Don't Pick An Evaporator Out Of Thin Air
Jeff Rothermal of Evapco, Taneytown, MD, spoke on “Air Unit Design and Application” at the recent Refrigerating Engineers and Technicians Association (RETA) conference. His objective was to pass along facts that can be passed along to customers, who can then make informed decisions.
SELECTING AN EVAPORATORRothermal focused on these “important parameters for the contractor as well as the owner.”
Also take into account the size of room, air throw distance, and air distribution.
SYSTEM DESIGN PARAMETERSNow that some of the basics have been determined, it’s time to apply those to various system design options.
Types of feed can be:
The liquid recirculation system’s goal is to “overfeed” the supply of liquid refrigerant to the coil, Rothermal explained. The overfeed or recirculation rate is typically 2:1 to 5:1 — two to five times more liquid than is evaporated for the capacity. This ensures that the coil tubes are fully wetted on the inside for continuous (maximum) heat transfer.
Liquid recirculation is the most common form of refrigerant feed, he said. Liquid is mechanically pumped to the coil from the recirculator vessel, or liquid is supplied to the coil via a controlled pressure receiver (gas pumper system), which uses system refrigerant (gas) pressure to move the liquid.
Bottom-feed liquid recirculation systems typically are used for hot gas defrost applications, where hot gas is fed into the open suction header (or pan). Condensed gas drains via gravity to the liquid side. The hot gas cycle also helps purge the coil of oil.
Top-feed liquid recirculation systems are typically used for higher temperatures, and use air or water defrost. Circulating allows for gravity draining of refrigerant liquid prior to defrost, Rothermal pointed out. Hot gas requires an extra header on the liquid side to bypass the orifices for hot gas feed.
In flooded-feed systems, liquid is fed to the coil via a surge drum located above the coil. Liquid feed is accomplished by gravity head pressure acting on the coil, and convection (rise) of the refrigerant gas, Rothermal explained. A liquid level control feeds the surge drum. Evaporated liquid (gas) returns to the surge drum and then back to the compressor or to an intermediate trap.
Coil tubes remain fully wetted for maximum heat transfer (as in recirculated), he said. Pressure drop is minimized, and “No orifices in the liquid header are required to meter the flow,” he said.
The coil in a flooded-feed system is typically sized with larger liquid and suction headers to minimize any pressure drop (temperature) penalty, Rothermal stated.
Flooded-feed systems are typically used in smaller systems or isolated coils operating under “unique” conditions.
DX-feed systems are typically used in high-temperature applications. They use high-pressure liquid from the main system receiver through a thermostatic expansion valve (TEV), Rothermal explained. Pressure and temperature are reduced through the TEV.
Distributor tubing is typically 3/16 to 3/18 inch; all refrigerant leaves as superheated gas. The TEV uses a sensing bulb on the suction line to meter refrigerant flow and ensure that superheated gas returns to the compressors.
Heat transfer of the coil is diminished since the circuits are not fully wetted inside, Rothermal added. DX feed is not recommended for low-temperature ammonia. It is more commonly used with halocarbons.
DX- or TX-feed system advantages include:
The next item to consider is the type of defrost required.
Air defrost can be used in rooms operating above 35 degrees. The refrigerant supply is shut off by the liquid solenoid. Warm room air melts frost off the coil. The fans remain off until the refrigeration cycle refreezes moisture on the coil. (An option is to select a unit with low face velocity to prevent spitting.)
Water defrost can be used for any range of room temperatures, Rothermal said. Typical water supply temperature is about 60 degrees. The water flow rate can vary, but it ranges from 1 to 3 GPM/square foot face area. Drain lines should be sloped and heat traced, he said. Traps should be located outside the refrigerant space, if possible. Multiple units sharing a common drain return line should be individually trapped and heat traced to the unit.
The following is the water defrost cycle:
1. Shut off refrigerant flow to the unit(s).
2. Allow fans to operate for a period of time in order to “pump out” or boil off any remaining liquid refrigerant in the coil.
3. Shut off fans after pump out.
4. Suction line remains open during defrost.
5. Water flush the coil for as long as it takes to defrost.
6. Shut off water and allow water to drain off.
7. Open the liquid feed, allow remaining moisture to refreeze.
8. Energize fans for operation.
Rothermal called hot gas defrost “the most commonly used method of defrost for any application.” It can be applied to the coil only, referred to as hot gas coil, or applied to the coil and pan, referred to as hot gas unit.
Hot gas from the compressor discharge is diverted to the evaporator. The evaporator functions like a condenser; hot gas condenses back to a liquid. The tubes and fins conduct the heat removed from the gas to warm and defrost the heat transfer surface areas.
Condensate is relieved through a pressure regulator valve. Effective defrost requires a sustained 70 to 80 PSI of hot gas measured at the coil, Rothermal said. Roughly 20 to 40 Btu/square foot of coil surface area are used.
Rothermal offered the following rule of thumb: “Defrost a maximum of one-third of the total system capacity at a given time, provided the other two-thirds are under load to generate enough hot gas.”
There are a variety of piping and valve arrangements for a hot gas defrost system. The specific arrangement is influenced by the application. Three- and four-pipe systems are most common.
Hot gas/water combination defrost also is available. In some applications, water is used to provide a secondary defrost and flushing of the coil, Rothermal said.
The hot gas cycle is performed as described. However, prior to opening the liquid line solenoid valve, a water defrost cycle is initiated to flush the coil and drain pan. The liquid line solenoid is opened, and the fans are energized.
Sidebar: Evaporator TerminologyTo get everyone in his RETA seminar audience on the same page, Evapco’s Jeff Rothermal gave the following evaporator terminology.
First of all, when he referred to evaporators (air units), he specifically meant:
Removal of heat produces a cooling effect. This change of state is referred to as evaporation.
Face area and face velocity is figured by multiplying finned height by finned length.
Fin spacing is the number of fins per unit of length, or it may be expressed as the space (or distance) in between adjacent fins.
A frosted coil occurs when an evaporator operates below freezing. Moisture in the airstream freezes (or frosts) on the coil.
Frost acts as an insulating barrier to heat transfer, which reduces capacity. “Most manufacturers publish frosted ratings, which are the most conservative,” Rothermal commented. Frost must be routinely removed through a defrost cycle — usually hot gas or water, he said.
A wet coil occurs when an evaporator operates above freezing. Moisture in the airstream condenses on the coil.
Wet coils have a higher capacity rating than frosted coils, he pointed out. Also, the face velocity must be less than 625 feet per minute (FPM) to prevent moisture carryover (“spitting”)
TD is the temperature difference between the coil (refrigerant) temperature and the room (entering) air temperature. The coil or refrigerant temperature is commonly referred to as the saturated suction temperature (SST).
External state pressure (ESP) is the resistance to airflow outside or external to the evaporator unit. ESP is measured in inches of water column, kPa, and millimeters of mercury (mm Hg).
Load is the total heat to be absorbed from a product and area to be cooled (refrigerated), measured in tons or Btuh.
Publication date: 11/18/2002