Direct Expansion Evaporator Coil with Ejector Capacity Boost for Low Liquid Temperature and Economized Systems

Description

FIELD OF THE INVENTION

This invention relates to low liquid temperature and economized refrigeration systems.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 11,493,245 B2 discloses a DX refrigeration system that provides a capacity boost with a vapor ejector (vapor only) that uses flash gas produced after the expansion valve to drive recirculation of unevaporated liquid from the suction header.

SUMMARY OF THE INVENTION

An apparatus and method is presented for improving the performance of a direct expansion refrigeration system, the apparatus having

• • an inlet separator adapted to be connected to an expansion device outlet of a direct expansion refrigeration system,
  • the inlet separator having a liquid refrigerant outlet and a two-phase refrigerant outlet, an evaporator having an evaporator inlet directly connected to the inlet separator liquid outlet,
  • an ejector having a motive flow inlet directly connected to the inlet separator two-phase refrigerant outlet,
  • an evaporator outlet refrigerant line connected at a first end to an outlet of the evaporator, the evaporator outlet refrigerant line bifurcating into an evaporator outlet liquid refrigerant line and an evaporator outlet vapor refrigerant line,
  • the evaporator outlet liquid refrigerant line connected to a side port of the ejector, the evaporator outlet vapor refrigerant line connected to a compressor,
  • the inlet separator configured to simultaneously and continuously deliver two-phase refrigerant to the ejector and refrigerant liquid to the evaporator.

According to alternative embodiments, the inlet separator may be dispensed with, and the two-phase refrigerant sent directly to the ejector. In other embodiments, the inlet separator may send liquid refrigerant to the ejector and vapor refrigerant to the evaporator inlet.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a representation of an economized direct expansion (DX) refrigeration system.

FIG. 2 is a representation of an economized direct expansion evaporator with two-phase ejector capacity boost according to an embodiment of the invention.

FIG. 3 is a representation of an economized direct expansion evaporator with two-phase ejector capacity boost according to another embodiment of the invention.

FIG. 4 is a representation of an economized direct expansion evaporator with two-phase ejector capacity boost according to another embodiment of the invention.

Features in the attached drawings are numbered with the following reference numerals:

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an economized direct expansion (DX) refrigeration system. High pressure, high temperature liquid from high pressure receiver enters a subcooler that reduces the temperature of the liquid refrigerant which then enters the evaporator through a thermostatic expansion valve and a distributor. The thermostatic expansion valve regulates (opens or closes) based on the superheat of the outlet vapor with the goal of generating superheated vapor (superheat ≥6° F.) to ensure dry suction for the compressor. However, this is not the case in practice, as unevaporated liquid tends to escape the evaporator resulting in reduction in superheat and closing of the thermostatic expansion valve to reduce the refrigerant flow rate. This reduces refrigeration capacity.

An economized DX system which uses a distributor to distribute liquid to all circuits of the evaporator is also sensitive to maldistributions. Non-uniform distribution results in excess liquid flowing out of some circuit outlets, which will reduce superheat below target. This causes the thermostatic expansion valve to close reducing the amount of liquid refrigerant entering the evaporator, causing an increase in the superheat back to target at the cost of reduced capacity.

Economized systems are used to reduce compressor work and improve overall system efficiency, due to reduction in mass flow of flash gas to be compressed from low evaporator outlet pressure (e.g. freezers). To address lower liquid temperature feed, the present invention delivers both refrigerant phases to the ejector instead of only saturated vapor or only subcooled liquid. As a result, the minimum requirement for liquid temperature can be dropped significantly, making it suitable for economized systems.

The invention features delivery of two-phase or saturated liquid refrigerant to the ejector after being throttled through a motorized expansion valve. For the purposes of this invention, x is the quality (vapor mass/total mass) of a refrigerant. By definition, then, the quality of saturated liquid is X=0 since the vapor mass is zero. The present invention is suitable where 0.95≥x≥0, preferably 0.75≥x≥0 and more preferably 0.5≥x≥0, saturated liquid being x=0, two-phase being x>0, and saturated vapor is x=1. The upper bound for x according to the invention may be about 0.95, which provides good flexibility for the current application.

The two-phase ejector of the present invention generates low pressure and entrains adequate liquid flow from the suction header to produce a recirculating flow that increases the heat transfer capacity of the evaporator. Use of a nozzle with a large vapor velocity/liquid velocity ratio will generate annular/mist flow through the ejector sufficient to manage two phases. The nozzle is configured to provide time for flashing so the vapor quality increases through the nozzle and can achieve the minimum pressure and accelerate the velocity in the range of 250 ft/s to 1000 ft/s depending on the refrigerant type and conditions.

FIG. 2 shows the portion of an economized DX refrigeration system of the invention which replaces the portion of a prior art economized DX refrigeration system that is enclosed in dashed lines in FIG. 1. Referring to FIG. 2, refrigerant liquid is delivered to expansion device 3. Outlet 5 of the expansion device 3 is connected via refrigerant line 7 to the inlet 9 of a vapor-liquid separator 11 (also referred to herein as inlet separator), which sends two-phase refrigerant received from the expansion device to inlet 31 of ejector 33, while liquid refrigerant is sent to the inlet 17 of distributor 19 via refrigerant line 16. Distributor outlets 21 are connected to the evaporator coil 25 via refrigerant line 26 for delivery of refrigerant liquid to the evaporator coil 25. While an evaporator coil is used as an example herein, any type of evaporator may be used in connection with the invention. Outlet 27 of the evaporator coil 25 produces both superheated vapor and unevaporated liquid. The superheated vapor is sent to the suction trap and/or compressor via refrigerant line 29, and the unevaporated liquid is sent to the side port 35 of the ejector 33 via refrigerant line 30. Sensor 100 measures the temperature and pressure of the superheated vapor and sends it to controller 102 to determine whether superheat has been achieved. Controller 102 causes the expansion device to open or close depending on the superheat determination.

Meanwhile, ejector 33 uses two-phase refrigerant L′+V received from the outlet 13 of inlet separator 11 as the motive flow for the unevaporated liquid L1, and the outlet 37 of the ejector 33 delivers the entrained refrigerant to the distributor 19.

While the inlet separator and the ejector are shown in FIG. 2 and described above as constituting separate structure elements, they may be optionally combined into an integrated refrigerant recycling device which carries out the functions of both devices.

According to an alternative embodiment of the invention, represented in FIG. 3, the two phase refrigerant leaving the expansion valve may be sent directly to the ejector, dispensing with the inlet separator.

According to a further alternative embodiment represented in FIG. 4, saturated liquid (x=0) from an inlet separator may be used as the motive flow for the ejector. After the throttling process through the motorized expansion valve, as in a standard economized refrigeration cycle, the two-phase mixture of liquid and vapor enters the inlet separator at pressure P1 as shown in FIG. 4. The saturated liquid (L) from the separator enters the ejector at inlet pressure of P1. The small amount of vapor (V) from the inlet separator is routed to the side port of the distributor through a fixed regulating orifice that can be calculated for a given application along with the size of the ejector. This ejector can be the same two-phase ejector described above even though saturated liquid (x=0 as referenced above) is used as motive, as even saturated liquid inlet will result in flashing and generate a two-phase mixture as it moves through the ejector. Hence, the nozzle design needs to handle two-phase flow as described above.

According to all embodiments described herein, the ejector accelerates the velocity (drop in enthalpy=increase in kinetic energy) resulting in a low pressure P_min, which is indicated by the location of the side port showing liquid L1. P_min is designed to be below suction pressure Ps (pressure in the suction header). As a result, refrigerant fluid (mostly liquid) is entrained from the bottom of the suction header to the side port of the ejector shown by L1. The sum L′+L1+V enters the distributor which is then evenly distributed to the various circuits of the evaporator. Note that the standard distributor nozzle or orifice is removed for the embodiments in FIGS. 3 and 4 so as to not cause additional back pressure to the ejector. The velocity of the refrigerant mixture at the exit of the ejector is sufficient to evenly distribute the flow rate. While only one distributor is shown in the Figures, multiple distributors may be used, which can be the case for high cooling capacity coils.

As can be seen, all the excess liquid flow L1 from the coil is continuously recirculated and only refrigerant vapor in a superheated state (like a Direct expansion evaporator) leaves the evaporator coil. The degree of super heat can be about 3° F. or less, while in a conventional DX it is >6° F. If conventional DX evaporator is operated below 6° F., there is liquid carryover to the suction, which is undesirable. This is the benefit of the ejector recirculation method, since it actively removes any unevaporated liquid/liquid carry over from the suction header and increased wetting inside the coil tube, which results in cooling capacity boost. It is also a regenerative method since the ejector is powered by entering enthalpy of the system and requires no additional energy. When used with economized systems, there is a double benefit, as enabled by this invention. The recirculated liquid can increase the cooling capacity of the coil significantly up to 38% over a traditional DX evaporator and the economizing can further increase system efficiency.

Similar to a conventional DX coil, superheat is measured at the outlet of the coil on the suction connection (as shown in figures) that regulates the opening of the motorized expansion valve to target a specified super heat of 2° F. to 3° F.

The ejector is capable of using two-phase refrigerant flow to produce minimum pressures less than suction and achieve a recirculation flow. Design of the two-phase ejector of the present invention is based on inlet pressure P1, motive mass flow rate fraction, quality of motive and back pressure P2. P1 and P2 are shown in FIGS. 1 and 2.

Notwithstanding the specific embodiments, features, elements, combinations and sub-combinations disclosed herein, it is expressly considered and here disclosed that every single element, every single feature, and every combination and sub-combination thereof disclosed herein may be combined with every other element, feature, combination and sub-combination disclosed herein.

It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as outlined in the present disclosure and defined according to the broadest reasonable reading of the claims that follow, read in light of the present specification.