Multi-Evaporator (Mini) Split System

Multi-Evaporator (Mini) Split System Points : multi-evaporator (mini) split system, parts of pump, circulating pumps, centrifugal pumps, reciprocating pump, heat exchanger, compression tank In some installation we use one compressor serves more than two evaporators, an example three different rooms and air conditioning for comfort in an office. Each coil of multiple evaporator normally requires its own metering device frequently, each coil will also be controlled by a solenoid shut off valve the diagram shown below shows a typical thermostatic expansion valve under similar load conditions while each of these coils act more or less as an independent entity and the expansion valves are installed in a manner similar to that of a single coil, special care must be taken to insure that each valve will be sensitive to the condition of the coil it controls and will not be effected by the other coil since suction lines have relatively little pressure drop, these connections are generally made at the most convenient spot on the suction line after the metering device bulbs. Pump It is a circulating part, which circulate gas or liquid. Parts of Pump 1. Nozzle
2. Runner and Buckets
3. Casing
4. Braking jet
5. Suction pipe
6. Delivery Pipe Circulating Pumps Circulating pumps from an important part of any commercial refrigeration system. Pumps for this type of service may be classified according to service requirements and use as
1. Rotary
2. Centrifugal
3. Reciprocating 1. Rotary Pumps Rotary pumps are usually employed for pressure lubrication service on ammonia compressors. Then deliver a continuous liquid flow like the centrifugal type, but operate at much slower speeds. Rotary pump should be operated within the assigned speed limits to avoid rapid ware and depreciation.
Rotary pumps sometimes called gear pumps or positive displacement pump, depends for their operation on the principle that a rotating plunger, impeller, gear or cam traps the liquid in the suction side of the pump easing and forces it to the discharge side. 2. Centrifugal Pumps A centrifugal pump is a pump in which the pressure is developed principally by the action of centrifugal force. Such pumps are commonly used where large quantities of water or brine are to be delivered at moderate pressure.
The rotating member of a centrifugal pump gives a rapid rotary motion to a mass of water contained in a surrounding case; the centrifugal force forces the water out of the case through the discharge outlet. The vacuum thus created, makes available atmospheric pressure to force in more water through the center. The process continues as long as motion is given to the rotor and there is a supply of water to draw upon. From this, a centrifugal pump may be define as one in which vanes as impellers, rotating inside a close fitting casing, and draw, in ‘the liquid at the center and by virtue of centrifugal force, throw out the liquid through an opening at the periphery of the casing.
Centrifugal pumps operate at high speeds and may therefore be connected directly to electric motors, where noiseless operation is desired and little space is available, the centrifugal pump is usually the most logic choice. Reciprocating Pump This type of pump may be gear driven by electric motor or other prime mover. They may be single acting, single- double acting, duplex double acting, triplex single acting and triplex double acting. The first two types should not be geared direct on an electric motor. Reciprocating pumps are generally employed where the amount of liquid (water or brine) is moderate and where the delivery pressure is high. They should be connected to secure a full and uniform supply and where the suction lift seldom exceeds 22 feet. Heat Exchanger Although sometime a subject of controversy in high-temperature application, it is generally agreed that in medium and low temperature refrigeration system, heat exchangers (when properly applied) perform the following function of most important to overall system performance.
1. Sub-cooling the liquid refrigerant entering the thermal expansion valve reduces the flash gas load at the evaporator inlet. It like wise increases the heat content difference of the refrigerant during its evaporating phase, which produces more useful work in the evaporator.
2. In the process of sub-coding, the heat extracted from the liquid refrigerant is transferred to the suction gas, thereby insuring a dry suction return to the compressor at an entering superheat level. This will produce the best possible volumetric efficiencies for the refrigerant used.
3. The increase in suction line temperature will likewise reduce the possibility of sweating.
4. The use of a heat exchanger will permit more open adjustment of the thermal expansion valve without, the risk of serious flood back of liquid to the compressor under light or variable load conditions, and at the same time assuring the maximum utilization of evaporator surface.

The advantage of a heat exchanger may be cancelled out if its use increases the suction line pressure drop by an excessive amount on a theoretical heat exchanger without pressure drop, designed to superheat the suction gas to 65 F from a normal 20 F suction temperature. It is entirely possible to obtain as high as a 16 percent increase in the overall efficiency of the machine as compared to a system without a heat exchanger. There are many factors involved, however, to obtain such a high and for practical purposes, it may readily be said that the overall efficiency of a system may increased 8 to 10 percent by the proper adaptation of a heat exchanger. Compression Tank Water expands when its temperature increases, unless it is restricted. In. a hydronic system an allowance must be made for this. If the piping system is completely filled and there is no space for the water to expand, the piping or equipment might break. An open expansion tank can be provided at the highest point in the system ‘to solve this problem. As the water temperature increases, the total volume of water in the system increases, the effect being a rise in the water level in the tank (Figure 1) because the tank is open to the atmosphere, however the system has some of the defects of open hydronic system. Particularly undesirable is the continual exposure to air and its possible corrosive effect.

A much better solution is to use a closed expansion tank containing a gas (air or nitrogen) when the water expands, it partially fills the tank, compressing the gas, for this reason the closed expansion tank is usually called a compression tank. The compression tank serves an additional purpose beyond that of providing for the water expansion it aids in controlling system pressure. For these reasons compression tanks have largely replaced open expansion tank in hydronic system.