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Integrated energy recovery systems

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20120292008 patent thumbnailZoom

Integrated energy recovery systems


An energy efficient laundry facility comprises a space heating system, an air conditioning system, a plurality of clothes washers, a plurality of clothes dryers, a hot water storage tank supplying the washers, a source of cold water and conduit means interconnecting the aforesaid. Dedicated heat recovery units are associated respectively with the dryers and employ high temperature dryer exhaust air to heat water for supply to the storage tank. A controller and temperature sensing means regulate the flow of hot water from the dryers to the storage tank. High temperature water is directed to the top of the storage tank and lower temperature water to an intermediate point in the storage tank. Hot water from the dryers is also directed to the space heating system. Each heat recovery unit comprises a heat exchanger, a blower, a water circulation pump, a water temperature sensor, and a dedicated controller. The controller may also include a dryer exhaust pressure monitor, a pressure controller, and a variable speed blower drive. The heat exchanger is of the counterflow type and each recovery unit has its heat exchanger and blower arranged to provide for right angle air flow resulting in a compact unit for mounting atop a clothes dryer. Hot water may also be drawn from the storage tank for use in either an air or hydronic space heating system.

Inventor: Michael Goldberg
USPTO Applicaton #: #20120292008 - Class: 165287 (USPTO) - 11/22/12 - Class 165 
Heat Exchange > With Timer, Programmer, Time Delay, Or Condition Responsive Control >Temperature Responsive Or Control

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The Patent Description & Claims data below is from USPTO Patent Application 20120292008, Integrated energy recovery systems.

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REFERENCE TO RELATED APPLICATION

Reference is had to Provisional Patent Application entitled Integrated Energy Recovery Systems, inventor Michael Goldberg, filed Jun. 18, 2009, hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

Energy cost and availability are currently forefront issues well known to all. Research and investment are focused intensely on many facets of this dilemma, and much is being accomplished. In many industries, energy recovery is a key area of development. Typically, unused process heat that has historically been vented to the atmosphere or discharged into waterways, is now being captured. The captured heat may be used to preheat a target process and reduce energy consumption, or it may be used as the source heat for other processes in the same facility.

While heat recovery is an elegant way to reduce energy consumption, it can only work where the waste heat is discharged at sufficient temperature to be useful. It is also essential that the beneficiary process consume energy at least substantially equal to the available waste heat. For example, supermarkets commonly capture the heat from their chillers and central A/C systems and use it to preheat domestic hot water. However, their domestic hot water consumption is low, and only a small portion of the A/C waste heat is recovered. This is unfortunately often the case.

The laundry industry, however, is unique. The key processes that discharge and employ waste heat occur at temperatures that are compatible with each other, and laundries consume large quantities of water. This presents a unique opportunity to recover a relatively high level of waste heat, materially reduce carbon load, and provide significant financial benefits to the laundry owner. Laundries are extremely energy intensive, so much so that utility costs present an imminent threat to the industry\'s survival. Despite the urgency of its need, the laundry industry has largely fallen under the research radar, and little energy efficiency innovation has occurred to date. Mainstream laundry equipment manufacturers have recently begun to recognize the issue, and have engaged in extensive promotion of ostensibly energy efficient products. However, the improvements offered by these products are incremental and do not offer tangible relief to the industry. This profound stress on the laundry industry, and the nearly total solution vacuum, presents an urgent need for innovation.

The general object of the present invention is to provide an improved heat recovery system particularly adapted for use with clothes dryers and washers in commercial laundries but which may also have broad application in other environments.

SUMMARY

OF THE INVENTION

In accordance with the present invention and in fulfillment of the foregoing object, the improved heat recovery system takes advantage of two key factors; compatible energy consumption and temperature in the separate processes, and very high water consumption. One function of the disclosed systems is recovery of waste heat from the dryer exhaust. This recovered heat is preferably used for hot water production, and may also be used for heating dryer makeup air and/or for facility space heat. In the event the facility Air Conditioning is water cooled, additional waste heat may be recovered, and Air Conditioning electrical consumption may be materially reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration in block diagram form of a conventional commercial Laundromat including a plurality of washers, a plurality of dryers, a boiler and hot water storage tank feeding the washers, a space heating system, and an air conditioning system,

FIG. 2 is a schematic illustration in block diagram form showing the-Laundromat of FIG. 1 equipped with an improved heat recovery system of the present invention,

FIG. 3 is a schematic illustration of a heat recovery unit mounted atop a commercial stacked dryer assembly comprising upper and lower dyer units,

FIG. 4 is schematic illustration of a heat recovery unit on a dryer assembly similar to FIG. 3 but from a different angle,

FIG. 5 is an enlarged fragmentary schematic illustration of a heat recovery unit constructed in accordance with the present invention,

FIG. 6 is an enlarged fragmentary schematic illustration of a heat recovery unit similar to FIG. 5 but with arrows indicating the direction of air flow through the unit,

FIG. 7 is a schematic diagram illustrating the controls for the heat recovery units,

FIG. 8 is a schematic diagram of a “straight through” configuration of the heat exchanger and blower in a recovery unit,

FIG. 9 is a schematic illustration of an overall system which might be installed in a laundry and is similar to FIGS. 1 and 2 but includes additional conduit means,

FIG. 10 is a fragmentary schematic view showing additional conduit means connecting a plurality of dryers to a number of different headers, and

FIG. 11 is a schematic illustration illustrating temperature gradients in a storage tank.

DESCRIPTION OF PREFERRED EMBODIMENTS

In a typical commercial laundry of the vended type as shown in FIG. 1, the key energy consuming processes are the dryers 10,10, washers 20,20 hot water boiler 12, the facility space heating system 14, and the facility air conditioning system 16. Each of these processes consumes significant energy, and discharges waste heat to the atmosphere and/or down the drain. Laundries generally employ a boiler that consumes fuel, typically natural gas or oil, and heats water, which is then stored in a separate tank as at 18. The washers 20, 20 draw on the tank as needed, and when each cycle is completed, the water is discharged, still substantially hot, down the drain. The dryers 10,10 draw ambient air, and consume substantial energy to heat the air to very high temperatures, ranging from 325 F. for 30 lb. dryers, to 550 F. for larger machines. This energy is most commonly supplied as natural gas, although propane and oil fired dryers, as well as electric and steam heated dryers are also in use. Drying textiles is an adiabatic process, and except for heat loss directly through the housing walls, substantially all of the energy consumed is discharged out of the building via the dryer vent.

The building heating system 14 consumes energy as fuel, typically natural gas or oil, to heat the interior space. Furnace efficiency is typically about 80%, and approximately 20% of the heat is discharged to the atmosphere via the flue. Modern condensing furnaces are available with efficiencies as high as 97%, but these are uncommon in vended laundries. Furnace efficiency notwithstanding, energy consumed for space heat is substantial, and presents a significant corresponding cost burden to laundry facilities.

Air conditioning 16 consumes electrical energy to drive the refrigerant compressor and ancillary components. Air conditioning typically pumps heat from the air-conditioned space to an outdoor condenser coil, which is cooled by ambient outdoor air. All of the energy removed from the conditioned space, plus all of the energy consumed, is discharged to the atmosphere via the outdoor condenser.

A typical conventional hot water boiler and storage tank system operates as follows: An Aquastat controls the temperature of the water in the tank. The Boiler includes an internal fuel burner, which is automatically controlled by an internal thermostat (not shown). As washers draw hot water from the top of the storage tank 18, cold makeup water is introduced at the bottom of the tank as from a city water supply. The influx of cold water gradually lowers the water temperature in the tank. When the Aquastat calls for heat, a circulator pump operates causing water to flow between the tank and the boiler. The boiler internal thermostat automatically cycles the burner on and off as needed to maintain a desired discharge water temperature. When the tank Aquastat is satisfied, the pump stops, and the burner subsequently shuts down.

Referring to FIG. 2, storage tank 18 includes a separate recovery circulation loop 21 that communicates with a heat recovery unit 22 at each dryer. Most laundry venues have multiple dryers, in which case each dryer preferably has its own individual heat recovery unit 22. In this embodiment, the heat recovery units are connected to common manifolds or headers 23 a,b,c (FIGS. 10, 11) which communicates with the recovery circulation loop. Dryers are typically installed in banks, with their exhaust ducts coupled in common to overhead manifolds or headers such as 23 a,b,c. In an alternative embodiment, a recovery unit 22 may be located at the common exhaust point of a bank of dryers. This embodiment, while potentially simpler, will typically deliver lower temperature water, as the air stream comprises exhaust air from a plurality of dryers at different points in their drying cycle, and thus at different temperatures.

As shown in FIGS. 3, 4, 5, and 6, in a system with a plurality of dryers and dedicated individual heat recovery units 22-22, each heat recovery unit comprises a housing 24, which is preferably mounted on top of its respective dryer 10. The housing 24 encloses a blower 27 and an air to liquid heat exchanger 28, preferably of the tube and fin type commonly found in conventional air conditioning equipment. Dryer exhaust air passes through the blower and heat exchanger, where substantial heat is transferred to the water circulating through the recovery loop. The dryer exhaust air, now at lower temperature, then exits the recovery unit at 30 and leaves the building via conventional ductwork.

In operation, heat collected from recovery units via the recovery circulation loop 20, heats the water in the storage tank 18. The heat collected from one dryer typically exceeds the heat required to supply hot water to a corresponding washer. In normal operation therefore, the aggregate heat recovered is at least sufficient or more than sufficient to maintain the temperature of the tank at the desired level, and the boiler circulator rarely or never operates. Correspondingly, the boiler burner rarely or never operates, and substantial fuel is not consumed.

Drying Thermodynamics

In normal operation, drying occurs in three stages, rising rate or warm-up phase, steady rate phase, and falling rate phase. During the warm-up phase, the dryer exhaust air temperature starts out relatively cool, near room ambient temperature, and slowly rises until it reaches the steady rate phase. During steady rate, the dryer exhaust temperature is relatively constant. In most vended commercial dryers this temperature is in the range of 160° F. to 180° F. at the highest heat setting. During this phase, the dryer exhaust heat content is at its maximum. Falling rate phase commences when the residual moisture remaining in the fabric is sufficiently low to limit capillary action that brings moisture to the surface, and the drying rate is correspondingly constrained. This causes the internal dryer controls to throttle or cycle the dryer burner (or other heat source) in order to maintain constant exhaust temperature. As a result, the heat output of the dryer is reduced.

Controls

It is desirable that the temperature of the water discharged from the recovery heat exchangers be relatively constant, and the same or higher than the desired wash water temperature. To ensure constant recovery water temperature over a wide range of dryer exhaust temperatures during the drying cycle, a novel closed loop temperature control algorithm is employed. A variable speed circulator comprising water pump 31 and variable speed drive 31a, a temperature sensor 33 that monitors discharge water temperature, and a closed loop control 35 delivers a proportional output to control pump speed FIG. 8. The control is preferably a Proportional, Integral, Derivative (PID) type. When the recovery water temperature is below the target set point, the pump is effectively cut off and no water flows. As this temperature rises, and approaches the set point, the controller starts the pump at low speed, and varies the pump speed as needed to maintain constant water temperature. During warm-up, and falling rate phases, the pump speed is relatively slow. During the steady rate phase, when dryer heat output is at its highest, the pump speed is correspondingly at or near maximum.

In order to accurately monitor the temperature of the recovery discharge water, it is preferable that the temperature sensor be placed at a location immediately downstream of the heat exchanger discharge. While this presents the most accurate location for temperature measurement, it is only accurate when water is flowing. If there is no flow, this sensor simply measures the temperature of the standing water in the pipe, which over time will approach that of the ambient surroundings. The sensor is preferably insulated, but regardless will not provide accurate information unless there is some water flow. Water flow sufficient to provide accurate temperature sensing may be achieved by setting the temperature control device to provide a minimum pump speed signal that is slightly above zero, even when no flow is called for. The resulting water flow rate is preferably just enough to provide proper exposure to the temperature sensor, yet injects a negligible volume of low temperature water into the recovery loop. While this approach works well in principal, most commonly available circulation pumps, such as those used for hydronic heating systems, do not function well at very low speeds, and become very sensitive to output head pressure. When multiple dryers are connected to a common manifold, it is very difficult to reliably maintain very low flow rates.

In another embodiment, the temperature controller provides a true zero pump speed signal when no flow is called for, and instead generates a pulse train that jogs the pump at normal speed for a suitable brief period, at a suitable repetition rate. This ensures substantial flow and sensor exposure during the pulse dwell time, regardless of how many other pumps are operating, but in the aggregate, injects negligible low temperature water into the recovery loop. In practice, a pulse of approximately 1.5 seconds, at nominal 30 second intervals, has proven very effective. As the temperature of the water rises, the controller will call for increased pump speed, and the pulse train will become irrelevant. The control algorithm may be configured to stop the pulsing at this time, or it may simply continue. In practice it has negligible effect when the controller is calling for substantial water flow.

Dryers are extremely sensitive to vent back pressure, and will typically tolerate ductwork pressure drop of only 0.3″ H2O. It is therefore desirable that the dryer heat recovery unit not introduce additional pressure drop to the vent ductwork. This may be accomplished by an integral fan or blower 100 and automatic pressure control, FIG. 7, comprising dryer exhaust pressure sensor 101, Proportional, Integral, Derivative (PID) closed loop control 104, and variable spped drive 102. The exhaust pressure sensor may be remotely located, and couples to the dryer exhaust via pickup tube 103.

Still another embodiment comprises a very low pressure drop heat exchanger, with widely spaced fins or finless configuration.

Pump operation is desirably enabled only when the dryer is operating and the exhaust air temperature has reached a suitable set point. It is desirable to determine this state without the need for connecting to the internal dryer controls, or otherwise modifying the dryer. Thus, a signal is provided by the blower speed controller, such as a dry relay contact or the like, that indicates the blower is operating. This will reliably indicate when the dryer is operating, with no need to invade the dryer itself. Dryer exhaust air temperature may be determined by a temperature sensor 200 or thermostat at a suitable location within the recovery unit housing, upstream of the heat exchanger.

Latent Heat

When the dryer stops, substantial latent heat remains entrained in the heat exchanger. It is desirable to recover this heat rather than allow it to dissipate while the dryer is idle. To advantageously recover this latent heat, the control algorithm of the disclosed embodiment does not inhibit pump operation when the dryer shuts down. It instead continues to operate the pump, preferably under PID control, until the output water temperature falls below 120° F.

The ability to recover latent heat from the heat exchanger after the dryer stops is advantageous, but presents a potential runaway condition. Toward the end of a typical laundry business day, the tank will become saturated, and the temperature of the return water from the tank to the dryers will be higher than 120° F. If the pump remains under PID control while the dryer is idle, a plurality of pumps may continue running through the night until the tank return water falls below 120° F. This is detrimental causing excessive run time on the pumps, and depletes a portion of the stored energy in the tank.

Thus, provision may be made to prevent the pump from running continuously when the dryers are idle, even if the return water is at or above the set point. This may comprise a time delay function that allows the pump to operate during a dwell interval, sufficient to extract any latent heat remaining in the heat exchanger, and then turns the pump off, independently of the water temperature. The algorithm may thus comprise an AND logic function; and the pump will continue to operate when the dryer stops IF the water temperature is above 120° F. AND the dwell interval has NOT elapsed.

When the laundry is closed and the dryers are not operating, wind impinging on the outdoor vent discharge can occasionally trigger a dryer vent pressure sensor, and cause the blowers to run briefly. This in turn can cause a false dryer run state, and enable the pump. In this state, the pump will cycle at its nominal temperature sampling pulse rate. While occasional events are not serious, in moderately windy conditions, pump cycling can be sufficient to partially deplete the water tank stored heat.

In a preferred embodiment, the controller includes a dryer exhaust air temperature sensor 200 in the Heat Exchange unit intake area, and disables the pump if the dryer exhaust air is below a desired setpoint, such as 120 F.

In this embodiment, the time delay function discussed above is not required. The algorithm may thus comprise an AND logic function; and the pump will continue to operate when the dryer stops IF the water temperature is above 120 F AND the dryer exhaust air is above the desired setpoint.

Storage Tank Configuration

The hot water storage tank 18 is described herein as a single tank. This tank may, however, advantageously comprise a plurality of tanks piped together in parallel and acting as a single tank. This embodiment offers easier installation and flexibility of placement in venues with limited space or access.

Low Grade Waste Heat Recovery

While recovery of waste heat at high or even moderate temperatures is well known and relatively straightforward, recovery of very low grade heat requires careful design. In the laundry under consideration the source hot air temperature is close to the target water temperature, providing a small delta T to drive the requisite heat transfer. The process heat exchangers must operate effectively with low delta T (approach), and very low heat loss must be maintained throughout the process.

Heat Exchanger Circuiting

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stats Patent Info
Application #
US 20120292008 A1
Publish Date
11/22/2012
Document #
13068594
File Date
05/17/2011
USPTO Class
165287
Other USPTO Classes
International Class
28F27/00
Drawings
11



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