SPM-8030 Ammonia Refrigeration System

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Process Description

Simtronics’ Ammonia Refrigeration System simulator enables comprehensive training and familiarization for a typical 2-stage reciprocating compressor-based ammonia refrigeration system used for chilling and freezing of packaged foods and other temperature-sensitive products. The Ammonia Refrigeration System employs circulating brine (salt solution) between the ammonia refrigeration equipment and two large warehouses (a freezer and a chiller) to minimize the risk of a toxic leak that would affect employees and property. This design allows the ammonia-containing equipment to be located at a safe distance from the warehouses.

Ammonia refrigeration systems are extremely common in the food industry because ammonia is an excellent refrigerant. The two stages of compression in the Ammonia Refrigeration System provide two separate temperature levels which can be independently controlled. The compressors have two basic forms of capacity control. The first is recycling gas for fine pressure control of the refrigerant evaporators. The second is suction valve unloaders on the head and crank ends of each compressor stage to change gross capacity (0, 50 and 100%). This allows optimizing the operation of the Ammonia Refrigeration System in accordance with chilling and freezing load changes in the warehouses.

A full range of operations can be learned and practiced on the Ammonia Refrigeration System simulator. These include normal, startup, shutdown, and emergency shutdown procedures.

Overview

The Ammonia Refrigeration System uses ammonia as the working fluid and provides chilling at two temperatures: -24.2 DEG F and 19.7 DEG F. Ammonia refrigerant liquid is used to chill circulating brine (a salt water solution with a low freeze point). The cold brine is circulated via independent circuits between the refrigeration equipment and two large warehouses: one for frozen foods and one for chilled foods (non-frozen).

The Ammonia Refrigeration System consists of:

  • Ammonia Compressor, K-601
  • 1st Stage Surge Drum, V-601
  • 2nd Stage Surge Drum, V-602
  • Oil K.O. Drum, V-603
  • Freezer Evaporator, E-601
  • Chiller Evaporator, E-602
  • Condenser, E-603
  • Accumulator, V-604
  • Freezer Warehouse, B-401
  • Freezer Brine Pump, P-401
  • Freezer Brine Expansion Tank, V-401
  • Chiller Warehouse, B-501
  • Chiller Brine Pump, P-501
  • Chiller Brine Expansion Tank, V-501

The main purpose of this process is to compress ammonia vapors from the two evaporators so that they can be condensed using cooling water. The liquid ammonia at high pressure serves as a refrigerant in the two evaporators. There are two operating pressures in the circulating ammonia system:

  • Low pressure (LP) which operates at 1.7 PSIG and -24.2 DEG F
  • High pressure (HP) which operates at 33.4 PSIG and 19.7 DEG F.

After compression of the ammonia vapors by the 1st and 2nd stages of Ammonia Compressor, K-601, they are condensed in Condenser, E-603. The condensed ammonia is collected in Accumulator, V-604. The liquid from the Accumulator is first distributed to the 2nd Stage Surge Drum, V-602. This vessel sits atop of the Chiller Evaporator, E-602, and supplies it with liquid ammonia which evaporates and cools circulating brine used to keep the Chiller Warehouse, B-501, cool.

The 2nd Stage Surge Drum also acts as reservoir of liquid refrigerant for the 1st Stage Surge Drum, V-601, and the Freezer Evaporator, E-601. As a result, refrigerant is supplied to the Freezer evaporator at a lower temperature than is the liquid ammonia from the Accumulator. The Freezer Evaporator, E-601, cools circulating brine used to keep the Freezer Warehouse, B-401, cool.

The vaporized ammonia from each evaporator rises and separates from liquid ammonia in its respective surge drum. The vapor from each surge drum is compressed in the respective stage of the Ammonia Compressor, K-601. The pressure in each Surge Drum/Evaporator determines the minimum achievable outlet temperature of the evaporator. By using two independent evaporator pressures, the Ammonia refrigeration system can optimally deliver cooling for freezing and chilling of the two warehouses.

V-601 1st Stage Surge Drum

The 1st Stage Surge Drum, V-601, sits atop the Freezer Evaporator, E-601. V-601 has three purposes:

  • To supply liquid ammonia to E-601 via two large channels connecting the equipment.
  • To separate rising ammonia vapor from E-601 out of the liquid ammonia for compression in the 1st stage of Ammonia Compressor, K-601.
  • To cool recycle vapor from the discharge of the 1st Stage of K-601.

Liquid ammonia is supplied to V-601 from 2nd Stage Surge Drum, V-602, under level control. In the event of an overfilling of V-601 which would inhibit the operation of K-601, a vent line is provided to vent off ammonia. This ammonia is vented to an ammonia vent system at battery limits.

Any liquid entering the Ammonia Compressor would lead to mechanical damage of the compressor internals and likely would require a shutdown to repair the compressor. In case of high liquid level in V-601, the Ammonia Compressor will be shut off.

To avoid a high 1st Stage compressor discharge temperature when most or all of the compressor flow is being recycled, the 1st Stage recycle vapor is injected into a sparger which is located in the lower section of V-601. The recycle ammonia vapor will mix with the ammonia liquid in V-601 and, therefore, effectively cool it before being recompressed. In the design condition, about 23% of the vapor flow through the 1st Stage of the Ammonia Compressor is recycled in order to regulate the pressure of V-601/E-601.

V-601 is protected from overpressure by safety valve PSV-601 set at 290 PSIG.

E-601 Freezer Evaporator

Freezer Evaporator, E-601, uses LP ammonia supplied from 1st Stage Surge Drum, V-601, to chill circulating brine for the Freezer from -9.3 DEG F down to -18.3 DEG F. The vaporized ammonia rises through the two large channels connecting E-601 and V-601. The pressure of V-601 determines the boiling temperature of ammonia in E-601. By adjustment of the pressure, the temperature of the circulating brine can be changed to suit the particular required temperature of the Freezer.

V-602 2nd Stage Surge Drum

The 2nd Stage Surge Drum, V-602, sits atop the Chiller Evaporator, E-602. V-602 has four purposes:

  • To supply liquid ammonia to E-602 via two large channels connecting the equipment.
  • To separate rising ammonia vapor from E-602 out of the liquid ammonia for compression in the 2nd stage of Ammonia Compressor, K-601.
  • To cool recycle vapor from the discharge of the 2nd Stage of K-601.
  • To cool discharge gas from the 1st Stage of K-601.

Liquid ammonia is supplied to V-602 from Accumulator, V-604, under level control. In the event of an overfilling of V-602 which would inhibit the operation of K-601, a vent line is provided to vent off ammonia. This ammonia is vented to an ammonia vent system at battery limits.

Any liquid entering the Ammonia Compressor would lead to mechanical damage of the compressor internals and likely would require a shutdown to repair the compressor. In case of high liquid level in V-602, the Ammonia Compressor will be shut off.

To avoid a high 2nd Stage compressor discharge temperature when most or all of the compressor flow is being recycled, the 2nd Stage recycle vapor is injected into a sparger which is located in the lower section of V-602. The net ammonia vapor from the 1st Stage of K-601 is also injected in this sparger. The combined ammonia vapor from the sparger assembly will mix with the ammonia liquid in V-602 and, therefore, effectively cool it before being recompressed. In the design condition, about 9% of the vapor flow through the 2nd Stage of the Ammonia Compressor is recycled in order to regulate the pressure of V-602/E-602.

V-602 is protected from overpressure by safety valve PSV-602 set at 290 PSIG.

E-602 Chiller Evaporator

Chiller Evaporator, E-602, uses HP ammonia supplied from 2nd Stage Surge Drum, V-602, to chill circulating brine for the Chiller Warehouse, B-501, from 32.7 DEG F down to 23.7 DEG F. The vaporized ammonia rises through the two large channels connecting E-602 and V-602. The pressure of V-602 determines the boiling temperature of ammonia in E-602. By adjustment of the pressure, the temperature of the circulating brine can be changed to suit the particular required temperature of the Chiller Warehouse.

K-601 Ammonia Compressor

Ammonia Compressor, K-601, is a two-stage reciprocating compressor. Compression occurs in both the head and crank ends of each stage. It is driven by an electric motor connected to a drive shaft which moves the pistons of both stages simultaneously. On each stage, the head end and the crank end of the cylinder can be unloaded by a suction valve unloader. These unloaders hold the suction valves open so that the gas cannot be compressed within the end of the cylinder the valves serve. In effect, the flow capacity of each stage can be set to 100% (no unloaders active), 50% (one unloader active) or 0% (both unloaders active).

Ammonia vapor from the 1st Stage Surge Drum, V-601, is compressed in the 1st Stage of K-601 and discharged into the 2nd Stage Surge Drum, V-602, via a check valve. Some of the 1st Stage discharge vapor is recycled back to V-601 for pressure control.

Ammonia vapor from the 2nd Stage Surge Drum, V-602, is compressed in the 2nd Stage of K-601 and discharged into the Oil K.O. Drum, V-603, to remove any entrained lubricating oil from the ammonia vapor. Most of the 2nd Stage discharge vapor passes on to the Condenser, E-603, via a check valve. The discharge line is protected from overpressure by safety valve PSV-603 set at 290 PSIG. Some of the 2nd Stage discharge vapor is recycled back to V-602 for pressure control.

The ammonia normally discharges from the 2nd (HP) stage of the Ammonia Compressor at 167.1 PSIG and 165.1 DEG F. Ammonia has a high heat of compression relative to other refrigerants so it is critical that the recycle vapors from the 1st and 2nd Stages of K-601 are quenched by passing through cold liquid ammonia in their respective Surge Drums, V-601 and V-602.

E-603 Condenser

Condenser, E-603, condenses ammonia vapor from the discharge of K-601 using cooling water. The design outlet temperature of E-603 is 90.0 DEG F. Most of the heat is used to condense the ammonia. A small amount of subcooling of the condensed ammonia normally occurs. Condensed ammonia flows into the Accumulator, V-604.

A small line connects the top of E-603 with V-604 to allow equalization of the pressures between the two. Without this line, the pressure in the accumulator would drop during times when the ammonia becomes significantly subcooled. This drop of pressure induces a surging of ammonia liquid from E-603 into V-604 and causes the pressure in the system to fluctuate. The small vapor line ensures that the pressure in V-604 is the same as in E-603 and avoids liquid surging.

V-604 Accumulator

Accumulator, V-604, is the main reservoir of ammonia liquid for the Ammonia Refrigeration System. Liquid from Condenser, E-603, is collected and distributed to the ammonia evaporators. V-604 normally operates at 163.1 PSIG which is the equilibrium pressure of ammonia at 90.0 DEG F.

Any non-condensables (e.g. air) present in the system will get trapped in the vapor space of the Accumulator. Non-condensables are vented from the top of V-604. A rise in the pressure indicated at PI-605 (top of V-604) above the equilibrium pressure of ammonia (determined by the temperature of ammonia in V-604) indicates that non-condensables are accumulating in V-604. A vent line and hand valve is provided in case V-604 needs to be vented. The vent can also be used to flash off ammonia liquid to the ammonia vent system at battery limits in case the V-604 is overfilled and the ammonia liquid cannot be transferred to the evaporators.

The liquid level in V-604 will rise and fall depending on the levels in the Surge Drums, V-601 and V-602, and depending on the pressures in the system. For example, if the discharge pressure of the Ammonia Compressor increases, say, due to an increase in cooling water temperature, the liquid level in V-604 will decrease because more ammonia is contained in the vapor space of the accumulator and in the piping of the system at the higher pressure. The volume of V-604 is sized to accommodate these fluctuations without overfilling or starving the flow to the 2nd Stage Surge Drum, V-602.

B-401 Freezer Warehouse

The Freezer Warehouse, B-401, is a large refrigerated warehouse for the storage of frozen foods. It is kept at -4.0 DEG F by its refrigeration system which consists of a fan, internal cooler and brine circulation system. The internal cooler exchanges heat between circulated air inside B-401 and cold brine solution returned from Freezer Evaporator, E-601. A thermostat controls the refrigeration system. The cold brine warms up in B-401’s internal cooler and is returned to the Freezer Brine Expansion Tank, V-401. The warmed brine is circulated from V-401 back to E-601 by Freezer Brine Pump, P-401, which is a centrifugal pump outfitted with a variable frequency drive (VFD) motor. This permits adjustment of flow of the brine flow by B-401’s thermostat by changing the speed of P-401.

The contents of B-401 gain heat through its exterior walls and roof. The amount of heat depends on the ambient temperature and how much warmer material is being added. B-401’s thermostat will regulate the brine flow to keep the interior temperature reasonably constant.

B-501 Chiller Warehouse

The Chiller Warehouse, B-501, is a large refrigerated warehouse for the storage of chilled (non-frozen) foods. It is kept at 37.4 DEG F by its refrigeration system which consists of a fan, internal cooler and brine circulation system. The internal cooler exchanges heat between circulated air inside B-501 and cold brine solution returned from Chiller Evaporator, E-602. A thermostat controls the refrigeration system. The cold brine warms up in B-501’s internal cooler and is returned to the Chiller Brine Expansion Tank, V-501. The warmed brine is circulated from V-501 back to E-602 by Chiller Brine Pump, P-501, which is a centrifugal pump outfitted with a variable frequency drive (VFD) motor. This permits adjustment of flow of the brine flow by B-501’s thermostat by changing the speed of P-501.

The contents of B-501 gain heat through its exterior walls and roof. The amount of heat depends on the ambient temperature and how much warmer material is being added. B-501’s thermostat will regulate the brine flow to keep the interior temperature reasonably constant.

Instrumentation

Ammonia Compressor & Evaporators

Ammonia Compressor K-601

The motor of Ammonia Compressor, K-601, is turned on and off using switch HS-601. In the event of a trip condition, interlock I-601 will lock HS-601 in the STOP state.

The suction valve unloader for the head end of the 1st Stage is operated by switch HS-603A and the unloader for the crank end is operated by switch HS-603B. The suction valve unloader for the head end of the 2nd Stage is operated by switch HS-604A and the unloader for the crank end is operated by switch HS-604B. Normally, the switches for all suction valve unloaders are in the LOAD state. Unloading a cylinder causes flow through the cylinder to stop because the suction valve is continuously held open. When a suction valve is held open, the gas that is pulled into the cylinder when the piston is moving in the intake direction will be pushed back out through the suction valve when the piston reverses to the compression direction. Therefore, it is possible to adjust the capacity of each compression stage at roughly 0, 50 and 100% to better match the flow requirements of the chillers. For example, if the heat loads of the Freezer and the Chiller were to be greatly reduced due to seasonal and/or daily ambient temperature changes, unloading one of the cylinders on each stage will save significant compression energy by reducing how much vapor is recycled for pressure control of the Surge Drums, V-601 and V-602.

The suction pressure of the 1st Stage is controlled by PIC-601 which adjusts the opening of control valve PV-601 to recycle more or less 1st Stage discharge vapors back to V-601. The 1st Stage suction temperature is indicated on TI-601. The 1st Stage flow is indicated on FI-601. The 1st Stage discharge temperature is indicated on TI-602 and the discharge pressure is indicated on PI-602.

The suction pressure of the 2nd Stage is controlled by PIC-603 which adjusts the opening of control valve PV-603 to recycle more or less 2nd Stage discharge vapors back to V-602. The 2nd Stage suction temperature is indicated on TI-603. The 2nd Stage flow is indicated on FI-603. The 2nd Stage discharge temperature is indicated on TI-604 and the discharge pressure is indicated on PI-604.

The level of lubricating oil in V-603 is indicated on LI-603. A switch, HS-603, is provided to drain any accumulated in V-603 oil back to the oil sump of K-601. Normally, HS-603 is in the CLSD state.

Ammonia Compressor & Evaporators

Surge Drums & Evaporators

The level of ammonia refrigerant in 1st Stage Surge Drum, V-601, is controlled by LIC-601 which adjusts the opening of control valve LV-601 to change the flow of refrigerant from V-602. The flow rate of refrigerant through LV-601 is indicated on FI-605. Ammonia vapor from V-601 can be vented using HIC-601 to adjust the opening of control valve HV-601.

The flow rate of Freezer brine to Freezer Evaporator, E-601, is indicated on FI-411. The temperature of the brine to E-601 is indicated on TI-411. The temperature of cold brine leaving E-601 is indicated on TI-412.

The level of ammonia refrigerant in 2nd Stage Surge Drum, V-602, is controlled by LIC-602 which adjusts the opening of control valve LV-602 to change the flow of refrigerant from Accumulator, V-604. The flow rate of refrigerant through LV-602 is indicated on FI-606. Ammonia vapor from V-602 can be vented using HIC-602 to adjust the opening of control valve HV-602.

The flow rate of chiller brine to Chiller Evaporator, E-602, is indicated on FI-511. The temperature of the brine to E-602 is indicated on TI-511. The temperature of cold brine leaving E-602 is indicated on TI-512.

Condenser & Accumulator

Condenser, E-603

The outlet temperature from the Condenser, E-603, is indicated on TI-605. Cooling water flow to E-603 is adjusted by HIC-801 which sets the position of control valve HV-801. The supply temperature of cooling water is indicated on TI-801 and the temperature of cooling water leaving E-603 is indicated on TI-802.

Accumulator, V-604

The level of ammonia liquid in Accumulator, V-604, is indicated on LI-604. Makeup ammonia is supplied to the system using FIC-604 which adjusts the position of control valve FV-604. The pressure of the vapor space of V-604 is indicated on PI-605. Non-condensables and ammonia vapor can be vented from V-604 using HIC-605 which adjusts the position of control valve HV-605. The temperature of ammonia refrigerant leaving V-604 and going to the 2nd Stage Surge Drum, V-602, is indicated on TI-606. The ammonia refrigerant isolation valve ESDV-601 is closed by interlock I-601 via a solenoid-operated positioner.

B-401 Freezer Warehouse

The temperature of the Freezer Warehouse, B-401, is regulated by thermostat TIC-401 which controls the following:

  • Motor of B-401’s circulating air fan (on/off)
  • Motor of Freezer Brine Pump, P-401 (on/off)
  • Speed of P-401 by varying the frequency of the electricity to the motor (VFD)

Switch HS-401 is used to turn TIC-401 on and off. In the OFF state the two motors TIC-401 controls will be turned off. In the ON state, they will be turned on.

TIC-401’s control mode is manually adjusted by the operator independent of HS-401. In MANUAL mode, the speed of P-401 can be manually set using the output of TIC-401. In AUTOMATIC mode, the speed of TIC-401 will be adjusted per the PID control action of TIC-401.

TIC-401 receives its setpoint from indicator TI-401SP. The setpoint is adjustable via a pair of raise and lower switches, HS-401R and HS-401L, respectively. Pressing either switch once will adjust the setpoint by +/-1.8 DEG F. These switches are accessible by clicking on the raise and lower arrows of TIC-401.

The flow rate of Freezer brine to Freezer Evaporator, E-601, is indicated on FI-411. The temperature of the brine to E-601 is indicated on TI-411. The temperature of cold brine returning from E-601 is indicated on TI-412.

Ambient temperature is indicated on TI-420.

B-501 Chiller Warehouse

The temperature of the Chiller Warehouse, B-501, is regulated by thermostat TIC-501 which controls the following:

  • Motor of B-501’s circulating air fan (on/off)
  • Motor of Chiller Brine Pump P-501 (on/off)
  • Speed of P-501 by varying the frequency of the electricity to the motor (VFD)

Switch HS-501 is used to turn TIC-501 on and off. In the OFF state the two motors TIC-501 controls will be turned off. In the ON state, they will be turned on.

TIC-501’s control mode is manually adjusted by the operator independent of HS-501. In MANUAL mode, the speed of P-501 can be manually set using the output of TIC-501. In AUTOMATIC mode, the speed of TIC-501 will be adjusted per the PID control action of TIC-501.

TIC-501 receives its setpoint from indicator TI-501SP. The setpoint is adjustable via a pair of raise and lower switches, HS-501R and HS-501L, respectively. Pressing either switch once will adjust the setpoint by +/-1.8 DEG F. These switches are accessible by clicking on the raise and lower arrows of TIC-501.

The flow rate of chiller brine to Chiller Evaporator, E-602, is indicated on FI-511. The temperature of the brine to E-602 is indicated on TI-511. The temperature of cold brine returning from E-602 is indicated on TI-512.