Print this page HVAC Psychrometric Analysis to Avoid Moisture Problems
Fred W. Dougherty, P.E.,B.A.E, M.M.E
Course Outline
Moisture may build up in a building when the mechanical cooling system is unable to remove latent heat from occupants and ventilation air. Simply matching the cooling system capacity to the building sensible and total cooling loads does not ensure that a selected cooling system can deliver design room temperature and relative humidity. This course explains how to set up psychrometric analysis and plot state points and process lines for room loads and cooling coil capacity on a psychrometric chart. Examples of systems with varying occupancy and outdoor air are presented using actual HVAC projects. These examples show how psychrometric analysis can spot potential moisture problems and lead to solutions.
1. Scope: explains the purpose and scope of the course.
2. The Design Iteration: shows how psychrometric analysis fits into the steps required to select the air conditioning system for a project.
3. Definitions of terms used throughout the course.
4. The Psychrometric Chart: a brief refresher of psychrometric processes and principles.
5. Cooling system psychrometric state points and processes: describes how to plot state points and calculate processes.
6. Load Variations: Analysis of actual projects. Shows examples of moderate occupancy and outdoor air (office), high occupancy with high outdoor air (church), high outdoor air with moderate occupancy (restaurant), and high occupancy with moderate outdoor air (lobby) . Shows application of reheat, energy recovery ventilator, dedicated outdoor air unit, and heat pipe.
7. Energy notes: discusses energy use of various humidity control and outdoor air pretreatment schemes.
8. Rules: review of requirements to achieve satisfactory occupied space humidity levels.
This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.
Learning Objective
At the conclusion of this course, the student will:
- Understand the need for psychrometric analysis to identify and solve potential moisture problems;
- Be able to relate state points on the psychrometric chart to physical locations within the air conditioning system;
- Be able to plot air conditioning system state points and process lines on the psychometric chart;
- Recognize when a selected system, though it matches the calculated total and sensible loads, cannot deliver the room design temperature and humidity;
- Understand reheat, dedicated outdoor air, energy recovery ventilators, and heat pipes and how to represent them on the chart; and
- Be able to remedy potential moisture problems by re-selecting systems or applying passive and active humidity control and outdoor air pre-treatment.
Intended Audience
This course is intended for design professionals that plan, design, and implement HVAC systems for non-residential buildings.
Benefit to Attendees
Attendee of this course will be able to apply psychrometric analysis to minimize the probability of moisture problems and sick building syndrome.
Course Introduction
The HVAC design process begins with calculating heating and cooling loads for the occupied space (rooms) and for the heating apparatus and cooling coil. Selection of a cooling coil and system affects room relative humidity because the coil de-humidifies as it cools. This course is therefore concerned only with the cooling process which if improperly selected can cause excessive build-up of moisture in a space. After cooling loads are calculated, a cooling coil is selected using either refrigerant (DX) or chilled water. The air flow across the coil must be selected to match as closely as possible the sensible cooling load of the occupied space plus any ventilation air required. (The procedure for determining required ventilation air is described in PDH course #M384, HVAC Ventilation for Indoor Air Quality) However, a system that will match the calculated loads may not actually be able to deliver the design temperature and relative humidity to the occupied space. Psychrometric analysis is therefore required to ensure that the supply air is delivered to the space at a temperature and dew point lower than that shown to be the maximum allowed by the space load. Thus, the space load process line, the mixed (return + ventilation) air point, and the cooling coil capacity process line must be calculated and plotted on the psychrometric chart.
Course Content
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HVAC Psychrometric Analysis to Avoid Moisture Problems
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HVAC design is a step-by-step process to find an air conditioning system that will match the calculated cooling coil sensible and total cooling loads. However in addition, the temperature and dew point of the coil leaving air (supply air) must both be below the required maximum to deliver the design temperature and humidity to the occupied zone. Psychrometric analysis is required to determine this maximum, and to compare that to the state of the air leaving the cooling coil.
When the system is ready for analysis, the known parameters will be the state points (dry bulb temperature, wet bulb temperature, enthalpy, etc) of the desired room (occupied space) air and of the outdoor ventilation air. Also known will be the calculated room and cooling coil sensible and total design cooling loads. From these parameters, the mixed air condition entering the coil, the room process line, and the coil process line can be calculated and plotted. The room process line represents the supply air as it enters the room and picks up heat and moisture. The mixed air condition is the state point of the outdoor air mixed with the return air either directly from the room or after picking up heat in a return air plenum. This will be the entering condition to the cooling coil. The cooling coil process line represents the supply air as it passes through the coil and gives up its heat and moisture.
For systems with moderate occupancy and ventilation air, a properly selected air conditioning system will deliver supply air to the room at a temperature and dew point that can satisfy the room design conditions. However, High occupancy or air flow can result in a required maximum temperature and/or dew point that an ordinary cooling coil cannot match. This will be evident with psychrometric analysis, and the designer can use an appropriate strategy to shift the supply air state point as needed to match the room load. This could be reheat, when the coil leaving dew point is acceptable but the leaving temperature is too low, which would cause short run times and moisture build-up. Other options are outdoor air pre-treatment or a heat pipe. These can be plotted to find a solution. In rare cases, it may be necessary to select a different cooling system and begin the iteration process over again.
Quiz
Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.
DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.
