined with flow rates on the Mix tab to get properties of the resulting air flow. Save entry parameters and calculations via email from direct entry or your contact list.

Features:

• Calculate multiple property states P1 and P3
• Imperial (IP) or Metric (SI)
• Properties - calculate based on Pressure, Dry bulb, and any of four other properties
• Properties for the adiabatic mixing of two streams
• Save properties - email report inputs and outputs

P1 - Process One

HVAC Psychrometric Buddy calculates psychrometric properties for each of two states labeled "P1" and "P2" corresponding to the same named tabs at the bottom of the application. To calculate properties press the appropriate tab and enter altitude, dry bulb temperature, and at least one of the following properties:

1. Wet Bulb Temperature
2. Relative Humidity
3. Humidity Ratio
4. Enthalpy

To enable the property tap the name of the property on the bar at the top of the window. Once three values are entered all remaining properties are calculated. If altitude is not supplied zero is assumed.

Example 1. Moist air exists at 100°F dry-bulb temperature, 65°F thermodynamic wet-bulb temperature, and 14.696 psia (sea level) pressure. Determine the humidity ratio, enthalpy, dew-point temperature, relative humidity, and specific volume.

Solution: Use the P1 tab and enter zero as altitude, 100°F dry-bulb temperature. Tap and make sure "Wet Bulb" is enabled. You can verify this two ways - it will show as enabled and a green dot will show in the table view on the Wet Bulb row. Note that all values you are expected to enter are green with output values being blue. Now enter 65°F as your wet bulb temperature. The resulting values will be displayed. Change any of the input values as many times as you like.

To drive the calculation off of another value such as Humidity Ratio select it from the list at the top of the screen. Notice the green dot moves from "Wet Bulb" to "Humidity Ratio" and Humidity Ratio is now editable.

To reset values to zero press the "Clear" button in the upper left corner. Note that zero actually is a value and you will see a difference in temperatures if you subsequently decide to switch between display units (Imperial (IP) / Metric (SI)) and the value is converted.

P2 - Process Two

P2 lets you record additional properties at the same altitude entered in P1. Multiple properties allow you to mix and compare.

Mix

Mixing of Adiabatic Streams is accomplished on the Mix tab by assigning flow rates to the property sets P1 and P2. All properties for this newly created airflow are automatically calculated along with some additional volume properties. To illustrate the process the following is a comparison of mixing streams using an example from ASHRAE Fundamentals 2009 (IP) and entering the same values in HVAC Psychrometric Buddy:

IP Example 4 pg 1.18 "A stream of 5000 cfm of outdoor air at 40°F dry-bulb temperature and 35°F thermodynamic wet-bulb temperature is adiabatically mixed with 15,000 cfm of recirculated air at 75°F dry-bulb temperature and 50% rh. Find the dry-bulb temperature and thermodynamic wetbulb temperature of the resulting mixture."

• The solution arrived at via the process of schematics is specific volume 12.65 ft³/lb for the 5000 cfm stream and 13.68 ft³/lb for the 15,000 cfm stream using equation (46) and ruler to arrive at a dry bulb temperature of 65.9°F and wet bulb of 56.6°F.

• To arrive at a solution using HVAC Psychrometric Buddy enter 40°F dry bulb and 35°F wet bulb on P1; 75°F dry bulb and 50% relative humidity on P2; On the mix tab enter 5,000 cfm and 15,000 cfm for streams one and two. The resulting specific volumes are 12.66 ft³/lb and 13,679 for the first and second sets of properties and dry bulb of 65.7°F, wet bulb of 56.5°F for the combined stream.

SI Example 4 pg 1.13 "A stream of 2 m³/s of outdoor air at 4°C dry-bulb temperature and 2°C thermodynamic wet-bulb temperature is adiabatically mixed with 6.25 m³/s of recirculated air at 25°C dry-bulb temperature and 50% rh. Find the dry-bulb temperature and thermodynamic wet-bulb temperature of the resulting mixture."

• The solution arrived at via the process of schematics is specific volume 2.535 kg/s for the 2 m³/s stream and 7.284 kg/s for the 6.25 m³/s stream using equation (46) and ruler to arrive at a dry bulb temperature of 19.5°C and wet bulb of 14.6°C.

• To arrive at a solution using HVAC Psychrometric Buddy enter 4°C dry bulb and 2°C wet bulb on P1; 25°C dry bulb and 50% relative humidity on P2; On the mix tab enter 2 m³/s and 6.25 m³/s for streams one and two. The resulting specific volumes are 2.533 kg/s and 7.283 kg/s for the first and second sets of properties and dry bulb of 19.6°C, wet bulb of 14.5°C for the combined stream.

Email

Email is used to save your input parameters and resulting calculations by sending a formatted report to anyone with an email address. This address can be entered on the fly or pulled from your contact list.

Instructions

This page!

Summary of Properties

Dry-bulb Temperature

Dry Bulb Temperature is that of an air sample, as determined by an ordinary thermometer, the thermometer's bulb being dry. It is presented in degrees Celsius (SI) or Fahrenheit (IP).

Wet-bulb Temperature

Wet Bulb Temperature is the temperature is the reading of a thermometer whose sensing bulb is covered with a wet sock evaporating into a rapid stream of the sample air. The wet bulb temperature is the same as the dry bulb temperature when the air sample is saturated with water. It is presented in degrees Celsius (SI) or Fahrenheit (IP).

Dew point Temperature

Dew Point Temperature is that temperature at which a moist air sample at the same pressure would reach water vapor saturation. The dewpoint temperature is not an independent property but presented as a convenience.

Relative Humidity

Relative Humidity is the ratio of the mole fraction of water vapor to the mole fraction of saturated moist air at the same temperature and pressure. Relative humidity is dimensionless, and presented as a percentage.

Humidity Ratio

Humidity Ratio is the proportion of mass of water vapor per unit mass of dry air at the given conditions (dry bulb, wet bulb, relative humidity, etc.). Although humidity ratio is dimensionless it is presented as grams of water per kilogram of dry air (SI) or grains of water per pound of air (7000 grains equal 1 pound) (IP).

Grains

Grains are a convenience calculation for Humidity Ratio. It is presented as grains of water per pound of air (7000 grains equal 1 pound) (IP).

Specific Enthalpy

Specific Enthalpy is the sum of the internal (heat) energy of the moist air in question, including the heat of the air and water vapor within. Enthalpy is presented in joules per kilogram of air (SI) or BTU per pound of dry air (IP).

Specific Volume

Specific Volume is the volume per unit mass of the air sample measured in cubic meters per kilogram of dry air (SI) or cubic feet per pound of dry air (IP).

Frequently Asked Questions

Q: Sometimes Enthalpy doesn't change when I switch between SI and IP?

A: This exists when you have selected Enthalpy as an input parameter. As it's value is dependent on other potential input values so therefore is taken literally and not changed for you.

Q: Why don't values show up on the "Mix" tab after I enter air flow?

A: Check to make sure you have valid properties on Tabs P1 and P2.