Mastering HVACR: Essential Concepts & System Operations

Refrigerant Definition

A refrigerant is a substance, usually a fluid, used in a refrigeration cycle. It absorbs heat from a low-temperature source (evaporator) and releases heat to a high-temperature sink (condenser) as it undergoes phase changes (evaporation and condensation).

Refrigerant Classification

Primary Refrigerants

These are the working fluids that directly participate in the refrigeration cycle by undergoing phase changes.

• Examples: Ammonia (NH3 - R717), R-12, R-134a, R-22

Secondary Refrigerants

These are fluids that are cooled by the primary refrigerant and then transport the cooling effect to the desired location. They do not undergo a phase change.

• Examples: Brine solutions (saltwater), glycol solutions, water

Classification by Chemical Composition

• Halocarbons: These contain carbon, fluorine, chlorine, and bromine. e.g., R-12 (Freon-12), R-22.
• Hydrocarbons: These contain only carbon and hydrogen. e.g., Propane, Butane.
• Inorganic Refrigerants: These are naturally occurring substances. e.g., Ammonia, Carbon dioxide.
• Azeotropes: These are mixtures of refrigerants that behave like a single substance.

Ideal Refrigerant Properties

An ideal refrigerant would possess the following properties:

Thermodynamic Properties

• High latent heat of vaporization: To absorb a large amount of heat during evaporation.
• Low specific heat: To minimize the amount of heat absorbed by the refrigerant itself.
• High thermal conductivity: For efficient heat transfer.
• Low boiling point: To enable refrigeration at low temperatures.
• High critical temperature: To allow for heat rejection at ambient temperatures.
• Moderate pressure and pressure ratio: To reduce compressor work.
• Low viscosity: To minimize flow resistance.

Chemical Properties

• Non-toxic and safe: To prevent health hazards.
• Non-flammable: To avoid fire or explosion risks.
• Non-corrosive: To prevent damage to system components.
• Stable and inert: To ensure long service life.
• Low ozone depletion potential (ODP): Environmentally friendly.
• Low global warming potential (GWP): To minimize the impact on climate change.

Standard Vapor Compression Refrigeration Cycle

The Vapor Compression Refrigeration (VCR) cycle is the most common refrigeration cycle.

Key Components

• Compressor
• Condenser
• Expansion Device (or Throttle Valve)
• Evaporator

Cycle Processes

• Compression (1-2): Low-pressure, low-temperature refrigerant vapor is compressed to a high-pressure, high-temperature vapor by the compressor.
• Condensation (2-3): The hot, high-pressure vapor rejects heat to the surroundings in the condenser and condenses into a high-pressure liquid.

• Expansion (3-4): The high-pressure liquid expands through an expansion device, causing a drop in pressure and temperature.
• Evaporation (4-1): The low-pressure, low-temperature liquid absorbs heat from the refrigerated space and evaporates into a low-pressure vapor.

Bypass Factor (BPF) Definition

The bypass factor (BPF) of a coil is the ratio of the amount of air that passes through the coil without undergoing a change in temperature to the total amount of air passing through the coil.

BPF Calculation Formulas

• Cooling Coil: BPF = (Leaving air temperature - Apparatus Dew Point Temperature) / (Entering air temperature - Apparatus Dew Point Temperature)
• Heating Coil: BPF = (Leaving air temperature - Entering air temperature) / (Coil surface temperature - Entering air temperature)

Effective Temperature (ET)

Effective temperature (ET) is a single value that combines the effects of air temperature, humidity, and air movement on the sensation of warmth or coolness felt by the human body. It's an index of thermal comfort.

Understanding Effective Temperature

• It's the temperature of still, saturated air that would produce the same thermal sensation as the given conditions.
• Human comfort is not just about air temperature; humidity affects how we perceive temperature because it influences sweat evaporation, and air movement increases convective heat transfer.

Psychrometric Processes Explained

• Sensible Heating: Increasing the dry-bulb temperature of air without changing its humidity ratio.
• Sensible Cooling: Decreasing the dry-bulb temperature of air without changing its humidity ratio.
• Humidification: Increasing the humidity ratio of air.
• Dehumidification: Decreasing the humidity ratio of air.
• Cooling and Dehumidification: Simultaneously cooling the air and removing moisture.
• Heating and Humidification: Simultaneously heating the air and adding moisture.

Heat Pump Systems

Introduction to Heat Pumps

• A heat pump is a device that transfers heat from a heat source to a heat sink.
• Unlike traditional heating systems that generate heat, heat pumps move heat.

Heat Pump Classification

Heat pumps can be classified based on the heat source and sink:

• Air-to-Air Heat Pump: Transfers heat between indoor and outdoor air.
• Air-to-Water Heat Pump: Transfers heat between air and water.
• Water-to-Water Heat Pump: Transfers heat between two water sources.
• Ground-Source (Geothermal) Heat Pump: Transfers heat between the ground and the building.

Industrial Applications of Heat Pumps

Heat pumps can be used for various industrial applications:

• Process Heating: Providing heat for processes like drying, washing, and sterilization.
• Waste Heat Recovery: Capturing and upgrading waste heat from processes to be reused for heating purposes.
• Boiler Feedwater Heating: Preheating water before it enters a boiler to improve efficiency.
• Evaporation and Distillation: Providing heat for separation processes.
• Industrial Drying: Efficiently removing moisture from products.

Heat Pump Working Principle

The basic heat pump cycle is similar to a refrigeration cycle but can be reversed to provide both heating and cooling.

Heating Mode Operation

• The heat pump absorbs heat from a low-temperature source (e.g., outdoor air, ground).
• A refrigerant is used as the working fluid to absorb and release heat.
• The refrigerant is compressed, raising its temperature.
• The hot refrigerant releases heat to the indoor space.

Cooling Mode Operation

• The cycle is reversed, and the heat pump extracts heat from the indoor air and releases it outdoors.

Key Heat Pump Components

• Evaporator
• Compressor
• Condenser
• Expansion valve
• Reversing valve

Thermal Comfort Explained

• Thermal comfort is the condition of mind that expresses satisfaction with the thermal environment.
• It is a subjective sensation, and it varies from person to person.

Factors Influencing Thermal Comfort

Several factors influence whether a person feels comfortable in a given environment:

• Air Temperature: The temperature of the air surrounding the body.
• Humidity: The amount of moisture in the air. High humidity reduces the body's ability to cool itself through sweating.
• Air Movement: Air movement enhances convective heat transfer, which can increase or decrease heat loss from the body.
• Radiant Temperature: The temperature of the surrounding surfaces. Radiant heat transfer occurs between surfaces at different temperatures.
• Clothing: Clothing provides insulation and affects heat exchange with the environment.
• Metabolic Rate: The rate at which the body produces heat, which depends on the level of physical activity.

Importance of Thermal Comfort

Maintaining thermal comfort is essential for:

• Productivity and performance in workplaces.
• Health and well-being.
• Occupant satisfaction in buildings.

Refrigerant Expansion Devices

The primary function of an expansion device is to reduce the pressure and temperature of the liquid refrigerant before it enters the evaporator.

Types of Expansion Devices

• Capillary tube
• Thermostatic expansion valve (TXV)
• Electronic expansion valve

Thermostatic Expansion Valve (TXV)

TXV Working Principle

• The TXV controls the flow of refrigerant based on the superheat of the refrigerant vapor at the evaporator outlet.
• The sensing bulb, attached to the evaporator outlet, senses the temperature of the refrigerant vapor.
• This temperature is converted into a pressure that acts on the diaphragm.
• The diaphragm balances the forces exerted by the bulb pressure, the spring pressure, and the evaporator pressure.
• By modulating the valve needle position, the TXV regulates the refrigerant flow rate to maintain a constant superheat.

Deep Sea Water Air-Conditioning (SWAC)

Deep Sea Water Air-Conditioning (SWAC) is a sustainable cooling technology that utilizes naturally cold seawater from deep ocean layers (usually around 1000 meters depth and 4-5°C temperature) to provide air conditioning to coastal buildings.

SWAC Working Principle

• Cold deep seawater is pumped to a heat exchanger.
• This cold water cools a secondary loop of freshwater without mixing.
• The chilled freshwater is then circulated to buildings for air-conditioning.

Benefits of SWAC

• Reduces electrical energy consumption up to 80-90%.
• Minimal use of refrigerants.
• Environmentally friendly and low operating costs.

Cooling Towers

Cooling towers are heat rejection systems that cool water by evaporative cooling, often used in HVAC, power plants, and industrial processes.

Cooling Tower Performance Factors

• Range: Temperature difference between hot water entering and cold water leaving.
• Approach: Difference between cold water temperature and wet-bulb temperature of air.
• Efficiency: Determined by how closely the cold water approaches the wet-bulb temperature.

Cooling Tower Selection Criteria

• Capacity and system cooling load.
• Type: Induced draft or forced draft; counterflow or crossflow.
• Climate conditions and water quality.
• Footprint and noise restrictions.

Types of Fans in AC Systems

Centrifugal Fans

Use rotating impellers to move air radially; high pressure, low noise; used in air handling units.

Axial Fans

Blades force air parallel to shaft; high flow, low pressure; used in condensers and exhausts.

Propeller Fans

Simple, high airflow at low static pressure; used in cooling towers and ventilation.

Crossflow Fans

Long, cylindrical fans; used in ductless units and compact systems for uniform air distribution.

Fan Selection Considerations

• Air volume (CFM)
• Pressure requirement
• Noise level
• Efficiency

HVAC & Building Insulation Materials

Insulation materials are critical for reducing heat transfer in HVAC systems and building envelopes.

Types of Insulation Materials

• Fiberglass: Low cost, non-combustible, commonly used in walls, attics, ducts.
• Polyurethane Foam: High R-value, sprayable or panel form; excellent moisture barrier.
• Polystyrene (EPS/XPS): Rigid foam boards with good moisture resistance and high compressive strength.
• Mineral Wool/Rock Wool: Excellent fire resistance, soundproofing properties; made from volcanic rock or slag.
• Reflective Foils: Aluminum-based, used to reflect radiant heat; ideal for roofs and ducts.
• Glass Wool: Lightweight, fire-resistant, widely used in HVAC ducts and false ceilings.

Applications of HVACR Systems

Residential HVACR Applications

• Heating and cooling of homes, apartments.
• Ventilation for indoor air quality.

Commercial HVACR Applications

• Office buildings, shopping malls, hotels.
• Restaurants, hospitals, schools.

Industrial HVACR Applications

• Manufacturing processes requiring specific temperatures or humidity control.
• Cold storage for food and other products.
• Data centers to cool heat-generating equipment.

Transportation HVACR Applications

• Air conditioning in vehicles (cars, buses, trains, airplanes).
• Refrigerated transport for perishable goods.

Food Preservation HVACR Applications

• Refrigeration and freezing to extend the shelf life of food products.

Food Industry HVACR Application

• Refrigeration is critical for preserving food and extending its shelf life. It slows down the growth of microorganisms and reduces the rate of spoilage.
• Different refrigeration technologies are used for various food products:
  • Cold storage warehouses for bulk storage.
  • Refrigerated transport for moving food over long distances.
  • Display cases in supermarkets to keep food at the proper temperature.
• Freezing is another important application, as it can preserve food for even longer periods.

HVAC Duct Design Methods

Duct design is a crucial aspect of HVAC systems to ensure proper air distribution. The goal is to deliver the required airflow to different areas while minimizing energy consumption and noise.

Common Duct Design Methods

• Velocity Reduction Method: The air velocity is progressively reduced along the duct run. This method is simple but may result in larger duct sizes and higher material costs.
• Equal Friction Method: Ducts are sized to provide the same friction loss per unit length. This method is widely used and provides a good balance between duct size and pressure drop.
• Static Regain Method: Ducts are designed to maintain a constant static pressure along the duct run. This method can result in smaller duct sizes.

Duct Design Considerations

• Airflow rate
• Pressure drop
• Noise levels
• Space limitations
• Cost

Ton of Refrigeration (TR)

• A ton of refrigeration is a unit of cooling capacity.
• It is defined as the amount of heat required to melt 1 ton (2,000 lbs) of ice at 32°F (0°C) in 24 hours.
• 1 TR is equivalent to 12,000 BTU/hr or approximately 3.5 kW.

Air Conditioning Definition

• Air conditioning is the process of simultaneously controlling the temperature, humidity, cleanliness, and distribution of air in an enclosed space.
• The goal is to maintain conditions that are most suitable for human comfort or for specific industrial processes.

Dry Bulb Temperature (DBT)

• The dry-bulb temperature is the temperature of air measured by a standard thermometer, shielded from radiation and moisture.
• It is the ordinary temperature of the air.

Wet Bulb Temperature (WBT)

• The wet-bulb temperature is the temperature measured by a thermometer whose bulb is covered with a wet wick and exposed to a stream of air.
• It indicates the amount of moisture in the air; the difference between DBT and WBT is used to determine humidity.

Humidity Ratio (Specific Humidity)

• Humidity ratio is the mass of water vapor present in a unit mass of dry air.
• It is expressed as kg of water vapor/kg of dry air or pounds of water vapor/pound of dry air.

Coefficient of Performance (COP)

• The coefficient of performance (COP) is a ratio that measures the efficiency of a refrigeration or heat pump system.
• For refrigeration: COP = (Refrigerating effect) / (Work input)
• For a heat pump: COP = (Heating effect) / (Work input)

Effective Temperature (ET) Revisited

• Effective temperature (ET) is an empirical index that combines the effects of temperature, humidity, and air movement on the sensation of warmth or cold felt by the human body.
• It is a subjective measure of thermal comfort.
• It represents the temperature of still, saturated air that would produce the same thermal sensation as the actual conditions.

Physiological Heat Hazards

Exposure to excessive heat can lead to a range of physiological hazards, some of which can be life-threatening.

Types of Heat-Related Hazards

• Heatstroke: A severe condition where the body's temperature regulation system fails. Symptoms include high body temperature, hot and dry skin (or sometimes sweating), rapid heartbeat, confusion, and loss of consciousness. It is a medical emergency.
• Heat Exhaustion: Caused by dehydration and overheating. Symptoms include heavy sweating, weakness, dizziness, nausea, headache, and muscle cramps.
• Heat Cramps: Muscle spasms, usually in the legs or abdomen, caused by dehydration and electrolyte imbalance.
• Heat Rash: A skin irritation caused by excessive sweating and blocked sweat ducts.
• Heat Syncope: Fainting or dizziness caused by overheating and low blood pressure.

Food Preservation Methods

• Food preservation is the process of treating and handling food to prevent or slow down spoilage.
• Spoilage is caused by the growth of microorganisms and enzymatic activity.

Key Food Preservation Techniques

• Refrigeration and Freezing: Low temperatures slow down microbial growth and enzymatic reactions.
• Heating: Pasteurization and canning use heat to kill microorganisms.
• Drying: Removing moisture inhibits microbial growth.
• Chemical Preservation: Using additives like salt, sugar, acids, or preservatives to inhibit spoilage.
• Irradiation: Exposing food to ionizing radiation to kill bacteria and insects.
• Modified Atmosphere Packaging (MAP): Changing the composition of the gases surrounding the food to extend shelf life.

Aircraft Air Cooling Systems

• Ram air cooling system
• Simple air cooling system
• Bootstrap air cooling system
• Regenerative air cooling system
• Reduced ambient air cooling system

Simple Air Cooling System

Processes in Simple Air Cooling

• Ramming: The process where the incoming air is compressed.
• Compression: Further compression of air.
• Cooling: Heat rejection in the heat exchanger.
• Expansion: Expansion of air in the turbine to produce cooling.

Applications

• Aircraft cooling

Sensible Heat Factor (SHF)

• The sensible heat factor is the ratio of sensible heat load to the total heat load.
• SHF = Sensible Heat Load / (Sensible Heat Load + Latent Heat Load)
• Sensible heat is the heat associated with a change in temperature of the air.
• Latent heat is the heat associated with a change in moisture content of the air.

Room Sensible Heat Factor (RSHF)

• The room sensible heat factor is specifically the ratio of the sensible heat load to the total heat load within the conditioned space (room).
• RSHF = Room Sensible Heat Load / (Room Sensible Heat Load + Room Latent Heat Load)

Grand Sensible Heat Factor (GSHF)

• The grand sensible heat factor considers the total sensible heat load, including that from both the room and any fresh air introduced into the system.
• GSHF = Grand Sensible Heat Load / (Grand Sensible Heat Load + Grand Latent Heat Load)

Effective Room Sensible Heat Factor (ERSHF)

• The effective room sensible heat factor is a modified version of RSHF that takes into account the bypass factor of the cooling coil.
• The bypass factor represents the fraction of air that passes through the cooling coil without being cooled to its design temperature.
• ERSHF is used in more precise calculations to determine the actual condition of the air supplied to the room.

Summer Air Conditioning

Also known as summer comfort air conditioning, its goal is to cool and dehumidify the air to maintain comfortable indoor conditions during hot weather.

Functions of Summer AC

• Lowers indoor air temperature.
• Removes excess humidity (dehumidification).
• Filters air to remove pollutants and dust.

Summer AC Process

• Hot, humid air is drawn into the air conditioning system.
• Air passes through filters, then cooling coils (evaporator).
• Moisture condenses and drains away.
• Cool, dry air is supplied to indoor spaces.

Typical Applications

• Homes, offices, shopping centers during summer months.

Winter Air Conditioning

Also known as winter comfort air conditioning, its main aim is to heat and humidify the air to make indoor environments comfortable in cold weather.

Functions of Winter AC

• Increases air temperature.
• Adds moisture to the dry air (humidification).
• Filters and circulates the air.

Winter AC Process

• Cold, dry air is drawn into the unit.
• Air is passed over heating coils (electric heaters, hot water, or steam).
• A humidifier adds moisture.
• Warm, humidified air is distributed indoors.

Typical Applications

• Residential and commercial buildings in cold climates.

Air Handling Unit (AHU)

An Air Handling Unit (AHU) is a central component of an HVAC system used to condition and circulate air as part of heating, ventilating, and air-conditioning systems. It is typically a large metal box containing various components that regulate and distribute air throughout a building.

AHU Components

• Filter Section: Removes dust, pollen, and other airborne particles.
• Cooling Coil: Cools the air by circulating chilled water or refrigerant.
• Heating Coil: Heats the air using hot water, steam, or electric elements.
• Blower/Fan: Circulates air through the system and the building.
• Humidifier/Dehumidifier: Controls humidity levels.
• Mixing Box: Mixes return air with fresh air for energy efficiency.
• Dampers: Regulate airflow and direction.

AHU Functions

• Maintains indoor air quality.
• Controls temperature and humidity.
• Ensures proper ventilation.
• Provides filtered and conditioned air to various zones of a building.

AHU Applications

• Commercial buildings, hospitals, malls, industrial facilities.