Sentry Equipment - Sampling Systems - Sampling Equipment

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TCU Frequently Asked Questions (FAQ)

 Why doesn't Sentry use an accumulator in the suction line?

An accumulator is a safety device, eliminating the possibility of liquid refrigerant traveling to the compressor. Sentry's TCU is designed so it does not require an accumulator. The TCU uses a thermal expansion valve in conjunction with a crankcase pressure reducing valve. The combination adjusts refrigerant flow through the evaporator ensuring the refrigerant at the evaporator outlet is gas. The thermal expansion valve adjusts the flow of liquid refrigerant through the evaporator by holding the superheat constant. Superheat is the difference between the temperature at the evaporator outlet and the temperature implied by the refrigerant pressure at the evaporator outlet. Consistently superheating the refrigerant ensures the evaporator is never flooded. If the load decreases and the superheat falls, the TEV allows less liquid into the evaporator until the superheat climbs.

Because of the "high" temperature in the evaporator the refrigerant pressure and temperature at the outlet of the evaporator is not ideal. Sentry uses a crankcase pressure reduction valve to decrease the refrigerant pressure to 80 psig (5.5 bar). A suction pressure of 80 psig (5.5 bar) takes advantage of the optimal operating range of the compressor increasing efficiency.

 Why is there no liquid receiver?

A liquid receiver is a storage tank for liquid refrigerant. The Sentry designed TCU is not a "critical charge" refrigeration unit. A critical charge unit relies on the refrigerant charge to control the evaporator pressure. Critical charge units generally have constant loads. Typical TCU loads vary when sample lines are enabled or as primary cooling water characteristics change. Sentry uses a thermal expansion valve to control evaporator pressure. The TEV throttles the refrigerant flow into the evaporator to hold evaporator pressure and superheat constant. Adjusting the refrigerant flow in the evaporator allows the load to change without changing the evaporating temperature. Sentry sized the condenser such that the refrigerant is completely condensed and subcooled. Before feeding any gas to the TEV, high discharge pressure trips the unit.

 Does Sentry use a MOP (maximum operating pressure) TEV to prevent liquid entering the compressor?

A MOP style thermal expansion valve limits the refrigerant pressure available to the evaporator. Often at startup, the process water is hot. Elevated temperatures on the process side of the evaporator elevate the temperature and pressure inside the evaporator. A MOP limits the maximum refrigerant pressure in the evaporator. As the evaporator pressure reaches the MOP set point the TEV closes, starving the evaporator thereby limiting the pressure. A Sentry TCU does not us a MOP type thermal expansion valve. Typical MOP valves have a charge in the bulb to control pressure at or less than 100 psig (7 bar). The evaporator on the TCU is controlling process water temperature at 77° F (25° C). The superheat is designed to be 12° F, implying an evaporating temperature of 65° F (18° C). Evaporating at 65° F corresponds to a R-22 pressure of 110 psig (7.5 bar). An off-the-shelf MOP will not work in our system. Instead Sentry uses a crankcase pressure regulator to limit the pressure at the suction of the compressor. The crankcase pressure reduction valve reduces the evaporator pressure of 110 psig (7.5 bar) to 80 psig (5.5 bar). A suction pressure of 80 psig corresponds to a refrigerant temperature of 53° F (11.7° C). The compressor uses the refrigerant to cool itself. It is necessary to keep the refrigerant cool to operate the compressor inside its optimal range and prevent overheating. The TCU is designed to operate at a condensing temperature of 130° F (54.5° C) and an evaporating temperature at 65° F (18° C) at full load.

 Why does my TCU trip on high head pressure?

The TCU uses a discharge pressure switch to protect the compressor from overpressure. The switch is reset manually by pressing the knife edge switch on the front of the pressure switch. Resetting the switch restarts the TCU. Head pressure created by the compressor is controlled by conditions in the condenser. Condenser problems are created a couple of different ways.

The condenser on a water cooled TCU requires cooling water above a minimum flow rate and below a maximum temperature. Cooling water flow rate and temperature control the discharge pressure of the compressor. To diagnose the discharge pressure problem, begin by measuring cooling water flow rate and temperature. The GA of the TCU included in the manual lists the required flow rates for each size of TCU. An adequate amount of cooling water flow is critical for keeping the discharge pressure down. The discharge pressure switch is set to trip at 400 psig (27.5 bar), which corresponds to a refrigerant temperature of 150° F (65° C). The TCU is guaranteed to operate at cooling water temperatures up to 105° F (40° C). The flow rate required rises as the cooling water temperature rises. Flow rate can either be directly measured or inferred from the pressure drop across the condenser. Contact Sentry for assistance calculating the flow rate using inlet pressure and pressure drop. Suspended solids in the cooling water collect in the inlet port of the condenser and block flow through channels of the heat exchanger. Using low quality cooling water to supply the condenser requires periodic backflushing to clean the condenser. An air cooled condenser requires the same amount of care. It is imperative to allow adequate room on each side of the condenser for proper air flow (see the manual for specifications). Periodically the underneath of the condenser should be cleaned of dirt. An accumulation of dirt will degrade the performance of the condenser.

Two other refrigeration problems occasionally cause high head pressure. One is too much refrigerant. A combination of high load and an overcharge can trip the unit. Every TCU is factory charged and tested, the refrigerant charge is etched on the nameplate of the TCU. A malfunctioning TEV may also affect the discharge pressure. This particular problem is addressed in question 5.

 The evaporator inlet line is iced up and the TCU is tripping on high head pressure - what can I do?

Ice buildup between the evaporator inlet and the TEV is a sign of a malfunctioning TEV. The ice occurs from condensation freezing on the tubing because the refrigerant is too cold. The TEV is stuck closed; this drops the pressure so much that the refrigerant temperature is below freezing. It is beneficial to test the TEV. To test the valve, pull the bulb from the evaporator outlet and immerse in hot water. A warm bulb opens the valve allowing more liquid refrigerant into the condenser; this in turn increases the suction pressure balancing the valve. Immersing the bulb in ice water reverses this reaction. If there is no reaction by changing the temperature of the bulb, the TEV is bad. Occasionally the TEV can recover. The valve may need to shock to break it loose.

 I shut my TCU down and when I try to restart it runs for a couple of seconds and shuts down on high head pressure.

A condenser filled with liquid refrigerant causes this problem. Sometimes it is difficult for the compressor to clear a completely liquid filled compressor. The refrigerant becomes completely condensed because of cooling water around or below 50° F (10° C). To restart the TCU, begin by throttling back on the cooling water. Limiting the cooling water will allow the refrigerant to heat up quicker. Then, turn on the TCU and watch the discharge pressure. Only run the unit until it is near the trip point (400 psig / 27.5 bar). Cycle the TCU on and off until it stays in operation. Every time the unit is cycled it will run for a longer period. A water-saver valve automatically limits the flow of cooling water in the condenser. Contact Sentry for more details.

 I think my TEV may be bad - how can I measure the superheat to check it?

Superheat is defined as the difference between the evaporator outlet temperature and the saturation temperature at the current evaporator outlet pressure. The TEV holds the superheat constant at 12° F (6.6° C) by adjusting refrigerant flow in the evaporator. Sentry provides a suction pressure gauge but it is downstream of the crankcase pressure reduction valve (CPR). Thus, it is not possible to directly measure evaporator outlet pressure. Fortunately, the CPR valve has a constant Cv and the evaporator outlet pressure can be inferred. The suction pressure is set at the factory to 80 psig (5.5 bar). The pressure drop across the CPR is 30 to 35 psig (2 to 2.5 bar). To measure the superheat, add 35 psig (2.5 bar) to the suction pressure and lookup the saturation temperature. Ideally, the superheat is constant over changing load conditions. To test whether the TEV is actuating, pull the bulb off the evaporator outlet line and immerse in ice water. In ice water the TEV closes and the suction pressure falls. Immerse the bulb in hot water and the TEV opens, raising the suction pressure.

 My compressor runs but the chilled water is not cooling and the discharge pressure is low - why?

Inability to control at temperature with a low discharge pressure is a sign of a low refrigerant charge. Typically the problem is that the condenser is working well, but there is not enough refrigerant volume to feed to the evaporator. Thus, the suction pressure can be a little high even though the discharge pressure stays low. The refrigerant is lost due to a leak. Depending on the severity, the TCU can run for quite some time before shutting down. Evenutally, the low pressure switch trips on low suction pressure. The TCU can be restarted after the leak is repaired and the unit recharged.

 The chilled water temperature is cycling more than 1 degree above and below the set point temperature. How can I improve the control?

Sentry specifies the ability to control chilled water temperature to 1 degree (.5° C) of the set point. A PID controller operates a hot gas bypass valve to achieve this accuracy. The PID parameters have been calculated and programmed at Sentry. Unfortunately, there is no way to predict every possible operating scenario. If the chilled water temperature is cycling too much, letting the controller auto-tune the PID parameters can help. It is beyond the scope of this FAQ to discuss PID control, but it is easy to let the controller calculate the parameters. A Partlow 1/6 DIN Temperature Controller manual is included with the TCU. The auto-tune procedure is included in the manual (click here to see a list of manuals).

 Could the water on the closed loop side damage the heat exchangers?

Often condensate is readily available to use in the closed loop side of the TCU. Condensate is clean and a very good choice of water for a closed loop. But, condensate's purity can be detrimental to the copper in the closed loop. A TCU uses copper tube to feed the heat exchangers and the heat exchangers have a copper braze. Ultra pure water can cause premature failure of a heat exchanger. Sentry recommends condensate as long as the water is treated with a corrosion inhibitor. A corrosion inhibitor negates the water's reactivity.

The heat exchangers used as condensers and evaporators are built of stainless steel plates and copper braze. Chlorides in the water, especially at elevated temperatures, attack stainless steel. Sentry has collected a great deal of information about chloride levels and their interaction with stainless steel. If there are any questions about the chloride levels, please measure current levels and then contact Sentry for help.

 Should the compressor cycle?

Never. The TCU is designed with a hot gas bypass valve (HGBV). The HGBV bypasses the condenser to control the refrigerant temperature to the evaporator. The compressor on a TCU runs continuously; cycling the compressor or allowing it to cycle will limit its life. A compressor can cycle if the refrigerant charge is too low. The low pressure suction switch is set to cut out at 45 psig (3 bar) and cut in at 60 psig (4 bar). There is a chance that a slow refrigerant leak can cycle between the cut out and cut in pressure. If this occurs, shut down the TCU and have the refrigerant cycle checked by a HVAC technician.

 My chilled water is below set point, why is the TCU sub cooling my samples?

Generally the problem in this scenario is not the sub cooling at the TCU but sub cooling at the primary coolers. Sentry's coolers are capable of cooling the hottest samples to within a couple of degrees of the cooling water. If the primary cooling water is significantly below 77° F (25° C), the sample entering the secondary cooler is below 77° F (25° C). Controlling the sample to temperature requires heat. A TCU is capable of providing some heat, but not enough to raise sample temperature 5 or 10 degrees. Regulating the flow of cooling water through the primary coolers raises the sample temperature to the secondary coolers. Many times the cold cooling water is used as primary cooling and as condenser cooling water. Flow rate in the condenser should be restricted if the cooling water is significantly below 85° F (30° C). Sentry offers a water-saver valve that senses the cooling water temperature and restricts flow based upon that measurement. Limiting the amount of condenser cooling water injects more heat in the refrigerant loop, allowing the TCU to provide more heat.

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