HVAC and Refrigeration Systems Notes

Learning Objectives

  • Psychrometrics and air conditioning processes

  • Types and applications of HVAC (Heating, Ventilation, and Air Conditioning) and refrigeration systems in industry

  • Comparison of energy performance of various HVAC and refrigeration systems

  • Performance assessment of room air conditioners

  • Factors affecting performance and energy efficiency

  • Applications of heat pumps

  • Ice bank systems

  • Humidification systems

  • Standards and labeling program for air conditioners

  • Opportunities for energy saving

HVAC & Refrigeration Systems

Introduction

The HVAC and refrigeration systems are essential for transferring heat energy either from or to products or building environments. These systems utilize energy in the form of electricity or heat to power mechanical equipment that transfers heat from colder areas (low energy level) to warmer regions (high energy level). Refrigeration specifically deals with heat transfer from low temperature sources to high temperature sinks using a refrigerant that has a low boiling point.

Heat Transfer Loops in Refrigeration Systems
A refrigeration system includes several loops or components which facilitate heat transfer:

  1. Indoor Air Loop: Indoor air is driven by a supply air fan through a cooling coil, transferring heat from the air to chilled water.

  2. Chilled Water Loop: Returned from the cooling coil by a chilled water pump, this water goes back to the chiller's evaporator for re-cooling.

  3. Refrigerant Loop: The compressor pumps heat from the chilled water to the condenser water.

  4. Condenser Water Loop: Condenser water absorbs heat from the chiller and is pumped to a cooling tower.

  5. Cooling Tower Loop: Here, a cooling tower fan drives air over the hot condenser water, expelling heat to the outdoors.

Psychrometrics and Air Conditioning Processes

Psychrometrics is the study of properties of moist air and is instrumental for analyzing air-conditioning processes. It converts knowledge of heating or cooling loads into correct volume flow rates for air.

  • Key Psychrometric Quantities: Dry bulb temperature, wet bulb temperature, dew point, specific humidity, relative humidity.

  • Psychrometric Chart: This chart represents the properties of air, allowing engineers to determine the required processes in air conditioning. The chart provides variables such as enthalpy and specific volume.

Comfort Zone

Comfort is typically experienced within an air temperature range of 22°C to 27°C and relative humidity of 40% to 60%. This comfort zone assists in managing required heating or cooling activities to maintain occupant comfort by adjusting indoor conditions accordingly.

Types of Refrigeration Systems
Vapour Compression Refrigeration (VCR)

  • This system causes heat to flow from cold to hot bodies by using a refrigerant. The refrigeration cycle involves several stages:

    1. Low-pressure liquid refrigerant in the evaporator absorbs heat and evaporates into gas.

    2. The compressor increases the gas's pressure and temperature.

    3. The condenser cools the gas back into a liquid.

    4. The expansion device reduces the pressure of the liquid before it re-enters the evaporator.

Vapour Absorption Refrigeration (VAR)

  • Absorption chillers produce chilled water using heat sources like steam or hot water. They are designed to operate with a refrigerant and an absorbent working fluid.

Common Refrigerants

Refrigerants such as R-11, R-12, R-22, and R-134a are commonly used. Guidelines such as the Montreal Protocol have led to the phasing out of some refrigerants due to their environmental impact, pushing the adoption of alternatives such as hydrocarbons and HFCs.

Energy Efficiency in HVAC Systems

  • Energy Saving Measures: Improving insulation, maintaining optimal temperatures, and ensuring efficient equipment operations can lower energy usage.

  • Performance Assessment: Valid measurements of TR, COP, and various operational parameters are crucial for assessing energy efficiency in HVAC systems.

Standards and Labeling

Energy standards for air conditioners help consumers select energy-efficient units. The Bureau of Energy Efficiency’s labeling program provides a star rating based on efficiency metrics, assisting users in making informed choices that reduce energy consumption.

Performance Assessment

Performance assessment methods include calculating the specific power consumption, EER (Energy Efficiency Ratio), and factors such as the cooling effect delivered versus power drawn. Efficient performance assessment can lead to significant energy savings for HVAC systems.

Heat Pump Applications

Heat pumps operate similarly to air conditioning systems, but they utilize low-quality waste heat to provide useful heat output. They exhibit high energy transfer efficiency and find applications in industrial heating, water heating, and even drying processes.

Ice Bank Systems

Ice bank systems store unused cooling energy created during off-peak electricity hours to be used during peak demand. This technique allows facilities to optimize operating costs by using stored ice during higher tariff periods.