8.1 DX vs Flooded Evaporators

Introduction to Evaporators in Industrial Systems

In industrial refrigeration, the evaporator is the heat exchanger where heat is absorbed from the refrigerated space, process fluid, or product and transferred into the refrigerant, causing it to boil and vaporize. The efficiency of this process directly dictates compressor runtimes, system power consumption, and room temperature stability.

In ammonia systems, evaporators are categorized by how they are fed with refrigerant and controlled. The two primary methods are Direct Expansion (DX) and Flooded designs. Because anhydrous ammonia ($NH_3$, R-717) has thermodynamic properties that differ significantly from synthetic halocarbon refrigerants, flooded systems are far more common in industrial plants, though DX has specific niche applications.

Direct Expansion (DX) Evaporators

In a Direct Expansion (DX) evaporator, liquid refrigerant is metered directly into the inlet of the cooling coil by an expansion device, typically a Thermostatic Expansion Valve (TXV) or an Electronic Expansion Valve (EEV). As the refrigerant flows through the tubes, it absorbs heat and evaporates.

By the time the refrigerant reaches the end of the coil, it must be completely converted into vapor to prevent liquid droplets from entering the suction line and returning to the compressor. To ensure this, DX systems are designed and adjusted to superheat the vapor at the coil outlet.

The Role of Superheat

Superheat is the temperature of the refrigerant vapor above its saturation temperature at a given pressure.

$\text{Superheat} = T_{\text{actual}} - T_{\text{saturation}}$

• Example Calculation: If the suction pressure at the evaporator outlet is $20 \text{ psig}$ (which corresponds to an ammonia saturation temperature of $5.5^\circ\text{F}$) and the actual thermometer reading of the suction gas at that point is $15.5^\circ\text{F}$, the superheat is:

$15.5^\circ\text{F} - 5.5^\circ\text{F} = 10^\circ\text{F} \text{ of superheat}$

Most industrial DX evaporators operate with a target superheat of $8^\circ\text{F}$ to $12^\circ\text{F}$ at the suction outlet. While superheat is vital to protect compressors from liquid slugging, it represents an efficiency penalty because the last $15%$ to $25%$ of the coil is occupied by dry vapor, which has a much lower heat transfer coefficient than boiling liquid.

Limitations of DX in Ammonia Systems

Direct expansion is the standard feed method for halocarbon commercial refrigeration, but it is rarely used in large ammonia systems for several key reasons:

1. Massive Latent Heat of Vaporization: Ammonia has a latent heat of vaporization of $589 \text{ Btu/lb}$ at $5^\circ\text{F}$, compared to only $65 \text{ Btu/lb}$ for R-404A. Consequently, an ammonia system requires a much smaller mass flow rate of refrigerant to satisfy the same cooling load.
2. Extremely Small Orifices: Because the required mass flow rate is so small, the expansion valve orifice in a DX ammonia coil must be tiny. These small orifices are highly prone to clogging from pipe scale, rust, or trace moisture that freezes inside the valve port.
3. Valve Hunting: The combination of small flow rates and high latent heat makes control highly sensitive. TXVs often "hunt"—meaning they swing between overfeeding and underfeeding. This leads to alternating periods of coil starvation (capacity loss) and liquid carryover.
4. Heat Transfer Efficiency: Air-side heat transfer is maximized when the entire internal coil surface is wetted by boiling liquid. Dedicating a portion of the coil to superheating gas decreases the average heat transfer coefficient of the evaporator.

Flooded Evaporators

To achieve maximum heat transfer efficiency, industrial ammonia refrigeration systems utilize flooded evaporators. In a flooded evaporator, the internal heat transfer surfaces are completely wetted by liquid refrigerant throughout the entire operating cycle.

Instead of feeding liquid directly through an expansion valve into the coil, flooded systems separate the expansion process from the evaporator. Liquid refrigerant is expanded into a separate vessel, known as a surge drum (or accumulator), which is maintained at low-side pressure.

Flash gas generated during expansion is immediately vented from the top of the surge drum directly to the compressor suction line. The remaining low-temperature liquid is then fed from the bottom of the surge drum into the evaporator coil. Because the coil is always flooded with liquid, boiling occurs across its entire surface area, and the refrigerant leaving the coil is a wet mixture of liquid and vapor. This mixture returns to the surge drum, where gravity separates the vapor (drawn by the compressor) from the liquid (which falls back to the bottom for recirculation).

Gravity-Flooded vs. Pumped Recirculation Systems

Flooded evaporators are classified into two main configurations based on how liquid is circulated:

1. Gravity-Flooded Systems (Thermosyphon)

In a gravity-flooded system, the surge drum is mounted physically above the evaporator coil. A downcomer pipe feeds liquid from the bottom of the surge drum to the bottom inlet of the evaporator coil. As heat is absorbed, vapor bubbles form within the coil tubes.

This liquid-vapor mixture has a lower density than the solid column of liquid in the downcomer. This density difference creates a natural siphon effect (the thermosyphon effect) that circulates the refrigerant up through the coil and back into the surge drum without mechanical pumps.

2. Pumped Liquid Recirculation Systems (Liquid Overfeed)

In large industrial plants with many evaporators spread across long distances, gravity circulation is impractical. These facilities use mechanical liquid recirculating pumps to pump liquid ammonia from a central low-pressure receiver (LPR) to the individual evaporators.

[Low-Pressure Receiver (LPR)]
       | (Liquid Outlet)
       v
[Liquid Recirculating Pump]
       | (Pumps Liquid)
       +-------------------------> [Liquid Feed Regulating Valve]
       |                                       |
       |                                       v
       |                             [Evaporator Coil (Boiling)]
       |                                       |
       v (Wet Return: Liquid + Vapor)          |
[Return Header] <------------------------------+
       |
       v
[Low-Pressure Receiver (LPR)]
       | (Vapor Separation)
       v
[Compressor Suction Line]

Circulation and Overfeed Ratios

Pumped overfeed systems are designed to supply more liquid refrigerant to the evaporators than can actually be vaporized. This is quantified by the overfeed ratio (also known as the recirculation rate or circulation ratio).

$\text{Overfeed Ratio} = \frac{\text{Mass flow rate of liquid pumped to coil}}{\text{Mass flow rate of liquid evaporated}}$

• 4:1 Overfeed Ratio: Under a $4:1$ ratio, $4 \text{ lbs}$ of liquid ammonia are pumped to the coil for every $1 \text{ lb}$ that vaporizes. The remaining $3 \text{ lbs}$ of liquid exit the coil as liquid, keeping the inner pipe surfaces fully wetted and helping wash out oil back to the LPR.
• Typical Ranges: Industrial systems typically operate at overfeed ratios of $2:1$ to $4:1$ for standard coolers and cold storage rooms. For low-temperature blast freezers, ratios may be increased to $6:1$ to ensure adequate wetting and velocity to move oil through the system.

Liquid Level Controls

Maintaining the correct liquid level in surge drums, accumulators, and low-pressure receivers is critical for safe operation. Level control systems must prevent coil starvation while guarding against liquid carryover to the compressor suction.

Safety Features: High-Level Cutouts

If liquid level control fails and liquid rises too high in a surge drum or LPR, liquid carryover ("slop-over") can occur. Liquid ammonia is incompressible. If liquid enters the compressor cylinder, it will cause hydraulic lock, destroying valves, pistons, screw rotors, and potentially cracking the compressor housing, leading to a major ammonia release.

For this reason, every flooded accumulator and LPR is equipped with a high-level cutout float switch. If the liquid level exceeds the safe limit, this switch immediately closes the main liquid feed solenoid valve, sounds an alarm, and trips the associated compressor to prevent damage.