We’ve often referred to the importance of HVAC systems to every layer of the cultivator’s business. One of the complications with selecting the right designer or technology is that few cultivators are mechanical engineers. This really complicates the process of deciding which provider or technology is best for your facility. When it all sounds good, how do you make the right decision for your grow? The truth is, there are a number of technologies that can successfully manage the climate in an indoor facility. The trick is understanding the pros and cons of each and working with a mechanical design partner who can clearly articulate them to help you make an informed decision.
Although there are a number of options within options within the various technologies that will do the job, they generally fall into two categories: those with integrated dehumidification, and those that rely on standalone dehumidification.“Standalone” describes climate control systems where the cooling function is decoupled from the dehumidification function, and integrated systems describe climate control systems where the cooling and dehumidification functions are coordinated, usually with the same pieces of equipment. In general, standalone systems are lower upfront cost but provide less precision and fewer energy-related benefits, and integrated systems may be more expensive, but with far greater precision and more energy saving options.
Standalone Systems Pros and Cons
DX with Standalone Dehumidification
DX with standalone systems mean the cooling and heating unit is separate from the dehumidification unit, usually located in the space it serves. The most common types of standalone DX systems are split systems, mini-splits and packaged roof top units (RTUs). Common types of dehumidifiers used are standalone electric, or desiccant in low humidity applications. These systems typically supply cooled or heated air to a single zone in grow applications. The typical capacity is from 1 to 10 tons (split units) or 10-30 tons (packaged units), but they can be larger in some applications.
Split systems are made up of two parts: an indoor fan coil unit (FCU) or small air handler, and an outside condensing unit. The FCU takes the warm air from inside your grow over cold evaporator coils, which contain refrigerant, absorbs the heat from the air inside the space, and transfers it to the condensing unit outside the grow to be rejected.
Packaged units operate similarly to split systems. The primary difference is that packaged units have all the components of the air conditioning system all in one place, contained in a “package” located outside the building (most commonly the roof).
DX with Standalone Dehumidification Pros and Cons
2-Pipe Chilled Water (also-known-as hydronic cooling and dehumidification)
Hydronic cooling is simply the removal of heat and moisture from a space utilizing chilled water as the heat exchange medium. People sometimes confuse chilled water systems with evaporative cooling which introduces humidity into the space and consumes water to cool, but they are quite different. Hydronic cooling systems are completely closed loop meaning no water is added to the space, much like the radiator in a car. In hydronic cooling, water is chilled by a chiller, dry cooler, or cooling tower and circulated via pump through the system into heat exchanger units in the space (air handlers or fan coils) and then back to the chiller to be recirculated again. Air handlers and/or fan coil units utilize a fan to pull warm air in and over the heat exchanger inside. As warm air in the room moves over the heat exchangers, heat is transferred from the air into the cool water inside the coils, pulling heat and humidity from the room and returning cool, dry air to the space. Since pumps keep water inside the system constantly moving, the warm water leaving the heat exchanger is immediately returned to a chiller, dry cooler or cooling tower.
Chiller systems are used worldwide for cooling in high-heat applications because of their top-of-the-line energy efficiency and flexibility for mechanical air conditioning and dehumidification. By decoupling the machine that is producing the cooling away from the indoor environment, it is possible to move cooling to the places that need it, when they need it, thereby “right” sizing the equipment for the overall building and not oversizing individual equipment for each space based on maximum loads in each.
2-Pipe Chilled Water Pros and Cons
Integrated Systems Pros and Cons
Refrigerant Based Systems (also-known-as DX)
The main difference between a DX (direct expansion) unit and a chilled water unit is that the DX system’s cooling, dehumidification and heating all happen directly to the airstream serving the space, whereas with chilled water systems, the unit producing the cooling (the chiller) is decoupled and cooling a recirculated water loop that is pumped to terminal units for heat exchange. The DX system uses a refrigerant vapor expansion and compression cycle to cool and dehumidify the air coming from the space and returns it to the room after conditioning. DX based systems described in the stand-alone section are different from the DX units described here, where dehumidification is coordinated with cooling functions.
Complex DX with Integrated Dehumidification via Hot Gas Reheat
As noted above, in this type of system, cooling, dehumidification and heating all happen directly to the airstream serving the space. The system uses a refrigerant vapor expansion and compression cycle to cool and dehumidify the air coming from the space and returns it to the room after conditioning. Here we will discuss the complex integrated system with the dehumidification/cooling/heating all in one unit.
These units cooling and heating functions are the same as standalone DX systems, however the dehumidification function is combined into the same unit, so they work in concert. There are two forms of hot gas reheat. There is an on/off type with poor discharge air temperature control (but cheaper) and complex modulating which offers better discharge air control but is more expensive.
Modulating Hot Gas Reheat
Modulating Hot Gas Reheat (MHGRH), the more complex form of reheat, allows the precise control of the HVAC system discharge air temperature and is typically used during dehumidification mode. During dehumidification, mechanical cooling lowers the temperature of the air to a specific dewpoint in order to lower the moisture content of the air. MHGRH is then used to temper (raise) the temperature of the air to a desired setpoint that is equal to the room’s temperature setpoint. A common solution is a sophisticated RTU with integral dehumidification via modulating hot gas reheat. These RTUs have a vast array of efficiency, sizes, features, and costs.
Modulating Hot Gas Reheat Pros and Cons
4-Pipe Chilled Water
The 4-pipe chilled water system is also a hydronic cooling system (like the 2-pipe system), but it utilizes both a chilled and hot water coil. The distribution piping runs to terminal fan coils or air handlers, which use chilled or hot water to change the air temperature by cooling, heating or dehumidifying it.
Traditionally these systems utilize air handler units (AHU’s) or fan coil units (FCU’s) which contain a blower, heating or cooling elements, and control valves (and optionally, filter racks, and modulating dampers) . AHU’s and FCU’s can either:
a) be connected to a ductwork system that distributes the conditioned air to the area served and reside outside the space, or
b) reside directly in and condition the space served without ductwork.
4-pipe chilled water systems provide the ability to economize via a dry cooler in colder climates saving significant amounts of energy by turning off the compressors in the chiller, which are the single largest energy consumer in the HVAC system.
The most energy efficient 4-pipe designs utilize heat recovery on the chiller plant to minimize or eliminate dehumidification reheat costs.
4-Pipe Chilled Water Pros and Cons
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