Introduction
Humidity control in buildings is a core component of occupant comfort.
Imagine stepping into a building on a sweltering summer day and feeling a refreshing cool breeze instantly. Now, picture the same building on a chilly winter day, where you are greeted with comforting warmth. This seamless temperature control is made possible by a technology known as hot gas reheat. In this blog, we’ll dive into the world of hot gas reheat, exploring how it works, its benefits, and why it is an excellent choice for efficient climate control in various settings.
Recent requirements to introduce large amounts of outside air into the workplace can result in the rise of the indoor humidity level in the space. People want to combat this by purchasing a unit with excessive cooling tonnage, a “bigger is better” mentality. The over-powered unit causes the system to short cycle, not running long enough to de-humidify. We’re left with excess moisture, which can lead to mold.
Dehumidification: When cooling air, HVAC systems often remove moisture to reduce humidity. In high-humidity environments, simply cooling the air isn’t enough. Excessive moisture can lead to discomfort and potential health issues, such as mold growth.
Balancing Act: Hot gas reheat allows the system to cool the air to remove moisture and then reheat it to a comfortable temperature without adding back the moisture. This maintains optimal humidity levels, enhancing indoor air quality and comfort.
Part-load operation: single zone constant volume systems:
The constant volume single-zone system delivers the same amount of air, regardless of any change in zone load. Therefore, during periods of reduced load, the system must deliver warmer supply air to prevent the zone from being overcooled. To accomplish this, a direct-expansion (DX) system cycles the compressor(s) on and off to maintain zone temperature.
When the zone setpoint temperature is met, the compressor(s) cycle off , but the supply fan typically remains on to continue delivering supply air and satisfy ventilation needs. If the outdoor air conditions are mild but moisture-laden, the zone loads will be impacted resulting in rising zone humidity.
Let see an example:
Outdoor air (OA’) mixes with return air (RA’), this mixture (MA’) then passes through the evaporator. At part-load, this air mixture is only cooled to 60°F DBT (CC’), compared to 52°F DBT at full load. While this elevated
supply air temperature is sufficient to satisfy the reduced sensible cooling load, the air leaving the unit (CC’) has only been dehumidified to a dew point temperature of 58°F compared to 51°F at design. As a result,
the supply air’s ability to off set latent load in the zone is reduced and the resulting zone humidity at part-load is 64 percent compared to 50 percent at full load.
How Hot Gas Reheat work?
Hot gas reheat allows the system to dehumidify the supply air to a sufficient dew point temperature for dehumidification, then reheat the air without the use of additional new energy, like fossil fuel heat or electric
heat.
Heat generated from the vapor compression refrigeration cycle can be recovered from direct expansion equipment rather than rejecting it to the ambient environment. This is typically accomplished by placing a hot gas reheat coil downstream of the unit evaporator (See below figure). Hot refrigerant from the compressor is routed through a valve to the hot gas reheat coil to reheat the airstream. Heat is transferred from the hot refrigerant vapor to the cooled and dehumidified supply air. The cooled refrigerant vapor continues its journey to the unit’s condenser coil for heat rejection.
In simple words, While dehumidification can be achieved by cooling the air to a lower dew point temperature, this very cold air isn’t suitable for supplying directly to a space. To address this, a reheat coil warms the air back up to a comfortable temperature before it’s discharged into the space.
The whole procedure is accomplished by having few additional component besides the core components of a hot gas reheat system (evaporator coil, hot gas reheat coil, refrigerant lines, and compressor), for proper operation and control.
Mixing Valve: This valve precisely regulates the ratio of hot gas from the compressor discharge and cool air from the evaporator coil outlet. By mixing these air streams, it achieves the desired temperature of the reheated air.
Diverting Damper: This damper controls the airflow path. During cooling mode, it directs cool air to bypass the reheat coil and directly supply the conditioned space. When heating is required, the damper adjusts to route air through the reheat coil for temperature increase.
Pressure Relief Valve: This safety valve protects the system from excessive pressure buildup. Hot gas injection can sometimes cause pressure spikes, and this valve acts as a safeguard by releasing pressure when it exceeds a safe limit.
Temperature Sensors: Multiple sensors monitor air temperatures at various points within the system. This information is vital for the control system to maintain proper operation and prevent overheating or overcooling. The sensors can be located at the entering and leaving air streams of the evaporator coil, reheat coil, and mixed air leaving the mixing valve.
Reheat Control System: This system regulates the operation of the hot gas reheat valve based on the temperature sensor readings. It ensures the reheated air maintains the desired temperature setpoint for the conditioned space. The control system can modulate the opening of the hot gas reheat valve to precisely control the amount of hot gas entering the reheat coil, achieving efficient temperature control.
Cost-Effective
Available as a factory-installed option on most light commercial rooftop products, the Packaged Rooftop Units With Hot Gas Reheat Dehumidification system provides a cost-effective packaged alternative for meeting latent load intensive applications and variable Sensible Heat Ratio (SHR) requirements. System installation costs are simplified and minimized by using a humidistat device with a thermostat for combined temperature and humidity sensing in the space.
Typical Applications
Health Clubs – Shower areas and human perspiration can cause uncomfortable and higher humidity space conditions. In addition to human discomfort, these conditions can propagate the growth of mold and mildew.
Schools – Due to variable student occupancy with constant changes in ventilation air change requirements in each classroom, the proportion of latent load may be high and humidity may rise. High humidity levels can damage computer equipment or building structural materials. In addition, students entering and leaving classrooms may result in a variation in latent load for each room, which requires maximum part load dehumidification control.
Convenience Stores & Supermarkets – High humidity levels can cause inefficient operation of freezer and refrigeration systems. Over-cooling can cause significant discomfort for customers.
Restaurants – The high degree of variable occupancy, along with kitchen areas of restaurants that have many humidity producing activities, such as dishwashing and cooking, can easily result in humidity control problems and over-cooling by conventional packaged rooftop units.
Humid Climates – In climates along the coast, when the temperature drops the outdoor wet bulb temperature may remain the same or higher. This results in a need to reduce the sensible capacity but yet provide more latent capacity to prevent humidity levels from increasing in the space.