Refrigeration – Automatic control – Of external fluid reheating
Reexamination Certificate
2002-07-02
2003-12-23
Jiang, Chen-Wen (Department: 3744)
Refrigeration
Automatic control
Of external fluid reheating
C062S238600, C062S090000
Reexamination Certificate
active
06666040
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention pertains to the art of refrigerating and heating systems, and, specifically, heat pump systems that use a liquid source as a thermal reservoir.
Refrigerant-based liquid water source heat pumps condition air by extracting heat energy from the liquid source or reservoir and transferring it to the conditioned air stream, or, in the opposite fashion, by extracting energy from the conditioned air stream and transferring it to the liquid. The liquid reservoir may be a groundwater loop, a heat pump loop, a pond, or a river.
Most heat pumps are known in the art as three element systems. That is, they consist of one or more refrigerant compressors, an air side heat exchanger, and a water side heat exchanger. When the conditioned air stream requires cooling and/or dehumidification, the air side coil functions as an evaporator. Refrigerant liquid circulating through the evaporator boils and absorbs energy from the air stream. The refrigerant compressor pumps the hot, energy-laden refrigerant to the water side heat exchanger, which functions as a condenser. The refrigerant gives up its energy to the body of water, and the process repeats until the cooling needs of the air stream are satisfied.
When the conditioned air stream requires heating, the water side heat exchanger functions as an evaporator. Refrigerant liquid circulating through the evaporator boils and absorbs energy from the body of water. The refrigerant compressor pumps the hot, energy-laden refrigerant to the air side heat exchanger, which functions as a condenser. The refrigerant gives up its energy to the air stream, and the process repeats until the heating needs of the air stream are satisfied.
Additionally, four element systems are also known in the art. A four element system is similar to a three element system, but with an additional air side heat exchanger. The additional heat exchanger is located downstream from the first air side heat exchanger. Often called a reheat coil, this additional coil functions as a condenser or desuperheater when the heat pump operates in the air dehumidification mode.
Whether a water source heat pump is a three element or four element system, most such systems use at least one refrigerant reversing valve to switch the system from the air heating to the air cooling mode of operation. Such systems are known as reverse cycle systems, and are quite common in the air conditioning field.
However, reverse cycle systems have several attributes that can hinder their reliability and energy efficiency. First, the air side and water side coils, or heat exchangers, must be capable of handling bi-directional refrigerant flow. Because an individual coil must function alternately as an evaporator or as a condenser, its design is a compromise.
For example, consider a typical air side coil functioning as a condenser. The majority of the refrigerant passing through its tubes exists either as a superheated vapor or a low quality liquid/vapor mixture. This mixture must flow with a velocity sufficient to “sweep” refrigeration oil back to the refrigerant compressor to ensure proper lubrication. When the system reverses and this same coil functions as an evaporator, the pressure drop of the refrigerant in the coil becomes much higher. This happens because the majority of the refrigerant passing through its tubes now exists as a subcooled liquid or a high quality liquid/vapor mixture.
Unfortunately, high evaporator pressure reduces the cooling capacity of a heat pump because its refrigerant compressor must work harder to overcome the friction between the liquid refrigerant and the tube walls of the evaporator coil. Although one can design a coil to reduce its refrigerant pressure loss when it functions as an evaporator, this same coil may not function well as a condenser. Its refrigerant velocity may then be insufficient to sweep lubricating oil back to the refrigerant compressor. In addition, refrigerant at low flow velocity tends to exhibit laminar rather than turbulent flow. This reduces its heat transfer capability. Finally, refrigeration oil tends to coat the inner walls of the coil, acting as a thermal insulator and further reducing heat transfer capability. High refrigerant velocities help “scrub” the coating of oil from the tube walls.
A second disadvantage of reverse cycle systems is that, like the coils, the internal refrigeration piping is the result of design compromises. Engineers select piping, valves, and refrigeration components that are small enough to minimize their cost yet large enough to prevent excessive refrigerant pressure losses. Pipes and components that handle refrigerant vapor are generally larger than those that handle only liquid. However, in a reverse cycle system, engineers must usually size components in a manner that they can conduct both liquid and vaporized refrigerant. This becomes even more difficult when a refrigeration system is subject to unloading, where it is made to operate at a reduced capacity to match a partial heating or cooling load.
Furthermore, refrigerant compressors can be damaged in traditional reverse cycle heat pumps when the system shifts from the air heating to the air cooling mode or vice-versa. This happens when a condenser suddenly becomes an evaporator, and the liquid refrigerant that collected in its final circuits is abruptly sucked into the crankcase of the refrigerant compressor. This liquid, which can be an effective solvent, displaces oil in the bearings of the refrigerant compressor, which could seize or damage the bearings. To prevent refrigerant compressor damage, most reverse cycle heat pumps are equipped with suction accumulators, large tanks designed to safely contain the slug of liquid refrigerant that occurs during system shifts.
Not only can the refrigerant compressors be damaged when the system shifts between a heating mode and a cooling mode, but the piping may be damaged as well. This happens when an evaporator suddenly becomes a condenser, and the liquid refrigerant that collected in its initial circuits is abruptly hit with hot discharge vapor from the refrigerant compressor. This causes violent expansion as a portion of the liquid refrigerant flashes into a vapor. In extreme cases, refrigerant piping may become fatigued or even rupture due to the force unleashed by this process.
SUMMARY OF THE INVENTION
The present invention presents a novel, non-reversible refrigerating and heating system that minimizes the disadvantages of the prior art while also having several advantages over the prior art. First, because it is not a reverse cycle system, it does not have the same risk of piping damage or refrigerant compressor bearing seizure when the system shifts from an air heating to an air cooling mode, or vice-versa. This invention does not require some of the specialized components that many reverse cycle systems use, such as suction accumulators, reversing valves, or bi-directional refrigerant filters.
Also, this invention operates more efficiently than existing art because its heat exchangers can be optimized for their intended function. For example, the air side evaporator coil of this invention can be designed specifically for high moisture removal without performance degradations caused by reverse-flow considerations. The reheat condenser can function efficiently during both summer and winter heating operations because it is designed and functions solely as a reheat condenser.
Moreover, the novel series arrangement of the water side heat exchangers permits more efficient heat extraction during air heating modes of operation. Because the water condenser is the upstream water side heat exchanger, it preheats the incoming water with any excess energy not required by the reheat coil. Preheating the water enables the downstream water evaporator to more efficiently absorb energy from that water. This is true because warmer water permits the refrigerant compressor to operate at a higher evaporating pressure, which increases the energy efficiency of the refrigerant compressor.
An additional bene
Groenewold Peter
Hall, Jr. Danny
Desert Aire Corp.
Jiang Chen-Wen
Ryan Kromholz & Manion S.C.
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