Hot Gas Reheat Coils

ENGINEERING PROCEDURE

Index:        Unit Design
Subject:    (1) Hot Gas Reheat Coils
Date:        Novermber 13, 2015
        Revised February 1, 2018
        Revised November 6, 2018
        

Hot gas reheat (HGR) coils are utilized to provide reheat to the unit airstream during compressor operation.  HGR coils are mounted downstream of the DX cooling coil for reheat.  In multi-zone applications, the HGR coils will be mounted in the hot deck, but can also be mounted downstream of the DX coil if specified by Sales.  For triple deck MZ units, the HGR coils will be mounted in the bypass deck, instead of the hot deck to temper the air that will be mixed with the conditioned cold deck airflow.  

Standard HGR coils for R-410a systems will be ½” O.D. tube x 1-1/4” x 1.0825 with corrugated fins.  Standard HGR coils for R-407c and R-134a systems will be 5/8” O.D. tube x 1-1/2” x 1.299” with corrugated fins.  HGR coils are typically designed for same-end connections, but opposite-end connections can be used if required for proper circuiting.

There are three typical HGR coil sizing selections used by Seasons-4:

 

20% THR (Desuperheating only):
The most common selection is sized for approximately 20% of the total heat of rejection (THR) of the circuit, with a maximum allowable heat recovery of 25% of the THR.  This type of HGR coil may have on/off controls (a two-position 3-way valve to divert all refrigerant flow from the condenser coil to the HGR coil), or modulating controls (a modulating 3-way valve to vary the amount of refrigerant flow that is being diverted to the HGR coil), normally controlling to a specified HGR coil leaving air temperature.  In this application, no additional refrigeration specialties/devices such as receivers, etc. are required for proper refrigeration management.  

 

50% THR:
There is also a HGR selection for 50% of the THR.  This is normally used in Supermarket units where more reheat is required than can be achieved with 25% of the THR.  In this selection, the HGR coil is selected for 50% of the circuit THR.  When the HGR coil is active, a condenser solenoid valve closes, shutting off half of the condenser surface.  An even number of condenser coils are required for each circuit.  A small bleed line is also installed to drain the liquid refrigerant out of the inactive condenser coil back to the suction line of the compressor.  This allows the refrigerant from the inactive condenser coil to be used by the HGR coil which is now active, so that a receiver is not required in the circuit for the additional refrigerant requirement of the HGR coil.  There is also a bleed line from the HGR coil to the suction line of the compressor to drain the refrigerant back to the system when the HGR coil is inactive.  This type of system cannot be used with modulating control.

 

100% (Part or Full Condensing):
For selections requiring more than 25% up to 100% of the THR for reheat, some condensing will occur in the HGR coil.  This will require an active receiver in the circuit to handle the difference in the charge required when the HGR coil is active and when it is inactive.  In addition to the active receiver, a subcooling device must also be added to regain the liquid subcooling that is lost in the receiver.  Either a subcool/reheat coil mounted in the airstream immediately downstream of the DX coil, or a plate subcooler installed in the liquid line if reheat is not needed in the airstream may be used (a system should never have both of these devices).  This system is typically provided with a modulating controls (a modulating 3-way valve to vary the amount of refrigerant flow that is being diverted to the HGR coil) to maintain a specified leaving air temperature.  

In this design, additional consideration must be given to operation in off-design conditions.  If the HGR coil is connected to a modulating capacity compressor (digital scroll, modulating screw, digital recip, etc.), the reheat capacity will diminish as the compressor unloads.  For systems where maintaining the reheat leaving air temperature is critical, additional modulating compressors should be provided to modulate the non-HGR circuit so that the circuit(s) with HGR can be operated at full capacity.  If it is not possible to keep the circuits at full capacity due to low suction concerns, multiple compressors may be modulated together with HGR on multiple circuits.  Since the amount of reheat is dependent on the evaporator load, an additional means of reheat will have to be provided to ensure that the desired leaving air temperature can be achieved under any condition.  Due to the added costs and labor associated with this set-up, you will want to maximize the reheat of each circuit to limit the number of circuits having hot gas reheat.  

 

Other considerations (system design):

The amount of reheat available is dependent of the load of the circuit, so the amount of reheat will decrease as the load decreases.  If the unit must meet a specified hot gas reheat or unit leaving air temperature, the hot gas reheat coil should be “oversized” to allow for off-design conditions when the reheat will be less than full load conditions.  If reheat is critical, other mechanical means of heating may have to be used at times.

Hot gas reheat coils on modulating compressor circuits (digital scrolls, VFD controlled compressors) will have an even lower reheat capacity as the compressors modulates down from full capacity to maintain discharge temperature.  Consider multiple modulating circuits and multiple HGR circuits.

Hot gas reheat coils provided in the hot deck (or bypass deck on triple deck units) of MZ units will provide passive dehumidification by raising the temperature of the unconditioned, bypass air that will then be mixed with the conditioned, dehumidified cold deck airflow.  The resulting airflow has a lower dewpoint temperature than it would if the unit did not have an HGR coil.  

For example:

MZ unit with 20% outdoor air, with the following conditions:

• OSA is 65°F db and 100% RH, and RA is 75° F db and 55% RH
• The DX cooling design and cold deck leaving air set point is 52.0 °Fdb/51.9°Fwb.
• At these conditions, the resulting mixed air will be 73.0° Fdb/ 64.1° Fwb/59.3° F dp/75.8 gr/lb
• For a particular zone, the required dry bulb temperature from the unit to meet the space load is 60.0 °Fdb (there is no zone humidity monitoring or control).  To meet this condition, the zone actuator will modulate to mix air from the hot and cold deck until this condition is met:
  • With no hot gas reheat, we will mix 38% hot deck air at 73.0° Fdb/64.1° Fwb with 62% cold deck air at 52.0° Fdb/51.9° Fwb, with the resulting condition of 60.1° Fdb/56.9° Fwb/54.9° F dp delivered to the space.
  • If we add 5° of hot gas reheat in the hot deck, we will mix 31% hot deck air at 78.0° Fdb/65.8° Fwb with 69% cold deck air at 52.0° Fdb/51.9° Fwb, with the resulting condition of 60.1° Fdb/56.6° Fwb/54.4° F dp delivered to the space.
  • If we add 10° of hot gas reheat in the hot deck, we will mix 26% hot deck air at 83.0° Fdb/67.4° Fwb with 74% cold deck air at 52.0° Fdb/51.9° Fwb, with the resulting condition of 60.1° Fdb/56.4° Fwb/54.0° F dp delivered to the space.
• So as we increase the dry bulb temperature in the hot deck, we are able to provide the same leaving air dry bulb temperature to the space, but at a lower dewpoint temperature.

This passive dehumidification is not a true dehumidification mode that can be controlled based on a humidity setpoint, and it cannot actively reduce the humidity level.  All HGR coils installed in the hot deck or bypass deck of an MZ unit will be installed with either on/off or modulating controls to shut the refrigerant flow to the coil off completely when heating to the hot or bypass deck is not required.  For DDC controls by Seasons-4, the HGR coil will be active based on the heating demand from the zones.  For controls by others, the HGR coil will be active based on ambient temperature.  Seasons-4 will provide a stand alone ambient temperature controller (Johnson A421 or similar), which will initially be set to allow HGR operation at ambient temperatures below 80°F db (adjustable).
  
If an active dehumidification mode is required for a MZ unit, the HGR coil will need to be installed downstream of the cooling coil, and will be a modulating HGR design as described above.  A specific sequence of operation will be developed to outline the conditions and operation of the unit during dehumidification mode.