- Direct-contact heat exchangers for highly efficient HVAC systems
- Variable-Conductance-Heat-Pipe (VCHP) heat exchangers that passively maintain outlet temperature in a small range for
widely varying inlet temperatures and flow rates
- Innovative heat exchanger designs for vapor compression systems with thermal storage to accommodate varying thermal loads
These programs utilize different thermal management technologies and demonstrate the breadth of heat exchanger technical expertise at ACT. A brief synopsis of each of these advanced heat exchanger projects follows.
Direct Contact Heat Exchangers for Highly Efficient HVAC Systems
ACT is developing a Vortical Direct-contact Heat Exchanger (VDHX) for higher efficiency, lower mass HVAC systems. The VDHX is a modification of the momentum-driven vortex phase separator currently under development at ACT for microgravity applications
The VDHX is shown in Figure 1. During operation, air enters through the inlet volute, which centripetally accelerates the air flow and forms a high speed, forced air vortex. In addition to the induced vortical motion, the air flow is directed in the axial direction and moves from the inlet volute into the mixing chamber. Chilled water is introduced in the mixing chamber as droplets generated by the spray channels. The spray channels are oriented such that the droplets enter the spray chamber in cross-flow with the air vortex. During their transit through the spray chamber, the droplets exchange thermal energy with the air stream by direct contact. The length of the spray chamber is designed such that the air reaches thermal equilibrium with the water before exiting this section. This results in significant cooling of the air and slight warming of the water.
Much like a conventional cooling coil system, condensation occurs if the air is cooled below the dew point. However, in a VDHX, condensation occurs at the droplet surfaces rather than on copper fins. In either case, once condensation occurs, the outlet air will achieve almost 100 % Relative Humidity (RH). This air-water mixture, which maintains a strong vortical motion, then flows from the spray chamber into the separation chamber. As the water-air mixture travels through the separation chamber, the centrifugal acceleration field developed within the vortex separates water, including condensate, from the air stream. The centrifugal acceleration experienced by water droplets within the VDHX is over 100 times greater than gravity. As a result, droplet transit occurs within tenths of a second, rather than the tens of seconds typical of a conventional direct contact heat exchanger. This allows the VDHX to minimize volume while maximizing throughput.
Together, these advantages provide an energy-efficient, low-maintenance HVAC heat exchanger with the following benefits compared to conventional finned-tube evaporators.
- Minimum possible temperature potential for heat transfer. This reduces the temperature lift required of and power consumed by the heat pump.
- Air conditioning by evaporative cooling when inlet conditions are appropriate. This allows the heat exchanger to provide cooling by latent heat exchange with the air. Operating in this mode will significantly reduce the heat load and power consumption of the heat pump.
- Freedom of material selection. High thermal conductivity materials are no longer necessary and noncorrosive, lightweight, recyclable, or low cost materials can drive the design instead.
- Continual recycling of the heat transfer surface. Particulate deposition, condensate build up, and biological growth, as well as the performance degradation associated with these, are eliminated.
- Filtration of submicron and larger particles. These particles are filtered by the water recirculation system, which greatly reduces the power consumption of the air recirculation system. Similar water filtration systems have been shown to remove 99% of particles greater than 0.5 microns in diameter, 96% of those 0.3 to 0.5 microns in diameter, and 86% of those smaller than 0.3 microns.
- Biological filtration. Combined with the intrinsic filtration of the water spray, low-power ultraviolet filtration of the water system allows removal of biologically active material from the recirculating air flow. Typical UV filtration systems can destroy 99% of bacterial growth with less than a minute of exposure.
A VDHX sized to provide 2 tons of air conditioning (7 kW) was fabricated and tested. A schematic of the test setup is shown in Figure 2, while Figure 3 is a view through the volute top during operation. As shown in Figure 4, the experimental system provided more than 2 tons of air conditioning.
These data were used to evaluate potential VDHX performance at over 70 locations across the United States. The results of this evaluation demonstrate the potential for significant benefits in performance and electrical usage when compared with a conventional system;
see Table 1.
VCHP Heat Exchanger for Passively Maintaining Outlet Temperatures in Chemical Reactors
The Navy is investigating hydrogen fuel cells powered by reformed naval logistic diesel fuel as a means of providing distributed ship service electrical power. Hydrogen fuel cell operation using diesel fuel requires a reforming process to remove sulfur and steam reform the diesel fuel into a hydrogen rich stream. The operating temperature of the reactors must be closely controlled to maintain their chemical equilibrium. Temperature control is made more difficult than typical reforming systems because changes in the fuel cell electrical load and the resulting changes in reactant flow rates occur more frequently and drastically. The fuel reforming system must maintain inlet and outlet temperatures within ±30°C despite a turndown ratio of 5:1 in reactant flow rate. A passive control scheme is needed to control the reactor temperatures within operational limits over all anticipated reactant flow rates.
ACT has developed a Variable Conductance Heat Pipe (VCHP) Heat Exchanger to provide a roughly constant temperature feed to a reactor, despite variations in flow rate and outlet temperature from the previous reactor. A schematic of the VCHP heat exchanger is shown in Figure 5. Heat from the gas stream is transferred by the VCHP to the coolant stream. The non-condensable gas in the VCHP is used to passively control the outlet temperature of the hydrogen. If the hydrogen temperature is too low, the non-condensable gas in the VCHP expands, which blocks more of the condenser, and reduces the heat transfer. Similarly, if the hydrogen temperature is too high, the noncondensable gas in the VCHP expands, which exposes more of the condenser, and increases the heat transfer.
The VCHP heat exchanger is shown in Figure 6. Hydrogen entering the system first passes through a pre-heater, where it is heated to the desired inlet temperature. The hydrogen then passes through the VCHP heat exchanger, where it is cooled by a counter-current water flow past the top of the VCHPs.
Innovative Heat Exchanger Designs for Vapor Compression Systems with Thermal Storage to Accommodate Varying Thermal Loads
There are several cases in which vapor compression systems must accommodate highly variable thermal loads, for example:
- A low, steady-state load must constantly be removed by the vapor compression system
- A transient load that is much higher than than the steady state load must be removed for a short period of time. This transient load can be 20 times higher than the steady state load.
The brute force method to provide cooling for these cases is to size the vapor compression system for the highest thermal load experienced during the transient. However, this method has significant penalties in both size, mass, and electrical power. For example, both the required compressor size and the heat exchanger size will increase by a factor of almost 20 over the steady-state system. The approach ACT has taken is to add thermal storage to the system, which allows the compressor and primary heat exchanger to be sized for roughly ten-percent more capacity than the steady state case.
The system that ACT is developing has two key components (see Figure 8):
- An integral heat exchanger that combines the evaporator, condenser and recuperator into a single heat exchanger. This approach significantly reduces the refrigeration system volume and mass. The use of a recuperator between the cooler vapor at the evaporator outlet and the hotter liquid at the condenser outlet ensures that a superheated vapor enters the compressor and increases the subcooling of the liquid entering the expansion valve. Both of these improvements contribute to increased system Coefficient of Performance (COP).
- A Phase Change Material (PCM) heat exchanger that stores the large amount of waste heat generated during the short operating peaks and dissipates the heat during steady state operation. This approach eliminates the need to oversize the compressor and other components in the refrigeration system, which results in significantly reduced system mass, volume and power consumption.
A more detailed schematic of the integral heat exchanger is shown in Figure 9. Using an integrated heat exchanger:
- Reduces the number of flow connections and lines
- Reduces the system mass and size
- Improves System COP And Reliability
The system works as follows:
- During normal operation, the PCM in the heat exchanger is frozen.
- During high heat load operation, the phase change material melts, storing most of the thermal energy. The remainder of the transient energy load is accommodated by slightly oversizing the compressor and heat exchanger compared to the steady state case.
- Once the high heat load is shut off, the PCM gradually freezes and slowly releases thermal energy into the loop. This heat load is handled by the slightly oversized compressor and heat exchanger.
A system based on the schematic shown in Figure 8 was modeled, fabricated, and successfully tested. The inclusion of a PCM heat exchanger and a recuperating heat exchanger reduced the overall mass by 36% while providing increased reliability and system efficiency.