HVAC & Refrigeration Mechanical Engineering

How Do Air Conditioners Work in Winter?

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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.


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