Hot Gas Reheat Systems

An application spotlight on hot gas reheat systems.

A hot gas reheat coil is part of the unit refrigeration system. An additional refrigerant coil is mounted downstream of the DX evaporator cooling coil and a three-way valve diverts an amount of hot refrigerant gas from entering the condensing coil to the reheat coil to control the supply air temperature after the cooling load has been satisfied. This provides a dehumidification mode of operation and prevents over cooling of the space.  

All moisture removal is achieved with the evaporator coil, but Supply Leaving Air Temperature (LAT) and Relative Humidity (RH%) are controlled with the reheat coil while in dehumidification mode. 

Series Reheat systems are usually only designed to recover part of the total heat of rejection of a refrigeration circuit. The remaining capacity will be handled by the condenser coil.  Series systems can be simpler systems. 

Parallel Reheat systems can be sized to handle the full heat of rejection of the system, both cooling load and compressor heat, or may be limited to a lower value based on reheat coil sizing and other factors like oil return and suction temperature limits. 

ON/OFF Hot Gas Reheat: The reheat coil and condenser coils are generally piped in a series arrangement, with refrigerant flowing through the reheat coil and then the condenser coil when reheat is active or bypassing the reheat coil completely when inactive. The control valve is on/off only and is not able to modulate reheat capacity. Reheat MBH capacity is less than the full value of heat removed by the system. Since there is no modulation control of capacity, temperature swing within the dead band of the controller between on and off. Overall capacity can still be controlled by modulating compressors or condenser head pressure control. 

Modulating Hot Gas Reheat: The reheat coil and condenser coils can be piped in either a series or parallel arrangement. A 3-way modulating reheat valve diverts a varying percentage of the hot gas entering the condensing coil to the reheat coil to control reheat Leaving Air Temperature. 

A supply air temperature sensor and DDC controller are used to maintain the supply air temperature during the dehumidification mode of operation. 

Modulation is controlled by a 3-way stepper valve controlled to maintain discharge air temperature at the dehumidification setpoint. 

AAON’s Parallel Modulating Hot Gas Reheat with Microchannel Reheat Coil: The reheat and condenser coils are piped in a parallel arrangement. The reheat capacity is regulated by the DDC controls and modulating 3-way control valve that diverts a varying percentage of the hot gas entering the condensing coil to the reheat coil to control reheat Leaving Air Temperature. Refrigerant leaving the reheat coil is piped directly to the inlet of the expansion valve, bypassing the condenser coil completely. 

With the option to put the full load of the refrigerant circuit through either the condenser or the reheat coil as needed, the Parallel Microchannel Reheat coil is sized to handle 100% of the total heat of rejection of the refrigeration circuit. 

During operation, if the full reheat capacity is called for, the reheat coil in this configuration will function as the main condenser. When there is no demand for reheat, the full heat of rejection will be handled by the condenser. 

For most applications, the negative effects of moving reheat to the lag circuit are minimal. To maintain enough reheat capacity, the digital compressors on the reheat circuit have reduced turndown capability.  

Using an on/off compressor on the reheat circuit will further limit the turndown to that of a single compressor running at full speed, but most applications require at least this much cooling capacity when there is a call for dehumidification. 

If additional dehumidification capacity is needed, having the reheat on the lag compressor is a significant advantage.  

For applications with a high percentage of outside air, lag circuit modulating compressors, either digital (VCC) or 2-stage compressors, or hot gas bypass on the lag circuit should be considered. Head pressure control is required with modulating reheat. 

Advantages 

• Parallel Circuiting: Allows the full heat of rejection of the refrigeration circuit to be available for reheat, since refrigerant can fully bypass the condenser coil. It is possible to provide higher than neutral air temperatures due to the reduced mass of the air through condensation and the addition of compressor heat. Leaving air temperature is controlled by the modulating 3-way valve and the remaining capacity will be handled by the condenser coil with Head Pressure Control. 
• Lag Circuit: Operating reheat on the lag refrigerant circuit, allows the full reheat capacity through the full modulation range of the compressors. The lag compressor circuit will provide the reheat capacity while the lead circuit, digital (VCC) or inverter (VS), compressors are free to modulate over a wider range without losing reheat capacity. 
• Microchannel Reheat Coil: Microchannel reheat coils offer several advantages over traditional fin and tube coils. Advantages such as increased effective face area for the same size coil, due to not having a coil case. Reduced internal volume requiring less refrigerant. Increased capacity (MBH) for the same size coil.