Psychrometrics: What It Is and Why It Matters?

By: Joseph Hayden, PE

Figure 1: Psychrometrics of Standard Cooling Cycle

Psychrometrics. At its simplest refers to the study and application of the thermodynamic properties of moist air and how various HVAC processes impact the condition of that air. Psychrometric charts (see Figure 1) are graphical representations and tools used to understand, design and analyze the thermodynamic cycles moist air undergoes during various HVAC processes, and can be useful in determining what combination of heating, cooling, humidifying or dehumidifying sources should be used to reach optimal environmental conditions in a given facility.

Proper analysis and comprehension of this fundamental principle is particularly critical for those working in the healthcare AEC industry. Most in the field are likely to know, and consistently reference ANSI/ASHRAE/ASHE Standard 170 Design Values, but without a thorough understanding of psychrometrics, one cannot design HVAC systems to meet the requirements of a specific project. An engineer will eventually run into problems if they arbitrarily use 55F-58F as the apparatus dew point or design supply temperature for an HVAC system.

This is especially important when designing HVAC systems that serve critical spaces like hospitals, which often have atypical requirements relating to interior environments (i.e., space temperature and relative humidity). There are a multitude of areas within a healthcare facility such as operating rooms and compounding pharmacies, where having an in depth understanding of psychrometrics is paramount. Interior design parameters for healthcare occupancies/spaces are detailed Tables 7.1, 8.1, and 9.1 of ASHRAE 170. Figure 2 shows a section of Table 7.1.

Figure 2: ASHRAE 170-2021 Table 7.1

If the parameters described in the reference tables are not met, the associated spaces will not comply with code and the system design will not be code compliant. Beyond this, the real-world applications of a non-compliant system in a healthcare setting can be immensely consequential. In hospitals, building occupants are typically sick or immunocompromised – leaving them vulnerable to many things including airborne pollutants and microorganisms. The comfortability of patients as well as hospital staff is a healthcare engineers’ top priority. As such, concerns like building temperature, humidity, pressurization, ventilation and air distribution can negatively impact those occupying the space. The proper design of an HVAC system is no more predominant than it is in an operating room, where its sole purpose is to minimize infection, and maintain staff comfort. It is in this way, that exception design contributes to patient care.

When we are designing a new HVAC system or analyzing and existing system, the first thing we should do is grab our psychrometric chart and plot various points on the chart that are relevant to the specific project based on design set points or existing information from a previous design (this might include trouble shooting a design in which space conditions could not be met). My particular expertise is in designing and troubleshooting HVAC systems serving compounding pharmacies. If a pharmacy is performing high-risk sterile compounding, where certain mixtures are conglomerated, but not necessarily administered straight away, greater care would also need to be taken to maintain the integrity of the mixture. Pharmacy space design temperature setpoints often range from 70°F to 72°F [21°C to 22°C], although specific drugs may require a tighter temperature range. Furthermore, humidity control may be desired by the client based on the types of drugs stored in the pharmacy but is not specifically required by ANSI/ASHRAE/ASHE Standard 170, Table 7-1. For example:

The interior condition to be maintained in a compounding pharmacy’s buffer room is 68°F or less and less than 60% Relative Humidity, as prescribed in USP 797
Pharmaceutical Compounding – Sterile Preparations. At that indoor condition, the space dew point is approximately 54°F. The cooling coil design leaving air temperature must be below the dew point of the space set point in order for the HVAC system to be able to meet the requirements of the space. Right off the bat, the typical leaving air temperatures used for most non-critical comfort cooling applications is 55°F-58°F which is above the dew point temperature of the room. Regardless of the quantity of air supplied to the room from the HVAC system, the space relatively humidity set point cannot be maintained.

Again, the real-world consequence of this must always be taken into consideration. Below are several considerations when designing a USP 797/800 Compliant Compounding Pharmacy:

  • Space Temperature and relative humidity – Code requirement is less than or equal to 68°F and 60% Relative Humidity to minimize the risk of microbial proliferation and to provide comfortable conditions for compounding personnel who must where full body PPE.
  • Filtration and Air Change Rates – Air introduced into the clean room suite must be via HEPA filters located in the ceiling of the room and at specified rates (30 total ACH minimum). These requirements are in place to ensure the space maintains the appropriate ISO classification during compounding activities.
  • Pressurization – Continuous differential positive pressure is required to minimize airflow from an area with lower air-quality classification to an area of higher air-quality classification. In a cleanroom suite (Non-HD compounding), a minimum differential pressure of +0.020-inch water column is required between each ISO classified area (e.g., between the buffer room and anteroom). The pressure differential between the anteroom and the unclassified area must not be less than +0.020-inch water column. Pressure requirements for compounding HD CSPs is between -0.01 and -0.03 inches of water column relative to adjacent areas.
  • This is all relevant to creating a space that is compliant and giving end users the best quality system. But often times, an engineer may have to rectify a problem. I recently worked on a project to remedy an HVAC system that could not maintain the code required space set points for a compounding pharmacy. The equipment submittals were provided by the client for our review/analysis.
  • With the existing design, this HVAC system was only capable of removing sensible heat from this airstream – resulting in high relative humidity inside the pharmacy. When this system and associated cooling processes is plotted on a psychrometric chart [See Figure 3], it becomes apparent that the space set points will never be met. Another dead giveaway was that the cooling coils total capacity matched the sensible capacity, indicating that there was no latent load on the coil (no moisture removal; dry coil condition; no dehumidifying of airstream).
Figure 3: Sample DX Coil Performance Data

Given all of the above, how do we best fix HVAC psychrometric problems? The best way is to avoid them in the first place. We designers can thrive towards better design by committing and recommitting to fundamental principles. It cannot be emphasized enough. In our experience, young engineers rush into things to get lines on paper in order to hit a deadline. As engineers, we need to first make sure the fundamentals of our design make sense. Do not put anything on the drawings that you do not understand, prior to rushing into full blown production. The drawings can look amazing from an outside perspective but if you dig into the fundamentals, you start to understand where the issues are, and where they will be.

At the time of publication, the 2008 version of USP 797 is official/applicable. A latest copy of USP 797/800 is waiting imminently to be approved, and contains within its revised chapters requirements that would render numerous existing compounding pharmacies non-compliant. Though many pharmacies have begun modifying their existing spaces to comply with the latest version of Chapter 797/800, it is not technically adopted or has been made official. As the official release date for these revisions has changed several times, users should regularly check in with the applicable regulatory body to confirm when new requirements of a particular standard should be implemented. 

Joseph Hayden, PE is a licensed mechanical engineer with over 10 years of extensive project management experience in the healthcare and commercial construction industries. Adept at designing HVAC, plumbing and fire protection systems for a variety of healthcare facilities and commercial building types, Joseph has comprehensive knowledge of ASHRAE 170, FGI guidelines, International Building Code and associated reference standards.