Honeywell TP970 Series, TP9600 Series Engineering Guide

TP970 and TP9600 Series

Pneumatic Thermostats

	ENGINEERING DATA
	Contents
Introduction ..............................................................................................................................	2
Fundamentals of
Thermostat Operation ..............................................................................................................................	2
General.................................................................................................................	2
Flapper-Nozzle Operation ....................................................................................	2
Valve Unit Operation ............................................................................................	2
TP973A, B and TP9630 A, B
Low-Capacity, Single-Temperature
Thermostats ..............................................................................................................................	4
General.................................................................................................................	4
Operation..............................................................................................................	5
Direct Action ..................................................................................................	5
Reverse Action ..............................................................................................	5
TP970A-D and TP9600A, B
High-Capacity, Single-Temperature
Thermostats ..............................................................................................................................	6
General.................................................................................................................	6
Operation..............................................................................................................	6
Direct Action ..................................................................................................	6
Reverse Action ..............................................................................................	7
TP971A-C and TP9610A, B
High-Capacity, Dual-Temperature
Thermostats ..............................................................................................................................	7
TP971B and TP9610B .........................................................................................	7
TP971C ................................................................................................................	7
TP971A and TP9610A .........................................................................................	7
Daytime Operation ...............................................................................................	8
Nighttime Operation .............................................................................................	8
Manual DAY Override...........................................................................................	9
TP972A and TP9620A High-Capacity,
Heating/Cooling Thermostat ..............................................................................................................................	10
General.................................................................................................................	10
Operation..............................................................................................................	12
TP974A Room
Temperature Sensor ..............................................................................................................................	12
General.................................................................................................................	12
Operation..............................................................................................................	12

Copyright © 1997 Honeywell Inc. • All Rights Reserved

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TP970 AND TP9600 SERIES PNEUMATIC THERMOSTATS

INTRODUCTION

This Engineering Data sheet provides detailed information on the operation of TP970 and TP9600 Series Pneumatic Thermostats (Thermostats). These Thermostats use the force-balance design with high nozzle feedback for stability. The TP970 and TP9600 Series includes the following thermostat models:

—TP970A-D and TP9600A, B: High capacity

Proportional control Single temperature

—TP971A-C and TP9610A, B: Dual temperature

Day/night control (automatic switchover through diaphragm logic)

Two sensing elements Individual setpoint control

—TP972A and TP9620A, B: High capacity

Heating/cooling control (automatic switchover through diaphragm logic)

Two sensing elements (one for heating control, one for cooling control)

—TP973A, B and TP9630A, B: Low capacity

Proportional control Single temperature

—TP974A:

Room temperature sensor

Used as a remote temperature transmitter for the RP920 Pneumatic Controller

This Engineering Data sheet describes the TP973A, B and TP9630A, B Thermostats first, because they are the simplest.

FUNDAMENTALS OF THERMOSTAT OPERATION

General

In force-balance design, two forces oppose each other until they are equal, or balanced. The TP970 and TP9600 Series Thermostats use the force of the bimetal to close the flapper over the nozzle and the opposing force of the air pressure in the nozzle chamber to lift the flapper (see the FlapperNozzle Operation section). When the forces are equal, a force-balance condition exists.

The throttling range setting and the calibration reference temperature determine the Thermostat span and calibration point. At control point the nozzle-flapper-bimetal assembly (acting through the calibration screw, setpoint cam, and the

throttling range (TR) adjustment) has a fixed branchline pressure (BLP) for each temperature within the temperature and throttling range settings. The forces within the nozzle- flapper-bimetal assembly always seek a balanced condition; giving the same BLP for the same temperature regardless of fluctuations in main air or the relative positions of the nozzle, flapper, and bimetal.

Flapper-Nozzle Operation

Flapper-nozzle operation is generally the same for all TP970 and TP9600 Thermostats. The Thermostat provides a branchline air pressure that is a function of the ambient temperature in the room or controlled space. As shown in Figure 1, the force of the temperature-sensing bimetal acting on one side of the flapper (Force A) is balanced by the feedback force of the pilot pressure through the nozzle acting on the other side of the flapper (Force B).

		SETPOINT
		KNOB
THROTTLING RANGE		CALIBRATION
ADJUSTMENT
		SCREW
	FORCE A
FLAPPER		BIMETAL
	<![if ! IE]> <![endif]>B	SETPOINT
	<![if ! IE]> <![endif]>FORCE
NOZZLE		CAM
		IN BALANCED STATE,
		FORCE B EQUALS FORCE A
		C6046

Fig. 1. Flapper-Nozzle-Bimetal Assembly.

The position of the flapper over the nozzle changes and creates a new pilot pressure when the bimetal force changes (through temperature or setpoint change). This pilot pressure feeds into the valve unit, which converts the low-capacity pilot pressure to a high-capacity branchline change (see the Valve Unit Operation section). Feedback at the nozzle regulates the pressure to negate the effect of normal main air supply fluctuations on the branch line.

Adjusting the throttling (proportioning) range changes the flapper lever position. Moving the setpoint cam changes the bimetal operating force and thus the setpoint.

Valve Unit Operation

TP970, TP9600, TP971, TP9610, TP972, and TP9620 Thermostats use force-balance valve units to amplify airflow and minimize air consumption without loss of required device capacity. Figure 2 is a cross-section of a TP970 and TP9600 Thermostat showing the relationship of the valve unit to the bimetal, nozzle, and other components.

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2

TP790 AND TP9600 SERIES PNEUMATIC THERMOSTATS

	THROTTLING RANGE		CALIBRATION
	ADJUSTMENT
		BIMETAL	SCREW
	NOZZLE

BRANCHLINE	FLAPPER
PRESSURE TAP

	SETPOINT
BRANCHLINE	CAM
CHAMBER
	SETPOINT
	KNOB
	FILTER
BLEED
	RESTRICTOR

EXHAUST	PILOT	VALVE				MAIN
	CHAMBER		BRANCH LINE				C6047-1
						LINE

Fig. 2. Cross Section of TP970 and TP9600 Thermostat Showing Valve Unit and Airflow.

TP970, TP9600, TP971, TP9610, TP972, and TP9620 Thermostats are designed around a valve unit for flow amplification rather than conventional pressure amplification. Branchline chamber and pilot chamber design are such that branch pressure is equal to nozzle pressure at a higher capacity.

Figures 3, 4, and 5 are cross-sections of the valve unit only, showing air passages and the pilot-branch diaphragm relationship.

Figure 3 shows a valve unit in a strategic or balanced condition. All the forces are equal; BLP equals the pilot-line pressure.

No main air enters the branchline chamber and no exhaust air leaves the branchline chamber. In this static condition, the valve is sealed at both Points A and B, preventing airflow.

POINT A	POINT B
	MAIN LINE
	BRANCH
	LINE
BRANCH	CHAMBER
EXHAUST	VALVE
PILOT CHAMBER	PILOT AIR
	C6048-1

Fig. 3. Valve Unit Flow Amplifier in a

Balanced (Static) Condition.

Figure 4 shows the valve unit supplying air to the branch line. This condition occurs when the bimetal sensing element forces the Thermostat flapper toward the nozzle, decreasing the nozzle-flapper gap and increasing the pilot pressure.

The increased pilot pressure against the pilot diaphragm overcomes the force of the BLP on the branchline diaphragm. This change opens the valve unit at Point B, allowing main air to flow into the branch line. BLP builds until the pressure against the branch diaphragm again equals the pressure against the pilot chamber diaphragm. The main airflow then shuts off, bringing the valve unit into a balanced condition at a new pressure.

With direct-acting bimetal sensors, a temperature increase closes the nozzle-flapper gap; with reverse-acting bimetal sensors, a temperature increase opens the nozzle-flapper gap. The arrows in the air passages in Figure 4 show the direction of airflow.

POINT A	POINT B
	MAIN LINE
	BRANCH
	LINE
EXHAUST AIR	VALVE
	PILOT AIR
NOTE: THE SEAL AT POINT A DOES NOT
ALLOW AIR TO EXHAUST.	C6049-1

Fig. 4. Valve Unit Shown with Pilot

Chamber Pressure Increased.

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TP970 AND TP9600 SERIES PNEUMATIC THERMOSTATS

Figure 5 shows the valve unit bleeding down the BLP. This condition occurs when the bimetal sensing element relaxes its force against the flapper, allowing the nozzle-flapper gap to increase.

POINT A

	VALVE	POINT B
	PISTON
		MAIN LINE
		BRANCH
		LINE
	EXHAUST
	AIR
		VALVE
		PILOT AIR
	NOTE: THE SEAL AT POINT A DOES NOT
	ALLOW AIR TO EXHAUST.	C6050-1

Fig. 5. Valve Unit Shown with Pilot

Chamber Pressure Decrease.

The reduction in pilot pressure against the pilot diaphragm allows the BLP to overcome the pressure in the pilot chamber. This change moves the valve piston down, sealing off Point B and opening Point A. Branchline air bleeds off until the pressure against the branchline diaphragm equals the pressure against the pilot chamber diaphragm. When the pressures become equal, the exhaust air is shut off at Point A. The valve unit is again in a balanced condition at the new pressure. The arrows in the air passages in Figure 5 show the direction of airflow.

The preceding explanation of valve unit operation is very important to understanding TP970, TP9600, TP971, TP9610, TP972, and TP9620 operation. As can be seen from Figures 3, 4, and 5, pilot pressure changes affect BLP changes in the same ratio. There is no pressure gain to amplify errors as with other pneumatic Thermostats. Still, the main air supply being switched through the valve unit, provides fast, high capacity increase and decrease of BLP.

TP973A, B AND TP9630 A, B LOWCAPACITY, SINGLE-TEMPERATURE THERMOSTATS

General

The TP973A, B TP9630A, B (Fig. 6) are the simplest Thermostats in the TP970 and TP9600 Series. Every other model includes the basic TP973 and TP9630 assembly with additions. Air going to the controlled device from the TP973 and TP9630 passes through an internal restrictor. The TP973A and TP9630A are direct acting (signal pressure increases as the temperature increases); the TP973B, reverse acting (signal pressure increases as the temperature decreases).

THROTTLING		SETPOINT
		KNOB
RANGE
ADJUSTMENT	BIMETAL
	NOZZLE	CALIBRATION
		SCREW
FLAPPER
		SETPOINT
		CAM
		SETPOINT
		KNOB

AIR CONNECTION

FILTER AND

RESTRICTOR C6051

Fig. 6. Basic TP973 and TP9630 Thermostat.

The TP973 and TP9630 are used on oneor two-pipe systems. Connections are made to main and branch for twopipe applications (see Fig. 7). The main air connector is plugged when used on one-pipe applications (Fig. 8). This causes the Thermostat to operate like any other bleed-type thermostat with a remote restrictor.

SETPOINT

KNOB

	THROTTLING RANGE	CALIBRATION
	ADJUSTMENT
		SCREW
	BIMETAL
	FLAPPER

CAM

NOZZLE SETPOINT SLOPE

NOZZLE

CHAMBER

CAM

RESTRICTOR	BRANCH
	LINE
	BACKPLATE
	TO CONTROLLED
MAIN LINE	DEVICE	C6052

Fig. 7. TP973 and TP9630 Operating Section.

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