HVAC Energy Recovery

Everything You Need To Know About HVAC Energy Recovery

Can Heat Pipe Heat Exchangers be used in a retrofit application?

Yes, ACT’s Energy Recover Heat Pipe Heat Exchangers can be installed as a retrofit into an existing installation, as a factory installation (sent to ACT and then finished AHU sent to the job site), or as an OEM factory installation (Heat Exchanger is sent to the AHU OEM).

What types of reheat solutions are available for chilled water coil applications?

One solution is to include a hot water or steam coil after the chilled water coil as a source of reheat.  Unfortunately, during cooling periods, boilers are often taken out of service because they are no longer required for heat and/or they are too expensive to operate for small loads.

ACT’s wrap-around heat pipe heat exchanger (WAHX) can be used to both reduce the cooling tonnage requirement and provide the reheat passively – “for free.”

What functions do air conditioners provide in the summer?

Air conditioners provide two things.  First, they provide space cooling. The cool air delivered to the space quenches the heat being transferred from the outside and heat generating sources in the room.  The airflow rate and the temperature difference between the air delivered to the space and the desired space temperature determine how much heat can be quenched.

The second thing air conditioners provide is dehumidification. The air conditioner pulls in warm, moist air and lowers the temperature of the air to approximately 52.5°F.  Air at this temperature is capable of holding very little moisture so any excess water vapor will condense as liquid onto the cooling coil. When that 52.5°F air is warmed back up to the desired temperature for the space, the resulting relative humidity is a comfortable 50%.

What happens if the space temperature is reached before the dehumidification level is achieved?

In many applications, the thermostat set point is reached before the dehumidification level is achieved. The adage that “more is better” is not true when it comes to air conditioning capacity.  When the thermostat temperature is met and the unit shuts off, then there is no means of dehumidifying the space. The relative humidity increases. If the unit’s set point is lowered to address the high relative humidity, then the space becomes over cooled.  Typically, these building spaces have an unfavorable combination of cold temperatures and damp humidity (cold and clammy).

Can I use electrical heat or hot gas reheat to warm the air slightly, so that the unit continues to dehumidify but not over-cool?

Certainly, this is one method that can be used; however, using electrical energy to cool the air and then adding more electrical energy to reheat the air will consume a lot of energy. In fact, most building codes and energy conservation regulations prohibit this brute force method.

Hot gas reheat can also be used for more sophisticated refrigerant direct expansion (DX) air conditioners. In these units, there is an extra condenser coil that is placed in the supply air stream.  A fraction of the hot refrigerant gas generated by the compressor bypasses the primary condenser and goes directly to the hot gas reheat coil.  This hot refrigerant gas is used to provide the heating necessary.  While this is an energy-efficient method for some DX systems, some systems do not have this capability.

ACT’s wrap-around heat pipe heat exchanger (WAHX) can be used to both reduce the cooling tonnage requirement and provide the reheat passively – “for free.”

How is indoor air quality maintained in buildings?

One of the most common techniques to manage indoor air quality is to introduce a certain amount of outside air to dilute the inside air.  You may have heard the term “Air Turns per Hour”.  If the air turns per hour in a space is ten, that means that every hour ten times the volume of the space is brought in from outside. The same amount of indoor air is expelled through dedicated ventilation ducts, leakage through doors, windows, bathroom vents, etc.

Maintaining a favorable indoor air quality through managed ventilation sounds expensive, like leaving the windows open in the winter?

As energy conservation has become more and more important, for environmental and cost reasons, recovering energy from ventilation air streams has become the norm.  In fact, many building codes dictate when and how much energy recovery is needed in various building applications. By exchanging energy between the leaving ventilation stream and the entering outside air stream, the impact of ventilation can be minimized; and simultaneously, indoor air quality can be excellent.

What type of heat exchangers can be used to recover the energy between the outdoor air and ventilation exhaust?

There are three main types of heat exchangers; energy wheels, enthalpy plates, and heat pipes.  All are effective devices for exchanging energy between two air streams.

How does an energy wheel work?

When the ventilation duct and the outside air duct are side by side a wheel-type heat exchanger can be used.  Half of the wheel is in one duct and half of the wheel is in the other. As the wheel spins, the portion of the wheel that is in the warmer duct heats up the porous wheel structure.  The portion of the wheel that is in the cooler duct removes heat energy from the porous wheel structure.  Wheels can also be treated with a desiccant such that moisture can also be transferred from one stream to the other.

Wheels do not seal perfectly and the volume of the wheel itself leaks from one duct to the other.  So, applications requiring zero transfer (cross-contamination) between streams cannot use wheels.  Wheels require side-by-side and equal-sized ducts, limiting the packaging freedom that many applications require.  And last but not least, wheels are active moving machines with motors, belts, pulleys, bearings, etc. requiring highly skilled maintenance personnel to keep the wheels turning.

How does an enthalpy plate exchanger work?

Plate exchangers are a series of parallel plates stacked together with each passage sealed from the next such that one airstream passes through every other passage on one face and the other airstream passes through every other passage on a face 180° apart.  The warmer air stream in one channel conducts heat through the plate material into the cooler air stream.  Some plate exchangers have desiccant membranes so that thermal energy and moisture can pass from one stream to the other.

Enthalpy plates can be sealed fairly well such that cross-contamination is negligible.  Like wheels, they require side-by-side duct layouts. So, packaging limitations prevent plates from being used in many applications. Besides the limited packaging freedom of having to have the ducts side-by-side and equal size, the packaging freedom is further constrained by their large size and volume.  A typical plate core can be 36” x 36” and placed on a diagonal in an air handler.  That requires about 51” of air handler length to accommodate!

How does a heat pipe heat exchanger work?

Heat pipes are thermal superconductors that use the evaporation and condensation of a refrigerant inside of a closed tube to transfer heat from one end of the closed tube to the other.  As heat is applied to one end of a heat pipe, the liquid refrigerant inside begins to boil converting liquid refrigerant to vapor refrigerant.  The slightly warmer vapor refrigerant is at a higher pressure than the vapor refrigerant at the other end of the heat pipe.  This pressure difference causes the warmer vapor to move to the lower pressure, cooler end of the pipe.  Here the vapor condenses on the cooler surface giving up the latent heat of vaporization of the refrigerant as it converts back to a liquid.  The liquid returns to the warmer end by gravity flow for another round of evaporation and condensation.  In a properly sized heat pipe, this process can transfer large quantities of thermal energy (heat) over long distances with practically no temperature difference, typically one or two degrees Fahrenheit,  from end to end.  Therefore, this heat pipe element, or superconductor of heat, can be used to build a superconducting heat exchanger.

Heat pipe heat exchangers start out as conventional tube and fin coils, where each of the tubes is converted into a superconducting heat pipe element.  The warmer air stream passes over one portion of the coil and the cooler air stream passes over the remaining portion of the coil, usually in a counter flow arrangement.  The warm air evaporates the refrigerant on the one side and the cool air condenses the refrigerant on the other side transferring large amounts of heat from one air stream to the other.

Heat pipe heat exchangers can be any size that a conventional cooling coil can be built.  They can be long and short or short and tall.  The two airstream sizes (ducts) can be of different sizes and shapes. The two heat pipe coils can also be separated by tens of feet.   They can also be designed to have 0% cross-contamination and they only require a small section of air handler length to install – typically less than 12”.