Civil Engineering Reference
In-Depth Information
Based on the international mechanical code (ICC
2012
) defi nition modular
boilers are a group of individual boilers installed as a unit without any stop valve in
between. The individual modules input should not exceed four hundred thousand
British thermal units per hour (400,000 Btu/h) gas or one hundred and fourteen
kilowatts (114 kW) electric.
Fire tube versus water tube boilers are another well known categorization for the
boilers, which refers to the locations of water and fi re inside and outside of the
boiler tubes.
A force draft boiler utilizes a fan to generate required force for discharging the
combustion products into atmosphere, while a natural draft boiler relies on the
buoyancy of natural pressure differences to move the combustion product up the
fl ue stack.
A hot water boiler system (Fig.
8.2
) that is used for space heating purpose is
commonly consisted of a number of boilers that generate hot water, a group of
pumps that distributes the hot water through a network of hot water piping to air
handling units preheat and heating coils, or other heating equipment such as hot
water unit heaters. The common source of heating generation in gas-fi red boilers is
natural gas, and in electric boilers is electricity.
Therefore other than power required for pumping the hot water in a hot water
heating system, the electrical power required for this system are power required for
forced draft combustion discharge fan, electric power for burner ignition, and in
case of using an electric boiler is the boiler own power (KW). Heat transfer from
pipes can indirectly increase the energy consumption of the system also. The small
energy consumed by low-voltage power provided for control valves is another
source of energy consumption.
8.6
Direct Expansion (DX) Unitary Systems
In a direct-expansion unitary system, the evaporator of the refrigerant cycle is in
direct contact with the air stream. These units can be found in the forms of packaged
or split system units. If the evaporator and condenser are located in the same enclo-
sure, the unit is called packaged and if the evaporator and condenser are located
separately inside and outside of the building these units are known as split systems.
The main reason for using this type system is its lower cost due to its lower required
labor and also fewer components to install. The other factors that affect the decision
in selecting direct expansion versus other systems are less required space, its avail-
ability in a variety of small capacities, and simpler controls.
The main sources of energy consumption in direct expansion systems are power
required for running the direct expansion system compressor and condenser fan,
power required for running the airside supply fan, and power required for running
terminal unit fans (if any) (this will be discussed in more detail when I discuss the
airside systems later in this chapter). The small energy consumed by low voltage
power provided for control valves is another source of energy consumption.
