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In the realm of HVAC engineering, the quest for optimal indoor comfort extends its dominion into the cold and chilly winter months.  At the core of winter air-conditioning systems lies a meticulously designed network of components, strategically arranged for achieveing the desired thermal control.  Achieving the desired indoor air conditions in winter mirrors the requirements of summer as shown in How Do Air Conditioners Work in Summer? The standard configuration of essential equipment and psychrometric chart below, illustrates the winter air conditioning system. This setup, involves the sequential passage of air through a preheating coil, then a humidifier, and finally a second preheating coil.  As curious HVAC enthusiats, let us delve into the inner workings of these systems, dissecting the components and processes that orchestrate indoor comfort amidst the winter chill.

Winter Air Conditioning System
Winter Air Conditioning System and Psychrometric Chart. Source: Electrical Workbook.

Enhancing Winter Comfort: Double Reheat Coils and Air Washer

In times of severe winter, adjustments are imperative to elevate the Dry Bulb Temperature (DBT) and Relative Humidity (RH) of the air. This can be achieved by adding an air washer and double in reheat coils on a winter airconditiong system.  This is shown in the image below with its corresponding psychrometric representation.  Here, processes such as air mixing (Condition 4), sensible heating (Process 4–5), adiabatic cooling (Process 5–6), and additional sensible heating in the reheat coil (Process 6–1) collectively contribute to cooling, dehumidification, and compensating for heat and vapor losses in the conditioned room. In large systems, the incorporation of re-circulating air fans and supply air fans is common but does not alter the processes outlined in the psychrometric chart.

Air washer and double reheat coils on winter air conditioning system
Air washer and double reheat coils on winter air conditioning system. Source: EIT

Utilizing Outdoor Air Economically: 100% Outdoor Air with Pre-heating

Efficient design of air-conditioning systems mandates the exploitation of internal heat emissions whenever feasible. The system shown below exemplifies the use of waste heat from exhaust for preheating fresh air. As detailed in Audel HVAC Fundamentals, Volume 1: Heating Systems, Furnaces and Boilers  this arrangement employs air washers as humidifying devices, countering moisture losses in the conditioned space while purifying the air. The reheat coil assumes a crucial role in regulating heat supply, thereby controlling the DBT of the air-conditioned space.

Winter air conditioning employing 100% outdoor air with preliminary heating through the utilization of waste heat from the exhaust.
Winter air conditioning employing 100% outdoor air with preliminary heating through the utilization of waste heat from the exhaust. Source: EIT.

Detailed Processes: Preheating, Humidification, and Sensible Heating

Delving into the technical intricacies, Process 4–5 involves preheating fresh air using waste heat from the exhaust. Meanwhile, Process 2–3 signifies the cooling of exhaust air, Process 5–6 orchestrates humidification through steam, and Process 6–1 contributes sensible heating in the reheat coil. Process 1–2 encapsulates the cooling and dehumidification of air, offsetting heat and vapor losses in the conditioned space. Notably, in winter air-conditioning systems requiring heating, the use of outdoor air should be minimized, aligning with principles of energy efficiency and sustainability.

 

 

1. Introduction

Psychrometric processes are the building blocks of HVAC systems, orchestrating the intricate dance of air properties like temperature and humidity. For HVAC engineers and technicians, delving into these processes is not just an option but a necessity, serving as the key to achieving mastery in the realm of indoor environmental control.

Understanding psychrometric processes is vital on multiple fronts. It’s the cornerstone of efficient system design, allowing engineers to strike the right balance between comfort and energy efficiency. Technicians depend on this knowledge for troubleshooting and system maintenance. Moreover, it contributes to energy conservation and creates indoor spaces that are not just comfortable but also health-enhancing. In this guide, we’ll dive into the world of psychrometric processes, equipping you with the knowledge to master HVAC systems and create indoor environments that are efficient, comfortable, and conducive to well-being.

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2. Basic Processes

To understand the basic process, we will delve into the four fundamental psychrometric processes that form the cornerstone of HVAC systems. These processes are:

  1. Sensible Heating: Learn about the process of sensible heating, where the temperature of the air stream increases without altering its moisture content.
  2. Sensible Cooling: Explore how heat is removed from the air without changing its moisture content in the process of sensible cooling.
  3. Humidification: Understand the latent energy addition involved in humidification, where moisture is added to the air without changing its dry-bulb temperature.
  4. Dehumidification: Delve into the latent energy removal in the dehumidification process, where moisture is removed from the air without altering the dry-bulb temperature.

By understanding these basic processes, you’ll gain insights into how HVAC systems create and maintain comfortable indoor environments. These processes involve phase changes in water content, and we’ll explore their significance in the subsequent sections of this comprehensive guide.

Why do HVAC professionals always excel in psychrometric class?

Because they knew how to keep their cool during heated discussions!

 

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3. Sensible Heating

Sensible heating process.
Sensible heating process with air moving over a heating coil. Source: IIT Delhi

Dive into the fascinating process of sensible heating, a fundamental aspect of HVAC systems. Sensible heating involves increasing the temperature of the air stream without altering its moisture content. This process plays a crucial role in creating comfortable indoor environments.  To read sensible heating on a psychrometric chart, start by locating the initial air condition point. Identify the final condition point along the diagonal line representing sensible heating, with the vertical position indicating the dry-bulb temperature and the horizontal position showing the change in humidity ratio. Calculate the temperature change by subtracting the initial dry-bulb temperature from the final temperature, as sensible heating keeps humidity constant and only raises air temperature.

As we explore sensible heating, you’ll also discover the various heating devices that engineers and technicians use to achieve this vital HVAC function. These devices include:

  • Steam Coil: Learn how steam coils efficiently transfer heat to the air stream.
  • Hot Water Coil: Explore the use of hot water coils as a heating method in HVAC systems.
  • Heat Pipe: Understand the principles behind heat pipes and their role in sensible heating.
  • Air-to-Air Heat Exchanger: Discover how air-to-air heat exchangers contribute to the sensible heating process.
  • Sensible-Only Rotary Heat Wheel: Gain insights into the specialized rotary heat wheels used for sensible heating.
  • Electrical Heating Coil: Learn about electrical heating coils and their role in HVAC systems.
  • Furnace: Explore the traditional use of furnaces for sensible heating.
Sensible heating processes can be read off of the psychrometric chart be moving from point1 to point 2.
Sensible heating processes can be read off of the psychrometric chart be moving from point1 to point 2. Source: SlidePlayer

4. Latent Heating 

A latent heating process occurs when water (w) is evaporated without changing the dry bulb temperature (t).This process is represented as a vertical line on the psychrometric chart. When water is evaporated, it adds latent heat (L) to the air, increasing its enthalpy (h). The change in enthalpy during this process can be calculated using the following formula:

Δh = ma * L

Where:

  • Δh = Change in enthalpy (kJ/kg)
  • ma = Mass flow rate of dry air (kg/s)
  • L = Latent heat (kJ/kg)

During this latent heating process, the dry bulb temperature remains constant while the air’s moisture content increases.

5. Sensible Cooling

Sensible cooling
Sensible cooling process on a psychrometric chart can be read by moving from point A to point B. Source: CIBSE

Sensible cooling is a vital process in HVAC systems that involves removing heat from the air without altering its moisture content. This is achieved using various devices designed to carry out this specific task.

  1. Chilled Water Systems: Chilled water is a common medium for cooling. It circulates through coils or pipes, and air is passed over these cooled surfaces, resulting in heat transfer. Sensible cooling occurs as the air temperature decreases without any change in humidity.
  2. Refrigerant Cooling Coils: Refrigerant coils function in a manner similar to chilled water systems but use refrigerants instead. The air passes over these coils, and as the refrigerant inside evaporates and condenses, heat is absorbed and released, causing sensible cooling.
  3. Indirect Evaporative Coolers: These systems employ an adiabatic process that involves sensible cooling and humidification. Incoming hot and dry air passes through a heat exchanger where it indirectly cools down by transferring heat to a wet surface. This process maintains a constant wet-bulb temperature.
  4. Heat Pipes: Heat pipes are used to transfer heat efficiently. They operate by vaporizing and condensing a working fluid within a closed system. When air is passed over the condenser side of the heat pipe, sensible cooling is achieved.
  5. Air-to-Air Heat Exchangers: These devices transfer heat between two separate air streams, leading to sensible cooling in one stream while heating the other. They are valuable for energy recovery and maintaining indoor air quality.
  6. Sensible-Only Rotary Heat Wheels: Rotary heat wheels use rotating segments to transfer heat between incoming and exhaust air streams. This process allows for sensible cooling without altering humidity levels.
  7. Air Washers: Air washers use water spray systems to cool and clean the incoming air. Sensible cooling occurs as air is passed through the wetted surfaces.

On a psychrometric chart, the sensible cooling process is depicted as a horizontal movement to the left along a line of constant humidity ratio, towards the saturation line. During this process, there is no change in dew-point temperature, water vapor pressure, or humidity ratio, ensuring efficient cooling without introducing excess moisture into the air.

 

6. Heating and Humidification

Heating and humidification.
Humidification can be determined by moving from point A to B on the psychrometric chart. Source: CIBSE

Heating and humidification are essential processes in HVAC systems that work in sequence to prepare the air for comfortable and controlled indoor environments. In this combined process, air undergoes a two-step transformation to achieve the desired conditions.

First, the air enters a heating coil, where sensible heating takes place. This initial step elevates the air temperature without changing its moisture content. The energy added during this phase can be calculated using the following equation:

\[
Q = m_a \left({h_2 – h_1}\right)
\]

Where:

  • \(Q\) = Rate of energy added, KJ/hr
  • \(m_a\) = mass flow rate of dry air through the process
  • \(h_2\) = Specific enthalpy of moist air downstream of heating coil
  • \(h_1\) = Specific enthalpy of moist air upstream of heating coil

During the humidification process, the energy equation is:

\[
m_a \left({h_3 – h_2}\right) = m_w \cdot h_w
\]

Where:

  • \(h_3\) = The specific enthalpy of the moist air downstream of the humidifier
  • \(h_2\) = Specific enthalpy of moist air upstream of the humidifier
  • \(h_w\) = Specific enthalpy of the steam
  • \(m_w\) = Mass flow rate of the steam

The rate of moisture addition to the air, \(m_w\), is determined by a water vapor mass balance:

\[
m_w = m_a \left({w_3 – w_2}\right)
\]

Where:

  • \(w_2\) = Humidity ratio of the moist air upstream of the humidifier
  • \(w_3\) = Humidity ratio of the moist air downstream of the humidifier

Combining the equations:

\[
\frac{{h_3 – h_2}}{{w_3 – w_2}} = h_w
\]

This equation represents the slope of the humidification process on a psychrometric chart, allowing us to determine the direction of the process based on the enthalpy of the steam added to the air stream and the enthalpy-moisture protractor on a psychrometric chart.

It’s important to use dry and saturated steam during the injection process to prevent condensation. Steam can’t be sprayed below 100°C (at atmospheric pressure) due to nozzle requirements for higher pressure, and the lowest possible enthalpy carried with steam is the total heat of steam at 100°C when the steam is fully dry and saturated.

The amount of steam sprayed per kilogram of air is given by:

\[
m_w = m_a \left({w_3 – w_2}\right)
\]

7. Cooling and Dehumidification

Dehumidification is the process of removing moisture from the air, achievable by cooling the air below its dew point temperature. Effective dehumidification requires the cooling coil’s surface to stay below the dew point temperature of the air.

Dehumidification
Dehumidification can be represented by moving from point A to point B on the psychrometric chart. Source: CIBSE

For example, consider cooling air from 35ºC dry bulb (DB) and 24ºC wet bulb (WB) to 20ºC DB and 17.6ºC WB. Plotting these values on a psychrometric chart and drawing a line from one point to another reveals a 0.74 intersection on the sensible heat factor line, indicating 26% latent heat removal and 74% sensible heat removal.

The cooling and dehumidification process is illustrated in the chart below, starting at point 1 and ending at point 2.

To calculate the required refrigeration capacity (\(Q_R\)), an energy balance is employed:

Energy Balance:

\[
m_a \cdot h_1 = Q_R + m_a \cdot h_2 + m_w \cdot h_w
\]

The mass flow rate of water in the air is determined by:

\[
m_a \cdot w_1 = m_w + m_a \cdot w_2
\]

Combining the above two equations yields the refrigeration capacity (\(Q_R\)):

\[
Q_R = m_a(h_1 – h_2) – m_a(W_1 – W_2)h_w
\]

Where \(h_w\) represents the enthalpy of saturated liquid at temperature \(t_2\). The enthalpy related to liquid condensate is small in comparison to \((h_1 – h_2)\), which represents the enthalpy difference for cooling the air and condensing the water. The process is often approximated by dividing it into sensible (S) and latent (L) components:

\[
Q_{RS} = m_a(h_2 – h_a)
\]

and

\[
Q_{RL} = m_a(h_a – h_1)
\]

Thus,

\[
Q_R = Q_{RS} + Q_{RL}
\]

The sensible heat ratio for the process is then given by:

\[
SHR = \frac{Q_{RS}}{Q_{RS} + Q_{RL}}
\]

The rate at which moisture is removed from the air is calculated as:

\[
m_w = m_a(W_1 – W_2)
\]

8. Adiabatic Cooling

In the adiabatic cooling process, air is directed over a spray chamber equipped with nozzles that disperse water. The temperature of the sprayed water exceeds the wet-bulb temperature (WBT) of the incoming air but remains lower than the air temperature. As the air flows over this chamber, a portion of the water evaporates and is carried away by the air, thus increasing the specific humidity of the air. This phenomenon can be visualized in the figure below.

Crucially, the air supplies the necessary heat for water evaporation during this process, causing the air temperature to drop while maintaining the total enthalpy constant.

Complete air humidification is typically never achieved. Thus, we can define the effectiveness of the spray chamber using the following formula:

\[
E = \frac{{T_1 – T_3}}{{T_1 – T_2}}
\]

Where:

  • \(T_1 – T_3\) represents the actual drop in dry-bulb temperature (DBT)
  • \(T_1 – T_2\) represents the ideal drop in DBT

The humidification efficiency(%) can be calculated as:

\[
\text{Efficiency } = 100 \times E
\]

9. Chemical Dehumidification

In the chemical dehumidification process, when highly humid air is directed over a solid absorbent bed or subjected to a liquid absorbent spray, a portion of the water vapor is absorbed. This absorption reduces the water content in the air. The latent heat released during this process is absorbed by the air, leading to an increase in its dry-bulb temperature (DBT), while the total enthalpy of the air remains constant. Consequently, the chemical dehumidification process follows a path along a constant enthalpy line.

The effectiveness of the dehumidifier can be expressed as:

\[
E = \frac{{T3 – T1}}{{T2 – T1}}
\]

Where:

  • \(T3 – T1\) represents the difference in dry-bulb temperature (DBT)
  • \(T2 – T1\) represents the ideal change in DBT

10. Evaporative Cooling Systems

Evaporative cooling encompasses various types, including:

10.1 Direct Evaporative System

Direct evaporative cooling is a straightforward and efficient process where conditioned air directly interacts with a wetted surface for cooling and humidification. The steps involved are as follows:

  1. The hot and dry outdoor air undergoes filtration and then comes into contact with a wetted surface or water droplets within an air washer.
  2. The air experiences simultaneous cooling and dehumidification due to the exchange of sensible and latent heat (process o-s).
  3. The cooled and humidified air is delivered to the conditioned space, extracting sensible and latent heat (process s-i).
  4. Finally, the air is exhausted at state i.

In an ideal scenario, if the air washer were perfectly insulated with an infinite contact area, the process would follow a constant wet bulb temperature line, resulting in an adiabatic saturation process. In practice, the exit temperature may be slightly higher due to heat leaks and finite contact area.

The saturation efficiency (\(ε\)) of the direct evaporative cooling system can be defined as:

\[
ε = \frac{{T1 – T3}}{{T2 – T1}}
\]

10.2 Indirect Evaporative System

The indirect evaporative cooling process involves two separate air streams:

  • Primary air stream, which gets cooled and humidified through direct contact with a wetted surface (o-o’).
  • Secondary air stream, used as supply air to the conditioned space, only exchanging sensible heat with the cooled and humidified primary air stream (o-s).

Notably, in an indirect evaporative cooling system, the moisture content of the supply air remains constant, while its temperature drops. This can provide greater comfort in regions with higher humidity levels compared to direct systems. Commercially available indirect evaporative coolers can have saturation efficiency as high as 80%.

10.3 Multi-Stage Evaporative Cooling Systems

Multi-stage evaporative cooling systems offer advanced cooling solutions, with a typical two-stage configuration:

  1. In the first stage, the primary air is cooled and humidified (o -o’) by direct contact with a wet surface. Simultaneously, it cools the secondary air sensibly (o -1) in a heat exchanger.
  2. In the second stage, the secondary air stream is further cooled via direct evaporation (1-2).

In an ideal case, the final exit temperature of the supply air (\(t2\)) is several degrees lower than the wet bulb temperature of the inlet air to the system (\(t0\)). To enhance efficiency, it’s possible to sensibly cool the outdoor air before sending it to the evaporative cooler by exchanging heat with the exhaust air from the conditioned space.

11. Conclusion

In conclusion, this comprehensive guide has provided a deep understanding of psychrometric processes and their critical roles in HVAC systems. These processes are the building blocks for engineers and technicians, allowing them to create indoor environments that balance comfort and energy efficiency. From the fundamental principles of sensible and latent heating and cooling to the intricate details of heating, humidification, dehumidification, and advanced concepts like adiabatic cooling and chemical dehumidification, we’ve explored the complete spectrum of psychrometric processes. These processes are pivotal in optimizing indoor environments for human comfort and well-being.

Furthermore, we’ve unveiled the world of evaporative cooling systems, including direct, indirect, and multi-stage approaches. Each method presents its unique advantages and challenges, offering HVAC professionals a diverse toolkit to address varying environmental conditions and client requirements. By mastering these psychrometric processes, HVAC engineers and technicians can not only design efficient systems but also excel in troubleshooting, maintenance, and creating indoor spaces that promote both comfort and health. As the HVAC industry continues to evolve, this knowledge remains indispensable for achieving the perfect balance between human needs and energy conservation. Remember, HVAC professionals always know how to keep their cool, even during heated discussions, because they understand the science behind comfort and climate control. Whether you’re an experienced HVAC expert or just starting your journey in the field, the mastery of psychrometric processes will guide you towards a brighter and cooler future in indoor environmental control.