HVAC systems, when properly designed and maintained, can provide comfortable environments year round. For many facility owners, however, a conventional air conditioning system in certain applications creates a dilemma - either the temperature is at the desired level or the humidity is at the desired level, but not both. This is the case, because conventional air conditioning systems are not designed to properly handle humidity levels at varying conditions, while simultaneously keeping the space at the desired temperature.
The other issue facility owners face is the dilemma between running an efficient HVAC system that minimizes energy costs, and running a system that ensures the comfort of tenants or building occupants. Often times the two will clash because the facility owner wants to keep the system at one temperature and the occupant or tenant wants to vary the temperature based on their specific comfort level. As a result, the building environment may resemble a cave - cold and damp.
FINDING BALANCEConventional air conditioning systems are not designed to handle conditions when the temperature is moderate, but the humidity is high. In these conditions, facility owners have only two options. Their tenants can either endure uncomfortably high humidity levels with a moderate room temperature, or they can decrease the humidity level by lowering the thermostat and endure the space getting too cold.
According to ASHRAE Standard 62.1-2001, Ventilation for Acceptable IAQ, the optimum humidity level in an occupied space should be between 30-60 percent relative humidity (rh). Rh becomes an important consideration in the design of an air conditioning system and requires HVAC design contractors or design engineers to estimate both the peak sensible load and the peak moisture load.
A properly applied air conditioning system will continuously run at design conditions, removing both sensible and latent heat. The system will cycle on and off below design conditions with demand based on the thermostat set point. The problem is that the thermostat only measures sensible temperature and not relative humidity. As a result, the building may be at the correct temperature, but excess humidity can continue to make the environment uncomfortable for occupants.
MEASURED CONTROLOften times, in commercial applications, fans run continuously in the occupied mode, while supplying fresh air to the space, providing proper air quality. Sometimes the fans run continuously day and night, 24/7, which may not be the most desirable situation in an energy conscious society.
Short cycling in humid environments can also have undesirable effects within a building. One such effect relates to energy costs. Often times, building tenants or occupants of a facility will lower the thermostat set point in an attempt to provide a more comfortable, less humid environment. As a result, the air conditioning system runs longer and more frequently, increasing energy costs for that system.
Another option is to add a humidistat and reheat the air. This option for dehumidification addresses humidity issues without lowering the space temperature and provides a comfortable environment for the occupant.
There are several methods of reheating the air, including the use of electric heat, a gas furnace or a hot water coil.
NEW FRONTIERSEmerging technologies are improving conventional reheat systems. For example, Johnson Controls incorporates an advanced dehumidification system in its YorkÂ® MagnaDRYâ„¢ Series. This hot gas reheat unit removes moisture and uses a reheat mode of operation that does not rely on electric heat or natural gas, said the company.
The unit also has a quick refrigerant recovery time, which ensures the unit delivers the capacity the space requires efficiently. Because MagnaDRY can remove moisture without increasing sensible cooling, the unit helps to maintain temperatures and humidity levels within the comfort zone without significant added energy costs.
The hot gas reheat unit meets the ASHRAE 90.1 energy standard. The key to effectively utilizing hot gas reheat units is to understand the applications that best apply to this type of system. The perfect condition for using air conditioning with hot gas reheat is when the sensible load is low, and the latent load is high.
This technology is an excellent choice for critical applications, such as supermarkets, art galleries, museums, libraries, fitness centers, doctors' offices, printing facilities, laundromats, banquet halls, and pharmaceutical companies.
Publication date: 08/14/2006