Electric Power Loads Types

 In a previous Topic, “Electrical Load Classification and Types”,I show that the electrical loads can be classified into various categories according to various factors; one of these factors is according to load function/usage as follows:

Third Classification: Electrical Load Classification According To Load Function 
  1. Lighting Load. 
  2. Appliances Load. 
  3. Power Loads.

In the following previous Topics, I explained the first type; lighting load:

And in the previous topic “Appliances Electrical Loads Types and Classifications“, I explained the second type; Appliance load

And today, I will explain the third and last type of electrical loads according to the load function / usage as follows.

Third: Electric Power Loads 

Electric power loads in any building shall include all loads other than lighting loads and those served by general purpose receptacles and comprise the environmental system electric power requirements and the facility occupancy equipment electric power requirements.

In general, it can be divided to the following major loads:

  1. HVAC loads (Heating, Ventilation and Air Conditioning System Loads).
  2. Lifts Loads (Transportation System Loads).
  3. Pumps Loads (Fire Fighting, Plumping, Irrigation Systems, Etc.).

Since this Course EE-1 is for beginners in the electrical design field, I must explain in detail these loads or systems and indicate its construction, principals of operation and its different types.

Today, I will begin with the HVAC loads (Heating, Ventilation and Air Conditioning System Loads) as follows:

Introduction to Heating, Ventilation and Air Conditioning System Loads (HVAC) 

The field of heating, ventilation, and air conditioning—HVAC—is a science and practice of controlling indoor climate, thereby providing health and comfortable interior conditions for occupants in a well-designed, energy-efficient, and low emissions manner.

The term "H" in HVAC stands for heating that comprises of any number of heating systems from gas furnaces, electric furnaces, oil furnaces, oil and gas boilers, radiant heating systems, and heat pumps.

The Term “V” in HVAC describes ventilation. This can be ventilating the facility using ductwork or ventilating a kitchen using ductwork and fans with a hood. It can also refer to combustion air or the air needed to have combustion for various heating systems.
The Term "AC" in HVAC refers to air conditioning that comprises of 3 main methods – mechanical compression, vapor absorption and evaporative cooling. Air conditioners (direct expansion – DX systems) and chillers usually accomplish the job of air conditioning.

HVAC Systems overview

HVAC systems have the following elements in common:
  1. Equipment to generate heating or cooling: The equipment is selected with a capacity to offset the peak load of the space or spaces to be served. 
  2. A means of distributing heat, cooling, and/or filtered ventilation air where needed: air, water, or steam. 
  3. Devices that deliver the heat, cooling, and/or fresh air into the building: registers and diffusers, hydronic radiators or convectors, and fan coil units. 

Units of HVAC systems:

These units express the efficiency of a heating or cooling source.
The most important units of HVAC system from the electrical designer point of view are:
1- COP – “Coefficient of Performance”:
  • It is the measure chiller efficiency measured in Btu output (cooling capacity) divided by Btu input (electric power). Typical values are 2 – 4. 
  • Cooling capacity is specified in tons of refrigeration; 1 ton is equivalent to 12000 Btu per hour. 
  • 1 kWh of electric power is equivalent to 3412 Btu per hour; multiplying the COP by 3.412 yields energy efficiency ratio. 

2- BTUH – “British Thermal Units per Hour”:
  • It is a rate of heating or cooling expressed in terms of Btu per hour. (1kW = 3412 Btu) 

3- Ton :
  • One ton of cooling is the heat extraction rate of 12000 Btu per hour. 
  • Theoretically it is energy required to melt one ton of ice in one hour. 

HVAC System Classifications:

the HVAC system includes the following major parts:
  1. Heating systems.
  2. Cooling systems.

First: Heating systems:

A- Heat Sources 

Heat sources which can be employed in buildings can be categorized to (4) categories as follows:
  1. On-site combustion (coal, oil, natural gas, propane).
  2. Electric resistance.
  3. On-site energy collection (solar energy).
  4. Heat transfer (heat pumps).

The choice of the preferred heat source for a given building situation is usually based upon the following factors:
  • Source availability.
  • Required heat system capacity.
  • The fuel/equipment costs. 

B- Types of heating systems:

Heating systems may be classified to two types:

  1. Central Heating System.
  2. Local Heating System. 

1- Central Heating System 

Central Heating System  

In this system the heat generation is done by a boiler, furnace, or heat pump to heat water, steam, or air, all in a central location. And the delivery of heat is done via either ductwork, for forced air systems, or piping to distribute a heated fluid and radiators to transfer this heat to the air.

Every area in the space is heated to the same temperature, which is controlled by a single thermostat.

2- Local Heating System 

Local Heating System Units

A local heating system serves a single thermal zone and has its major components located within the zone itself. Serving only a single zone, local heating systems will have only one point of control, typically a thermostat for active systems. A local heating system will consist of one or more self-contained equipment units containing heat source, distribution, and delivery functions in a single package.

Portable electric heaters, built-in electric resistance heaters, electric resistance baseboard radiators, infrared heaters, fireplaces, and wood stoves are examples of local heating-only systems.

Note: In general central system is preferable for large buildings having multiple zones and local heating is a plus, if majority of areas remain unoccupied and if the people preferences require different temperatures or they disagree about the most comfortable temperature.

C- Equipment used to generate heating in central heating systems:
  1. Furnace.
  2. Hot water & steam boiler.
  3. Heat Pump.

1- Furnace: 

A furnace is a heating system component designed to heat air for distribution to various building spaces. All four heat source categories are used with furnaces.

1.1 Principle of operation: 

Regardless of the type of fuel the furnaces generally work on the same principle. The burning of fuel takes place inside an enclosed metal container (generally referred to as a fire box or heat exchanger), which warms the heat exchanger. The heat exchanger, now hot, radiates the heat into the air. This heated air is either circulated by density differential via a ‘Gravity Furnace’ or by aid of a blower via ‘Forced air Furnace’.

1.1.a Gravity Furnace:

Gravity Furnace 

Small-capacity furnaces that rely on natural convection for heat distribution are classified as gravity furnaces. The term gravity refers to the fact that the furnace has no blower to move the heated air around the room. They rely on the fact that heated (less dense) air rises and the cooler (more dense) air falls to circulate the heat. This is not a very efficient way to heat a space and generally can effectively condition only one space.

1.1.b Forced Air Furnace:

Forced Air Furnace 

Forced are furnaces are equipped with fans to circulate warm air over greater distance through a ductwork system. Cool return air from occupied spaces passes first through a filter, the blower, and the heating chamber, arriving at the supply ductwork at a raised temperature. The exhaust gases (including carbon-mono-oxide) are vented to the exterior of the building. The unit may also include a humidifier that evaporates moisture into the air as it passes through.

1,2 Types of Furnaces:

There are three types of furnaces: 

1- Single-Stage Furnace:
Single stage implies the furnace fan control is simply “on and off”.

Since every space has a unique "heat load” which varies throughout the day, the manufacturers wisely put options in the furnace fan speed.

2- Two-Stage Furnace:
Two-Stage furnaces were developed with comfort in mind. Here is how they function. When the thermostat activates the furnace, it comes on at 2/3 rd strength (burning gas at 65% of maximum). If, after 10 minutes of operation, the thermostat is still calling for heat - the furnace will switch to 100%. What that accomplishes is a uniform heating of entire space. Two-Stage furnaces are more efficient and more effective way than single stage furnaces.

3- Two-Stage Variable Furnace:
Two-stage variables furnace incorporates an additional feature of variable speed blower. In a single-stage furnace, the control is On-Off where as in the variable furnace, the fan turns over slowly and varies the air circulation per the load. This offers better energy efficiency.

1.3 Furnace Configuration

there are two major types of furnace configurations as follows:

a- Up flow Furnace: 

Up flow Furnace 

When a furnace is installed in a basement it is considered an "Up flow" furnace, meaning the cooler air is drawn at the base of the furnace, and the warm air exits out the top of the furnace.

b- Down flow Furnace:

Down flow Furnace 

If a furnace is installed on the main floor of a home and the heat comes from floor registers, it is a "Down flow" furnace. In a down flow furnace, the cool air enters the furnace at the top and the warmed air exits at the bottom.

in the next topic, I will explain other heat sources in central heating systems which is " the Boilers and Heat pumps". so, please keep following.

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