Dometic 39726.506 General Information, Basic AIR Conditioning Cycle, Function of a Duct System

INSTALLATION INSTRUCTIONS

2.0.0 APPLICATION MANUAL

2.1 AIR DISTRIBUTION

2.1.1GENERAL INFORMATION

The purpose of an air conditioning system is to provide environmental conditions in a space to keep its occupants comfortable.

The basic elements of a simple forced circulation air system consists of a cooling unit, a centrifugal blower, a temperature sensing device controlling operation of the compressor and blower, suitable air filters, and a duct system.

Air is filtered, cooled, and distributed to various areas of the vehicle. Duct work should deliver this conditioned air as directly, quietly, and economically as possible. If the distribution is not properly sized and balanced, flow of air will not be as calculated and the system will not function properly or efficiently.

2.1.2THE BASIC AIR CONDITIONING CYCLE

The components which are basic to air conditioning systems are illustrated by Figure 2.1. Air is treated at the conditioning unit, transferred to the conditioned space through the supply duct system and returned to the conditioning unit through the return system. The duct systems are also referred to as the distribution system.

2.1.3 FUNCTION OF A DUCT SYSTEM

A duct is a tube, or pipe, that carries air between two points. Strictly speaking, in air conditioning terms, a duct system is the arrangement of ducts between air conditioning equip- ment and rooms to be cooled, not including such items as filters, cooling coils, etc. However, we shall include in our use of the term “duct system”, every item in the air-passage network that offers resistance to air flow. From the standpoint of the blower it makes no difference whether a resistance is caused by filters or by the use of a small duct; effect will be the same. Resistance tends to restrict flow of air through the entire system.

A forced air system is only as good as its air delivery system. Comfort levels are affected by the quantity and velocity of air movement within the space and the proper mixing of the supply air with the room air. Supply air should be furnished in a manner that will direct the air to the sources

CONDITIONED	RETURN DUCT
SPACE	SYSTEM
SUPPLY DUCT	CONDITIONING
SYSTEM	UNIT

Block diagram of comfort air conditioning cycle.

Arrows indicate direction of air flow.

FIG. 2.1

of the greatest heat loss and/or heat gain. The effects of the gain or loss can then be offset by the supply air. It is important that the selection of the diffusers and grills for the supply and return systems receive careful attention to enable them to accomplish their purpose.

Consideration must be given to all aspects of the supply air distribution patterns: throw, spread, drop, etc. Also, the outlet and return grille velocities must be held within reason- able limits. Any noise generated at the grille is equal to or greater in importance than duct noise.

A window air conditioner (Figure 2.2) is essentially a device with a minimum duct system, since the only items that offer resistance to air flow are built into the cabinet.

2.1.4FACTORS AFFECTING RESISTANCE TO AIR FLOW

Common observations of daily events tell us something about factors affecting resistance to air flow. We know, for example, that more pressure is required to force a given rate of air flow through a small duct than through a large duct. We also know that streamlining of ducts means less resistance to air flow, and that sharp angled turns must be avoided.

As air passes through ducts, cooling coils, grilles, diffusers, and dampers, the static pressure is reduced by friction and turbulence losses. Good duct design minimizes the need to balance the duct system by sizing the ducts such that the designed pressure drop allows the desired airflow rate to be delivered to each room. An improperly sized duct system will require extensive balancing. Balancing is a procedure by which the air flow allotment is adjusted to supply the correct quantity of conditioned air to each room.

COOL AIR DISCHARGE	HEATEDAIRFROM
	CONDENSERDISCHARGED
INTOROOM
	OUTDOORS

	EVAP	COMPRESSOR,
ROOM		CONDENSER
	AND		OUTDOORS
		AND FAN
	FAN

OUTDOOR AIR TO

CONDENSER

WARM ROOM AIR

TO BE COOLED

FIG. 2.2

The Window Air Conditioner represents a minimum duct sys- tem, since air to and from the cooler is handled without external ducts.

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