One set of fixed-frequency outdoor unit and one set of fixed-frequency indoor unit send cold and hot air to every room through air hose, and introduce outdoor fresh air conveniently; they could realize centralized treatment of the air such as humidification, etc.. These are reasonably-designed low-cost machines, and the noise increased by them in every room is only 1~3 dB. It is unnecessary to construct suspended ceiling in bedrooms, and the temperature higher than that set up by the main thermostat in every room could be controlled. A proportion of energy is allowed to transfer, in order to achieve the effects of saving energy and accelerating the cooling or heating of air conditioner.The indoor unit produces relatively big top noise, which is actually measured to be 42~52dB according to different installation conditions at the site, so the design and installation are required to be very professional.

Multi-split machine unit

(1)Fixed-frequency VRF unit

Split air conditioners are centralized in an outdoor unit, and in multi-split unit, there are three sets of compressors at most, and cooling medium systems are independent respectively; open-mounted wall-hanging indoor unit is changed into hidden type; fixed-frequency VRF is another form of split air conditioner, and has difficulty in introducing fresh air; the noise produced by fan in bedrooms is increased by 7~14 dB, and reaches up to 50dB at the most. Every bedroom shall be lengthened by more than 1.2m, shall have a suspended ceiling of 0.6m wide and 0.3m high, and shall have an access hole; in every indoor unit, there shall be a pipeline for drainage of condensate water.

The cooling medium system is independent, but has shared point at electric circuit part; in case of failures with external fan, temperature probe of outdoor unit, pressure protection, or local short circuit of electric appliances, etc., the whole machine won’t work.

(2)Fixed-frequency and frequency-conversion multi-split machine unit

There are 1~2 sets of frequency conversion compressors or another 1 set of fixed-frequency compressor, and in the circuits, there is radio frequency interference, which has impact on computers. It is the same as VRF unit in terms of access hole, introduction of fresh air, suspended ceiling and condensate water; the splitter of Fluorine pipe is required to design reasonably; the upper and lower layers share 1 set of machine, and the requirements on pipeline are even higher; when all units are started, the situation will easily occur that the effect of indoor unit at the tail end is too bad.

☆ Cold and hot water units

Fixed-frequency or frequency conversion cold and hot water units

It is a scaled-down version of large-scale central air conditioning unit, and the condenser is changed from water cooling to forced air cooling; and water pump is used to send cold / hot water to the fan coil. It is the same as VRF in terms of fresh air introduction, access hole, discharge of condensate water from suspended ceiling, and noise index. But meanwhile, it adds cold and hot water pipes; for very big temperature difference, the sealing problem is much highlighted, and leakage will severely destroy decoration. In addition, the evaporator of large-scale central air conditioning units is cleaned and scoured periodically; but there is no good countermeasure for household cold and hot water units, and after long-term use, the efficiency of cool-heat exchanger will be greatly discounted. If it is combined with central water treatment system, the abovementioned difficulties may be overcome.

In case air conditioner is used in a single room, cold/ hot water will flow through the fan coils of other rooms and produce energy consumption; now, it is popular to close water path with electromagnetic water valve; besides the factor of construction cost, locally over-high flowing speed of water will produce noises.

II. Operational principle of residential central air conditioning unit

1. The basic work process of cold (hot) water unit: Outdoor refrigerator unit refrigerates and cools down (or heats and warms up) the cold (hot) medium water, then the water pump conveys the cooled-down cold (hot) medium water to the indoor fan coil unit, and then, the fan coil unit conducts heat exchange with indoor air by means of local return air to realize the objective of processing indoor air.

2. The basic work process of wind pipe (pipeline) type unit: In case of cooling, outdoor refrigerator unit will absorb the refrigerant vapour from indoor unit, compress and condense it, and then deliver liquid refrigerant to each indoor unit. In case of heating, outdoor refrigerant unit will absorb the refrigerant vapour from condenser, compress it, and then deliver gaseous refrigerant to each indoor unit. The indoor unit will suck air from the return air opening arranged on the ceiling, carry out heat exchange, and then send it to the wind pipe (pipeline) installed in the ceiling of each room, and deliver air to the interior of each room through the air diffuser at the air outlet. Fresh air door and exhaust door are designed on wind pipe (pipeline), and they could exchange air as per certain proportion, in order to guarantee the quality of indoor air.

3. The basic work process of frequency conversion multi-split unit: The basic work process of wind pipe (pipeline) type unit: In case of cooling, outdoor refrigerator unit will absorb the refrigerant vapour from indoor unit, compress and condense it, and then deliver liquid refrigerant to each indoor unit. In case of heating, outdoor refrigerant unit will absorb the refrigerant vapour from condenser, compress it, and then deliver gaseous refrigerant to each indoor unit. Each indoor unit is mounted on ceiling in a hidden way. They will suck air from return air opening, carry out heat exchange, then supply the air processed from the air outlet to the indoor by means of local return air.