NIELS VESTERGAARD
GLOBAL APPLICATIONS EXCELLENCE MANAGER
INDUSTRIAL REFRIGERATION
DANFOSS A/S
MORTEN JUEL SKOVRUP
GLOBAL APPLICATIONS EXCELLENCE MANAGER
INDUSTRIAL REFRIGERATION
DANFOSS A/S

ABSTRACT

Ammonia has over decades proven its value as an effective refrigerant, but choosing the right— and correctly sized—defrost and control methods is important to ensure high efficiency.

Traditionally one of two methods for controlling drainage of the evaporator during hot gas defrost is used: pressure control, which keeps the pressure in the evaporator constant during defrost, or liquid drain control, which uses a float valve to drain condensed liquid from the evaporator. Each method’s energy consumption is quite different, as the pressure-control method bypasses a certain amount of hot gas during the defrost period.

This paper is based on results from a research project focusing on energy savings potential during hot gas defrost in ammonia refrigeration systems (ELFORSK project 347-030).

In the ELFORSK project, an ammonia pumped circulation system was built at the Danish Technological Institute, enabling detailed measurements of the defrost system. Two methods of hot gas defrost were tested and analyzed (pressure control and liquid drain method), as were three evaporator designs (bottom feed, top feed, and side/bottom feed). A simulation model was also developed and validated using the measurements.

This paper will focus on the design requirements of the two most common defrost methods for ammonia systems (pressure control and liquid drain method) and describe the design requirements for both systems to obtain the highest efficiency. The efficiency of the two defrost systems will be analyzed and compared.

INTRODUCTION

Over time, air coolers in refrigeration systems, operating below the freezing point, will be covered with ice/rime. To ensure that the system is operating efficiently, the evaporator must be defrosted. The goal of a defrost is to remove the ice/rime from the heat exchanger surface. An effective defrost is a key feature of the system to preserve the plant’s overall efficiency and product quality. In an ideal defrost, all added heat will be used to melt the ice on the evaporator surface, with a minimum of heating of the evaporator coil and the cold room.

Several elements should be considered when evaluating the effectiveness of a defrost, including:

• Removal of all ice/rime from the air cooler surface with minimum energy consumption, including 
  • • Minimum heat transfer into the refrigerated space
    • Minimum transfer of moisture from the surface of the air cooler into the refrigerated space, and
• Electrical energy used for the defrost process;
• Defrost cycle duration; and
• Reliability and safety of defrost process.

Numerous defrost methods are known in the industry. Figure 1 shows the most common. The different systems have their pro and cons when in terms of effectiveness and cost.

Electrical defrost is the most common defrost method with an “external” heat source. From an application point of view, electrical defrost is an easy and attractive solution, but from an operational cost point of view it is very expensive— especially for low-temperature systems.

In hot gas defrost systems the heat comes from within the refrigeration system as “free energy.” However, selecting the right method to control the hot gas supply to the evaporator is important to ensure that energy losses are minimized. Losses typically come from flash gas and noncondensed hot gas passing through the evaporator.

Figure 2 illustrates two methods for controlling the hot gas supply to the evaporator that are traditionally used:

• Pressure control method: the pressure in the evaporator is controlled during defrost with a back-pressure control valve in the defrost drain line. The pressure control method is the most commonly used method in the industry, mainly due to the simple design, but the energy losses are a challenge.
• Liquid drain method: condensed liquid is drained from the evaporator using a float valve in the defrost drain line. The liquid drain method ensures that only liquid refrigerant is drained from the evaporator during defrost, thereby minimizing noncondensed hot gas flow.

Noncondensed hot gas bypass happens when the evaporator cannot condense all supplied hot gas while keeping the pressure at the set point of the pressure control valve. The result is that the pressure control valve will open (to keep pressure at the setpoint) and let the hot gas bypass to the compressor. This does not happen to the same degree with the liquid drain method. A small bleed is necessary in the float valve though to ensure that any flash gas generated in front of the valve can be released to the valve discharge, but this bleed will bypass only a small fraction of the gas that would be bypassed using the pressure control method.

Vestergaard et al. (2016) measured the energy consumption of a system in operation comparing the two defrost control methods. The results showed considerable energy savings using the liquid drain method (Figure 3).

To generalize these results and investigate the influence of the type of evaporator, a series of measurements on different evaporators was made at the Danish Technological Institute in the ELFORSK (347-030) project. In parallel with the measurements, a simulation model was developed and validated using the measurements.

The simulation model was thereafter used to investigate the influence of varying the operating conditions, but also to quantify some of the parameters, which can be difficult to measure on a real system—for example, the mass of refrigerant in the evaporator during defrost and the size of convection losses from the evaporator to the surroundings (i.e., how much the defrost process heats up the cold room).

The findings and experiences gained in this project were collected in a series of design recommendations for hot gas defrost systems, both related to practical issues (such as piping arrangements) but also to recommendations for sizing valves and line components.

TEST SYSTEM

The laboratory test system at the Danish Technological Institute consists of a pumped recirculated liquid ammonia system and a climate chamber.

The amount of ice added to the evaporator surface during normal operation is controlled, and during defrost, the amount of ice removed is also measured to make sure that the ice on the evaporator before defrost is controlled.

The condensing hot gas supply is regulated to a pressure of about 15°C (59°F) (inlet of hot gas defrost valve (4) or (5,6)) from a condensing temperature of 31°C (87.8°F) (see Figure 4 for position of valves). When the defrost starts, the hot gas flow is controlled by either a soft gas solenoid or a slow opening solenoid:

• Soft-gas solenoid: first the soft-opening valve (6) is opened for 10 minutes and thereafter the main defrost valve (5). The soft-opening valve has a capacity of about 10% of the fully opened defrost valve. Measurements in the following Test Results section are all taken with the soft gas solenoid.
• Slow-opening solenoid: the motor valve (4) opens slowly (from closed to fully open in 160 s). Measurements taken with the slow-opening solenoid are shown later in the Discussion section.

The temperature in the pump separator is approximately -22°C (-7.6°F) and is kept constant through all experiments.

When using the pressure control method (valve (7)) the defrost pressure is set to 7.3°C (45.1°F). Table 1 summarizes the operating conditions. When designing hot gas systems, considering the design of the actual evaporator type is important. Bottomfeed evaporators without distribution orifices are very common in Europe, whereas top feed and side feed are the most common types in the United States. Top-feed evaporators normally have distribution orifices at the inlet, which means that hot gas is injected through the orifices creating additional pressure drop. Side-/bottom-feed evaporators have distribution orifices in the liquid inlet/condensate drain outlet, which means that liquid drain during defrost must pass through the orifices creating additional flash gas before the drain valve.

All three types of evaporators were tested using both the pressure control and liquid drain methods to control the hot gas supply during defrost (Figure 5).

TEST RESULTS

Table 2 shows the results of the visual evaluation of the defrost process for the different evaporator types (Appendix A provides details about the evaporators).

Figure 6 supports the conclusion of the evaluation. Figure 6 shows the evaporators in the following conditions:

• The system ran for approximately 60 hours and 50 kg (110 lb) of ice formed on the surface of the evaporator.
• BF1 and SF1 are from the start of the defrost (50 kg (110 lb) of ice). • BF2 and SF2 are from after 12 minutes of defrost. SF2 shows the uneven defrosting with minor refreezing of ice on the bottom.
• BF3 and SF3 are from after 23 minutes of defrost (defrost completed).

Figure 7 shows the measured mass flow of hot gas for the different evaporator configurations and drain control methods.

For the liquid drain method, the shape of the mass flow curves differs slightly depending on the evaporator configuration, but the defrost duration does not seem to be affected by the evaporator configuration.

The peak mass flow for side-feed evaporator is higher than for the rest of the measurements. The operating conditions (pressures) were approximately the same for the different tests, so currently we believe that the higher mass flow for the side-feed evaporator is due to uncondensed gas mass flow passing through the evaporator in the top pipes, which have the largest orifice size. This gas flow continues while ice remains on the evaporator, and the gas must pass through the bleed when the drain float is installed. Whether more gas passes uncondensed through a side-feed evaporator still needs to be confirmed.

The effect of the distribution orifices on the evaporator outlet is evident when looking at the mass flow for pressure control and side-feed evaporator. The evaporator outlet pressure is kept constant by the pressure control valve, and as the pressure drop through the evaporator increases because of the orifices, the hot gas pressure at the inlet to the evaporator increases. At the same time, the pressure after the back-pressure regulator (valve (3) in Figure 4) is kept constant, so when the pressure at the evaporator inlet increases, the available pressure difference across the main hot gas solenoid valve (5) decreases, which means that the mass flow will also decrease (see also measured pressures in Figures 9 and 10).

The defrost time for the different evaporator configurations can be seen by looking at the mass of drained water during the defrost (Figure 8).

Looking at the drained water mass in Figure 8, concluding that the defrost time changes significantly by changing the evaporator configuration is difficult—especially given that the amount of ice on the evaporator was not exactly the same before a defrost was started. If anything, the side-feed configuration seems to defrost a bit faster, but this relates to the conclusion of the visual inspection (Table 2) where the defrost for the side-feed evaporator resulted in uneven defrost with minor refreezing. The top-feed evaporator also appears to defrost slightly more slowly than the others, but only by a few minutes.

Figures 9 and 10 show the measured hot gas pressure at the inlet and the outlet of the evaporator for the different configurations and control methods.

What should be noted when looking at the pressure curves is that

• The pressure increases more slowly for the liquid drain method during the filling period where the first step of the hot gas solenoid is opened. This is because the float valve opens as soon as liquid is present in the drain line. For the pressure control method, the pressure in the evaporator needs to build up to the set pressure for the control valve before it opens.
• For the pressure control method, the ice starts melting before the main step in the twostep solenoid opens.
• For the pressure control method and side-feed evaporator, the pressure drop through the distribution orifices is significantly higher than for the liquid drain method
• The pressure at the end of the defrost is higher for the liquid drain method than for the pressure control method, simply because the pressure in the evaporator rises to the regulated hot gas pressure as the flow decreases.

SIMULATION MODEL

Figure 11 illustrates the hot gas defrost system being modeled. The hot gas line directs hot gas from the compressor discharge to the hot gas valve and the soft-opening or slow-opening solenoid. Components located downstream from these valves such as pipes, stop valves, solenoids, etc., are collected into one inlet resistance.

After the evaporator, the drain line consists of an outlet resistance (collecting pipes, bends, stop valves, etc.) and either a pressure-controlled valve intended to keep the pressure in the evaporator constant or a liquid drain valve that opens only when liquid is present. As indicated in Figure 11, the liquid drain valve is equipped with a bleed to remove any gas in the drain line. The drain line leads to the lowpressure separator shown in Figure 4.

Skovrup et al. (2017) presents the details of the model, but the following explains the basic principles.

The valves in the hot gas and in the drain line are modeled using the valve equations from EN 60534 2011 (EN 60534-2-1 is identical to IEC 60534-2- 1 and ANSI/ASI-75.01.01). The control valves (3) and (7) are moreover modeled as proportional regulators, with a smoothened opening curve (to help the numerical solver).

The evaporator is modeled as one lumped refrigerant volume with thermal mass in the refrigerant, the evaporator wall, and the ice on the evaporator (Figure 12).

The evaporator arrangement (top, bottom, side feed) has not been included in the model, but any pressure drops in the evaporator inlet or outlet (from orifices) are included in the inlet or outlet resistances.

For the pressure control method, the drain valve (9) is modeled in such a way that first the condensed liquid (if any) passes through the valve, and then the flow is “topped up” with saturated gas (or superheated gas if no liquid is present in the evaporator).

For the liquid drain method, if the valve is large enough to handle the amount of liquid condensed in the evaporator, then the valve lets exactly this amount pass plus an amount of gas decided by the bleed in the drain valve. If the amount of generated liquid is larger than the maximum allowable mass flow through the fully opened drain valve, then only the maximum liquid mass flow passes plus the gas mass flow through the bleed (this situation will result in an increase of mass in the evaporator).

 

The simulation model has four states, which are used to shift logically among different sets of equations (defining the states enables the differential equation solver to handle discontinuities in the equations):

1. Filling of evaporator, either by soft-gas solenoid or slow-opening motor valve.
2. Heating of ice, where ice is heated from its initial temperature to 0°C (32°F). State 2 runs simultaneously with state 1. The model can change to state 3 either before or after state 1 ends.
3. Melting of ice continues until mass of ice on evaporator reaches 0 kg (can run simultaneously with state 1).
4. Heating of room, where all ice has been removed and heat is just added to the room. Continues until defrost ends.

Model Validation

The following model validation is done using the measurements from the bottom feed evaporator.

Figures 13 and 14 show the measured and simulated pressures in the evaporator for the pressure control and liquid drain methods and the mass flow into the evaporator. In each figure, the four states (1: filling, 2: heating of ice, 3: melting, and 4: heating of room) are indicated on the secondary yaxis on the right-hand side. Note that state 1 and state 2 run at the same time, and that the model shifts to state 3 for pressure control before the soft filling finishes at 600 s.

The qualitative shape of the simulated pressure curves follows the measurements satisfactorily. The pressure illustrates the difference between liquid drain and pressure control. At approximately 900 s, liquid generation in the evaporator starts to drop. The liquid drain method reacts by reducing the mass flow (it only allows liquid through the valve), and the pressure rises to the evaporator inlet pressure dictated by the hot gas line. The pressure control method, however, keeps the pressure almost constant, which means that the pressure control valve needs to allow an increasing amount of gas to flow out of the evaporator.

Note that in the simulations, the liquid drain method starts melting the ice later than the pressure control method but ends earlier, i.e., the defrost period is slightly shorter. This is also supported by the measurements, where the pressure for the pressure control method has a plateau of constant pressure from about 480 s to 600 s where the first step in the twostep solenoid ends, indicating that melting starts before the first step is finished.

Comparing the simulated and measured mass flow in Figure 14, the simulations appear to agree well with the measurements from the end of the filling time (where the main solenoid valve opens) to the defrost end. Agreement between measurements and simulations during filling is, however, not satisfactory. This is probably due to the fact that the drain pan is not part of the simulation.

For the liquid drain method, measurements using the slow-opening solenoid valve were also taken. Figure 15 shows the measurement and simulation results.

The results show that the simulation model can also reproduce—qualitatively—the pressures and mass flows in the evaporator if the two-step soft-opening solenoid is replaced by a slow-opening motor valve. The measurements show 0 mass flow the first 60 seconds of the defrost period. We have not been able to satisfactorily explain why this happens, but it might be because the motor valve does not open at low control signals.

DISCUSSION

The validated simulation model has been used to investigate and quantify in detail energy consumption, soft versus slow opening hot gas valve, importance of pressure drops and defrost temperature, and the refrigerant charge.

Energy Consumption

For both pressure-controlled and liquid drain, a mass flow of uncondensed gas passes through the evaporator. For liquid drain, the mass flow occurs through the bleed in the liquid drain valve, and for the pressure-controlled case it simply flows through the valve together with the liquid. This gas flows to the separator and then into the compressor and is essentially equal to a hot gas bypass or loss.

The pie diagrams in Figure 16 show the distribution of the energy consumption during defrost for simulations of two different defrost durations. The energy consumption is split into

• Compressor, i.e., hot gas flowing uncondensed through the evaporator and back to the compressor. To calculate the compressor power consumption, an isentropic efficiency of 0.7 has been assumed.
• Convection, the energy loss to the cold room by convection during the defrost. The convection loss will be negative in the beginning of the defrost (the ice is colder than the cold room) and will grow proportionally with the temperature of the ice/evaporator. Energy to remove the heat after the defrost is not included.
• Ice heating, the amount of energy necessary to heat the ice from initial temperature (-22°C/ -7.6°F) to the melting point. The initial temperature is chosen according to measured test conditions.
• Ice melting, melting of the mass of ice on the evaporator. In all simulations, 50 kg (110 lb) of ice on the evaporator has been assumed.
• Coil heating, the necessary energy to heat the coil during defrost. The necessary energy depends on the mass and specific heat of the evaporator material. Energy to cool the coil after the defrost is not included.

Ice heating and ice melting are constant no matter what defrost control method is used.

For the liquid drain method, the temperature of the refrigerant will end up being slightly higher than for the pressure control method, assuming that saturated conditions exist in the evaporator during the defrost (this will normally be the case as convection loss will keep refrigerant condensing even when no ice remains on the evaporator). This is because the liquid drain method allows the pressure to rise up to the hot gas supply pressure, whereas the pressure control method will keep the pressure at a defined level (see also Figure 13). This means that the coil heating and convection losses will be slightly higher for liquid drain than for pressure control, but as Figure 16 shows, the amount of gas bypass for the pressure control method dominates the losses and takes up a significant part of the difference in energy consumption between the two methods—especially if the defrost duration is longer than the minimum necessary to melt the ice.

Table 3 summarizes the calculated energy savings for the two control methods.

The amount of gas bypass in the pressure control method also has another interesting consequence. At the end of the defrost process, the gas mass flow can exceed the amount of gas generated in the evaporator during normal operation— i.e., the load from the evaporator on the compressor(s) is larger during defrost than during normal cooling operation.

In the simulation results shown in Figure 17, the total defrost refrigerant flow is separated into gas mass flow and liquid mass flow. The gas mass flow is then converted to compressor power (using an isentropic efficiency for the compressor of 0.7), which for a given gas mass flow can be read on the right-hand y-axis. The gas mass flow for the liquid drain method is significantly smaller than for the pressure control method, resulting in much less compressor power used for recompressing the gas.

The capacity of the evaporator used in the calculations is approximately 20 kW (5.7 TR) at 22°C (-7.6°F), so 16 kW used to recompress bypassed gas during defrost is certainly larger than the power need to deliver 20 kW (5.7 TR) cooling at -22°C (-7.6°F).

Soft- or Slow-Opening Solenoid

The simulation model has been used to investigate the pressure and mass flows during initial opening of the hot gas supply. The reason for this is that high mass flow peaks were observed both in measurements and in simulations when the second (large) step of the two-step solenoid was opened.

Note: The valves in the simulation have been sized according to the principle described in the section “Dimensioning of Hot Gas Defrost Systems” later in this paper.

When the first step in the two-step solenoid is opened, hot refrigerant flows into the evaporator and starts to condense. This liquid refrigerant is—in the case of pressure control—not drained from the evaporator, simply because the pressure has not increased above the set point of the controller yet. So, when the second step is opened, a chance exists that the observed peak in refrigerant flow can accelerate the condensed liquid in the evaporator causing safety problems downstream from the evaporator.

Typically, sizing the low step of the two-step solenoid to 10% of the size of the main step is recommended (this was also done on the test system). To investigate the consequence of the sizing, simulations were carried out varying the size of the first step from 10% to 50% of the main step and comparing to a slow-opening motor valve, where the opening time was varied from 2 to 10 min (Figure 18).

The simulations show that 10% of the main step does not increase the pressure in the evaporator enough during the first step to avoid the peak in mass flow. A better size would probably be 20–30%. Looking at the motor valve, the peak in mass flow disappears when the opening time is longer than 3 min. Also the pressure rises continuously without large gradients for the motor valve when opening time is longer than 3 min. So controlling hot gas supply with a slow-opening motor valve seems to be an attractive method from a safety point of view and suggests that starting the defrost may be possible without draining the evaporator prior to injecting the hot gas. This method may be relevant to consider when keeping the defrost time short is important.

Defrost Temperature

The saturated temperature of the hot gas has an influence on the defrost time: the hotter the gas, the shorter the defrost time and vice versa. Figure 19 shows the simulated defrost time as a function of the regulated hot gas temperature.

The plot is simulated using the liquid drain method with a soft-opening valve (two-step solenoid) where the first step is opened in 2 min.

Figure 19 shows that if, for example, you increase the regulated hot gas temperature by 5 K (9°F) from 15°C (59°F), then the defrost time is lowered by about 2 min. If you decrease the regulated hot gas temperature 5 K from 15°C, then the defrost time is increased by 5 min.

Refrigerant Charge

As noted earlier, the pressure control method does not start to drain liquid from the evaporator until the pressure reaches the set point of the valve. This means that the amount of refrigerant in the evaporator during defrost is higher for the pressure control method than it is for the defrost drain method.

Figure 20 shows the total amount of refrigerant in the evaporator during a defrost using the two control methods.

Both simulations start with the same initial amount of refrigerant. Clearly, the refrigerant mass in the evaporator is significantly higher for the pressure control method, where the amount of liquid refrigerant fills almost 20 % of the evaporator before the control valve opens. This indicates that the liquid drain method should be considered when designing systems for low charge.

Dimensioning of Hot Gas Defrost Systems

Several elements must be considered when designing hot gas defrost systems, but besides safety, energy efficiency and defrost speed are the two most important elements. If speed is the most important design criterion, the defrost components should be selected accordingly, but the penalty of high speed is reduced energy efficiency, depending on the control method.

Selecting the components for a defrost system—whether optimizing for speed, energy efficiency, or both—essentially means calculating the required capacity of the components. Calculating the capacity needs detailed information about the operating conditions the components will work under, and these are sometimes very hard to get. Estimating operating conditions includes calculating pressure drop and density of refrigerant before and after the components (including two-phase flow), but also estimating the required mass flow of hot gas to get a satisfactory defrost.

The following sections provide an overview of how to estimate the necessary capacity of components in a hot gas defrost system. Also included in the overview are practical items to consider and take care of when the system is designed.

Dimensioning Quality

The dimensioning quality is used to determine the position of point D at the inlet to the defrost drain line (see Figure 21).

The term “quality” is a measure of the mass flow of gas compared with the total mass flow of refrigerant. The dimensioning quality differs significantly based on the drain control method you select.

For the liquid drain control method, the dimensioning quality should always be 0.0; i.e., the refrigerant in point D is saturated liquid (Figure 21). The function—or purpose—of a float valve in the defrost drain line is to avoid (as far as possible) gas passing through the float valve and only letting liquid pass through. For the pressure control method, the defrost process will be quite different. Initially, all hot gas supplied to the evaporator will condense, and the valve will only see liquid at the inlet. Later in the process, some gas will not condense in the evaporator, and the valve will see a mixture of liquid and gas. This process is illustrated from D* to D in Figure 21.

Selecting the right dimensioning quality for pressure-controlled drain valves is very important for selecting the right valve size. If a dimensioning quality of 0.0 is selected (saturated liquid), then the resulting valve will be relatively small, which could mean that defrost will be prolonged at the end of the defrost cycle as gas cannot pass through the valve efficiently. It will also mean that pressure in the evaporator can rise to the hot gas supply pressure, which is not always wanted.

If a dimensioning quality of 1.0 is selected (saturated gas), then the resulting valve will be relatively large, meaning that a lot of gas will be bypassed (which equals larger energy consumption) and the valve can become unstable when pure liquid enters the valve in the beginning of the defrost cycle.

Using a relatively low dimensioning quality equal to 0.05 ensures that the valve is stable when liquid enters it and that the amount of bypassed gas is minimized.

Sizing a Hot Gas Defrost System

The defrost system is a very dynamic system, but applying appropriate design parameters simplifies the selection and calculation process significantly. The pressure drop in the hot gas line is often assumed to be less important, but calculating it as precisely as possible is strongly recommended, especially for systems with floating condensing pressure, where low condensing pressure may appear.

If the maximum supply pressure is significantly higher than needed, consider an outlet pressure regulator in the hot gas supply line (regulated hot gas) to reduce the pressure before the evaporator is good practice. Supply pressure that is too high may lead to increased pressure in the evaporator (for the liquid drain method it will lead to increased pressure) and significantly increased gas bypass mass flow in pressure-controlled defrost systems. Especially for large evaporators, regulated hot gas is recommended for safety reasons.

Evaporator Types

When designing hot gas systems, considering the design of the actual evaporator type is important:

• Top-feed evaporators normally have distribution orifices at the inlet, which means that hot gas is injected through the orifices during defrost, creating an additional pressure drop in the hot gas supply to the evaporator.
• Side-/bottom-feed evaporators have distribution orifices in the liquid inlet/ condensate drain outlet. The presence of these orifices needs to be considered when sizing the drain control device (the orifices will create flash gas before the drain control device).

For the pressure control method, the hot gas is injected into the evaporator, and the pressure is gradually built up. When the pressure reaches the set pressure of the regulator, the regulator will maintain the pressure in the evaporator by draining the condensate from the evaporator. After a few minutes, the hot gas mass flow into the evaporator is assumed to be equal to the combined liquid and gas mass flow out of the evaporator. The mass flow into the evaporator is then only a function of the pressure difference over the complete inlet line as shown in Figures 22 and 23, and the capacity of the pressure control valve needs to be selected so that the valve can maintain the pressure at the preset value (typically +10°C (50°F)).

Initially, the total mass flow out of the evaporator is liquid, but later in the defrost process a significant amount of gas will also pass through the pressure regulator. The capacity of the pressure control valve varies significantly depending on the quality of the fluid. Using a dimensioning quality of x = 0.05 leads to a valve size that ensures sufficient capacity for removing typical acceptable gas bypass (Table 4).

When sizing the pressure control system for the hot gas inlet, including all components and pipes is important. For evaporators, adding an additional pressure drop for any distribution orifices may be necessary.

According to the performed tests, the gas bypass for the pressure control method depends on the hot gas flow, but because predicting the exact pressure drop in the complete hot gas line precisely can be difficult, a manual adjusting valve may be considered a good feature (not shown).

Sizing Liquid Drain Systems

When sizing the valves at the inlet of the hot gas system controlled with the liquid drain method, the same dimensioning rules apply as described for pressure-controlled systems (compare Figure 22 and Figure 23). The mass flow in liquid drain systems is controlled by the amount of liquid condensate generated in the evaporator. When the condensate flow starts to decrease, the hot gas mass flow follows, lowering the pressure drop in the hot gas line. For liquid drain systems, considering the maximum supply pressure is important, which must be compatible with the design of the evaporator. An outlet pressure regulator is normally required to ensure that the inlet pressure is kept within acceptable limits. From a control point of view, the liquid drain method is very simple and “self-adjusting” according to the defrost drain demand.

An additional benefit with “self-adjusting” of the mass flow in the liquid drain method is the reduced pressure drop in the inlet line. For identical hot gas system designs, the liquid drain method can defrost at a lower discharge pressure compared with pressure-control systems.

Defrost Drain Line

Despite the simplicity of the liquid drain method, a couple of issues have to be considered carefully. Roof-mounted valve stations are very common, meaning that the liquid drained from evaporators is “lifted” to the liquid drain valve on the roof, which could be situated 5 m (16.4 ft) or more above the evaporator. It is therefore extremely important that the liquid drain valve has a bleed to remove any flash gas created due to pressure drop and liquid lift.

For side-/bottom-feed evaporators, the liquid drain must pass the distribution orifices, which creates additional pressure drop/flash gas, and this gas flow needs to be considered when dimensioning the gas bleed in the liquid drain valve.

A bleed with a flow coefficient of approximately 5–7% of the Kv-value (Cvvalue) of the float valve is normally sufficient for well-designed systems. The gas bypass is a loss, but the mass flow of gas is typically only around 10% of what the mass flow of liquid through a bleed of the same size would be.

Special attention needs to be paid to the liquid drain from the evaporator when it is lifted in a riser. For the liquid drain method, the velocity out of the evaporator decreases during the defrost process and the liquid is not “blown out” in the same way as it is for pressure control. Therefore the outlet of the evaporator must be designed so that liquid does not accumulate in the lower pipe. Therefore a “P-trap” near the inlet of the riser is strongly recommended (Figure 24).

CONCLUSIONS

Hot gas defrost is the most common method for defrosting evaporators in industrial refrigeration. Several elements are important to consider, when evaluating the effectiveness of a defrost:

• Reliable and safe defrost process;
  • Minimum heat transfer into the refrigerated space,
  • Minimum transfer of moisture from the surface of the air cooler into the refrigerated space, and
  • Minimum flash gas and noncondensed hot gas bypassing through the evaporator (gas will flow directly to the compressor for recompression); Removal of all ice/rime from the air cooler surface with minimum energy:
• Electrical energy to conduct the defrost process; and
• Defrost cycle time.

Different types or configurations of air coolers are used in the industry depending on accepted practice in different parts of the world. Performance testing and analysis shows that awareness of the actual design is necessary when sizing the defrost system, but also that defrost time (if system is properly sized) is largely unaffected.

The pressure control method is a commonly used method to control the defrost process, but the measurements show clearly that this method allows large amounts of noncondensed hot gas to bypass through the evaporator, increase the compressor load, and reduce the defrost efficiency.

The measurements show that the liquid drain method has a higher efficiency than the pressure control method because only liquid condensate is drained in this method, but measurements also show that the performance can be affected if the system is not configured properly.

The use of a “defrost capacity factor” has been found to be an easy and suitable tool to use when sizing inlet and outlet pipes. This factor allows easy selection of defrost controls, pipes, etc., based on evaporating capacity.

The simulation tool, calibrated with test results, has shown to be useful when analyzing the effect of sizing key elements when changing the boundary condition:

• The size of the first step of a two-step solenoid in a hot gas line must be bigger than 10%; 20–30% is a better fit.
• A slow-opening solenoid valve (motor valve) in hot gas lines is the perfect solution to ensure safe smooth pressure build-up.
• The efficiency of the pressure control method is significantly more sensitive to correct termination of the defrost cycle time than the liquid drain method.
• The hot gas temperature (saturated temperature) is the most important factor for the defrost cycle time. For example, if you increase the regulated hot gas temperature by 5 K (9°F) from 15°C (59°F), then the defrost time is lowered by about 2 min. If, however, you decrease the regulated hot gas temperature 5 K from 15°C, then the defrost time increases by 5 min.
• The load from an evaporator on the compressor(s) is larger during defrost than during normal cooling operation when using the pressure control method.

When sizing a pressure control system, considering that the hot gas mass flow is controlled by the actual set pressure of the control valve and the pressure drop in the inlet line is important; the whole hot gas inlet system, regarding pressure drop, needs to be considered.

In a liquid drain system, the hot gas mass flow is controlled by the actual condensate flow, and design considerations are less sensitive because of the self-adjusting control of the valve. In this project, many tests were carried out and analyzed, but a lot of work still remains in understanding the details of the hot gas defrost process—especially regarding automatic detection of when to start and when to stop the defrost.

APPENDIX A

Technical data on the tested evaporators:

BIBLIOGRAPHY

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ACKNOWLEDGEMENTS

We would like to thank the participants in the ELFORSK 347-030 research project (Danish Energy Association). Without the contribution from this project, it wouldn’t be possible to write this paper:

• Danish Technological Institute: Jóhannes Kristófersson and Lars Reinholdt
• IPU: Jorrit Wronski
• Johnson Controls: Alexander Cohr Pachai