Direct-Expansion Evaporator Coil Design
As summer approaches, it may be time to upgrade or design your process cooling or industrial refrigeration system. Many refrigeration systems incorporate direct-expansion evaporators, which can be one of the most challenging types of coils to design and operate successfully. In most cases, failure of the direct-expansion coil to perform as specified is a result of a poor thermostatic-expansion valve or distributor operation.
Direct-expansion evaporator coils are used in low temperature refrigeration applications to cool and sometimes dehumidify air. Air passing across the fins is cooled as the refrigerant flowing through the tubes absorbs heat and is boiled (evaporated). Refrigerant flowing through the coil tubes is controlled by a thermostatic-expansion valve.
The thermostatic-expansion valve is mounted at the coil just ahead of the refrigerant distributor, automatically feeding just enough refrigerant into the coil to be completely converted (boiled) from liquid to gas. The thermostatic-expansion valve is controlled by a temperature-sensing bulb mounted on the coil outlet (suction) connection. Proper operation of the thermostatic-expansion valve depends on the bulb sensing the required amount of superheat in the refrigerant gas at the coil outlet (superheat equals the number of degrees above the boiling point temperature of the refrigerant).
Most thermostatic-expansion-valve manufacturers recommend minimum superheat of 12oF (7oC). This means that the refrigerant must be heated 12oF above the saturated suction temperature (SST). Good direct-expansion coil design will put the suction connection on the air-entering side of the coil (counterflow), closest to the warmest air. The amount of superheat that can be generated is limited by the ∆T, or temperature difference between entering air and suction temperature. Typical direct-expansion refrigeration coil designs call for a ∆T of 10oF (6oC), which limits the maximum available superheat to 10oF. For example, if a coil is designed for 35oF (2oC) suction and 45oF (7oC) entering air, it is impossible to heat the refrigerant gas to more than 45oF. In other words, it is impossible to operate a thermostatic-expansion valve for this coil at more than 10oF superheat.
It is important to recognize two other problems that will result in poor thermostatic-expansion-valve operation and therefore poor coil performance: nonuniform refrigerant distribution and nonuniform circuit loading.
The use of properly sized multiple distributors and thermostatic-expansion valves controlled by load will solve this problem.
Nonuniform Circuit Loading. This is caused by differences in air velocity or by large temperature gradients across the coil face. Here, again, some of the coil circuits will receive more refrigerant than others, causing the thermostatic-expansion valve to close and starve the remaining circuits.
In this case, air-mixing devices and diffusion plates can be installed upstream of the coil to create uniform velocity and temperature profiles across the coil face. Alternately, the use of properly sized multiple distributors and thermostatic-expansion valves controlled by load will solve this problem.
There are three standard ways to design direct-expansion coils with multiple distributors and thermostatic-expansion valves, called “split circuits” (figure 1).
Row Split. Normally, this method limits the number of distributors and thermostatic-expansion valves to two or three. The entire face of the coil is active with this method, and air bypass through inactive circuits is not a problem. In some cases, though, it is not possible to match the required load split with the combination of rows available. For example, a six-row coil with a 2/4 row split produces an approximate 50/50 load split, but with a four-row coil, an exact 50/50 load split is not possible.
Interlaced. The entire face of the coil is active with this method, and air bypass through inactive circuits is not a problem. Almost any load split can be achieved for a given coil with this method, but the maximum number of distributors and thermostatic-expansion valves is limited to two or three.
When designing coils with multiple distributors and thermostatic-expansion valves, it is important to provide each thermostatic-expansion valve with its own suction header and connection for mounting the thermostatic-expansion valve bulbs. This allows for proper independent operation.
If your refrigeration system manufacturer follows these design guidelines, you are sure to experience successful direct-expansion coil operation.