HVAC SYSTEM DESIGN TUTORIAL- Psychrometric Chart- Cooling Load calculation methods

Summary

This HVAC system design tutorial focuses on calculating the sensible and latent heat load required to achieve desired indoor air conditions. The video outlines two methods for calculation: the enthalpy method and the separate latent and sensible heat load equations. It demonstrates how to use the psychrometric chart to derive enthalpy values and humidity ratios, calculate air mass rates, and compute heat removal for cooling systems. The video also addresses the feasibility of achieving the cooling load, prompting viewers to consider whether the scenario is psychometrically possible and invites feedback on the cooling coil's capacity.

Takeaways

• 😀 The tutorial covers HVAC system design with a focus on psychrometric charts, specifically calculating sensible and latent heat loads in air systems.
• 😀 The starting condition involves an outdoor air unit with 20,000 CFM, 95°F temperature, and 80% relative humidity, while the goal is to cool the air to 60°F at 50% relative humidity.
• 😀 Two methods for calculating the heat load are discussed: using enthalpy differences or calculating the sensible and latent loads separately using specific equations.
• 😀 The enthalpy method involves calculating the enthalpy at two points: the starting point (outdoor air) and the desired point (cooled air).
• 😀 The difference in enthalpy between the two points represents the total heat removal required from the air stream.
• 😀 The latent heat removal is determined by the change in moisture content, represented by the humidity ratio between the two points, and calculated using enthalpy.
• 😀 The sensible heat removal is calculated by the temperature difference between the starting and desired points and multiplied by the mass rate of the air.
• 😀 The second method for calculating heat loads uses the latent heat of vaporization of water and the change in humidity ratio between the two points.
• 😀 For the latent heat calculation, the mass of water removed is converted from grain per pound of dry air to pounds per pound of dry air.
• 😀 The sensible heat is calculated using a factor of 1.08, factoring in the temperature difference and the air stream's cubic feet per minute (CFM).
• 😀 Despite using different methods, the total heat load calculated from both methods yields results that are very close, with a negligible error of around 1-2%.
• 😀 The tutorial concludes by asking viewers to consider the possibility of such a cooling scenario and encourages them to comment on its feasibility and potential limitations.

Q & A

What is the focus of this tutorial on HVAC system design?

The focus of the tutorial is on calculating the sensible and latent heat load in HVAC systems using two different methods. It specifically covers how to use the psychrometric chart to determine the heat load from a given outdoor air condition to a desired discharge air condition.

What were the initial conditions given in the example scenario?

The initial conditions were a makeup air unit of 20,000 CFM with outdoor air at 95°F and 80% relative humidity. The goal was to calculate the heat load to achieve a discharge air temperature of 60°F at 50% relative humidity.

How does the enthalpy method help in calculating heat load?

The enthalpy method calculates the heat load by determining the difference in enthalpy (energy content) between the starting and ending conditions. The total heat removal is the difference in enthalpy values at the initial and final conditions, which is then multiplied by the air mass flow rate.

What is the significance of the enthalpy lines in the psychrometric chart?

Enthalpy lines on the psychrometric chart represent the energy content of air. By drawing parallel lines to these enthalpy lines, one can determine the enthalpy values at specific points, helping in calculating the sensible and latent heat load.

What difference between enthalpy points represents the latent heat removal?

The difference in enthalpy between points 1 and 2 on the psychrometric chart represents the latent heat removal, as this involves reducing the moisture content in the air while maintaining the temperature.

What formula is used to calculate the total heat removal in the enthalpy method?

The formula used is: Total Heat Removal = Mass Flow Rate of Air × (Enthalpy at Point 1 - Enthalpy at Point 3). The mass flow rate of air is determined by converting CFM into pounds of dry air per hour.

What is the latent heat of vaporization of water used in the second method?

The latent heat of vaporization used is 1060 Btu per pound of water. This value represents the energy required to evaporate and condense water in the air stream.

How is the mass of water removed from the air stream calculated in the second method?

The mass of water to be removed is determined by calculating the difference in humidity ratio between point 1 and point 2. This difference is then converted from grains of water per pound of dry air to pounds of water per pound of dry air.

What equation is used to calculate sensible heat removal in the second method?

Sensible heat removal is calculated using the formula: Sensible Heat Removal = 1.08 × CFM × Temperature Difference (in Fahrenheit). This equation uses an air factor of 1.08, which is adjusted based on geographic location or altitude.

What was the comparison between the results of the two methods in terms of heat removal?

The two methods produced very similar results. The total heat removal from the first method (using enthalpy differences) was about 56,000 Btu per hour, while the second method (using latent heat of vaporization and sensible heat calculations) came out to approximately 706,000 Btu per hour. The difference in calculations was negligible, with a small error margin of 1-2%.